chemistry librarian

Konsep Bilangan Oksidasi

2011-01-02T07:32:00.001-08:00

Penentuan bilangan oksidasi mengikuti beberapa aturan tertentu. Aturan tersebut dijelaskan sebagai berikut.

1. Bilangan oksidasi unsur dalam keadaan bebas adalah nol. (Unsur bebas diantaranya Na, Al, Fe, O₂, I₂, Br₂, S₈, H₂, atau Cl₂.)

Contoh :

Bilangan oksidasi Br dalam Br₂ = 0

Bilangan oksidasi Na dalam Na = 0

2. Bilangan oksidasi ion dari suatu atom (monoatom) sama dengan muatan ionnya.

Contoh :

Bilangan oksidasi Mg dalam Mg ²⁺ = +2

Bilangan oksidasi I dalam I^- = -1

3. Bilangan oksidasi unsur logam dalam senyawanya selalu bertanda positif yang besarnya tergantung elektron valensinya (letak dalam sistem periodik unsur).

Contoh :

Bilangan oksidasi golongan IA (Li, Na, K, Rb, Cs) = +1

Bilangan oksidasi golongan IIA ( Be, Mg, Ca, Sr, Ba, Rb) = +2

Khusus untuk beberapa unsur transisi memiliki bilangan oksidasi yang bervariasi.

Beberapa unsur transisi dalam senyawanya dengan variasi bilangan oksidasinya.

Unsur

Bilangan Oksidasi

+2 dan +3

+2 dan +4

+1 dan +2

4. Bilangan oksidasi atom H dalam senyawa adalah +1, kecuali pada senyawa hibrida (NaH, LiH, dan CaH₂), bilangan oksidasi H adalah -1.

Contoh :

Bilangan oksidasi H dalam H₂S = +1

Bilangan oksidasi H dalam LiH = -1

5. Bilangan oksidasi atom O dalam senyawa adalah -2.

Contoh oksida O dalam H₂O = -2

Pengecualian :

Bilangan oksidasi O dalam senyawa peroksida (H₂O_2,Na₂O_2,dan BaO₂)= -1

Bilangan oksidasi O dalam senyawa superoksida (KO₂ dan RbO₂) = -1/2

Bilangan oksidasi O dalam senyawa OF₂ = +2

6. Jumlah total bilangan oksidasi (BO) atom-atom dalam senyawa netral adalah nol.

Contoh :

Bilangan oksidasi HNO₃ = 0

Bilangan oksidasi H₂SO₄ = 0

7. Jumlah total bilangan oksidasi (BO) atom-atom dalam ion poliatom sama dengan muatan ion tesebut.

Contoh :

Bilangan oksidasi H₂PO₄^- = -1

Bilangan oksidasi SO₃^2- = -2

Berdasarkan aturan tersebut, bilangan oksidasi setiap atom dalam senyawanya dapat dihitung.

Contoh Soal :

Tentukan bilangan oksidasi S dalam H₂SO_4.

Jawab :

(2 x BO H) + (1 x BO S) + (4 x BO O) = 0

(2 x 1) + (1 x BO S) + (4 x (-2)) = 0

2 + BO S - 8 = 0

BO S = +6

Konsep Oksidasi Reduksi Berdasarkan Pengikatan dan Pelepasan Oksigen

2011-01-02T07:27:00.000-08:00

Reaksi reduksi merupakan reaksi pelepasan oksigen dari suatu zat (unsur maupun senyawa). Adapun zat yang mengalami reduksi (kehilangan oksigen) disebut oksidator.

Contoh :

a. Reduksi bijih timbal menjadi timbale murni

PbO_(s) + C_(s) → Pb_(s) + CO_(g)

PbO bertindak sebagai oksidator. PbO melepas oksigen menjadi Pb

b. Reduksi bijih besi menjadi besi murni

FeO_(s) + CO_(g) → Fe_(s) + CO_2(g)

FeO bertindak sebagai oksidator. FeO melepas oksigen menjadi Fe.

Bila sepotong besi dibiarkan di udara terbuka makin lama besi itu akan membentuk karat besi (Fe₂O₃). Reaksi perkaratan besi disebabkan terjadinya antara besi dan oksigen. Dapat dikatakan besi mengalami oksidasi.

Besi + Oksigen → Besi Oksida

4Fe_(s)+ 3O₂(g) → 2Fe₂O_3(s)

Pada reaksi oksidasi tidak hanya dengan unsur oksigen tetapi dapat pula dengan senyawa yang mengandung oksigen seperti reaksi antara hydrogen dengan oksida tembaga.

Hidrogen + Oksida Tembaga → Air + Tembaga

H_2(g)+ CuO_{(s) →}H₂O_(g)+ Cu_(s)

Oksigen terlepas dari senyawa CuO dan bergabung dengan hydrogen membentuk H₂O. Hidrogen yang mengikat oksigen mengalami oksidasi.

Jadi, menurut konsep pengikatan dan pelepasan oksigen, reaksi oksidasi adalah reaksi yang mengakibatkan pengikatan oksigen. Sedangkan reaksi reduksi adalah reaksi yang melibatkan pelepasan oksigen.

Di sisi lain, reaksi oksidasi melibatkan penggabungan oksigen pada suatu zat (kebalikan reduksi). Zat yang mengalami oksidasi (menerima oksigen) dinamakan reduktor.

Contoh :

a. Pembakaran metana

CH_4(g) + 2O_2(g) → CO_2(g) + H₂O_(l)

CH₄bertindak sebagairedoktor. Atom karbon dalam metana menerima oksigen membentuk CO₂. Sementara itu, atom hidrogen dalam metana juga menerima oksigen membentuk H₂O.

b. Pembakaran pita magnesium

2Mg_(s) + O₂ → 2MgO_(s)

Mg bertindak sebagai reduktor karena menerima oksigen menjadi MgO.

Reaksi reduksi oksidasi selalu berlangsung bersamaan. Dalam suatu reaksi, jika reaksi oksidator mengalami reduksi, pasti ada reaksi oksidasi yang mengiringinya.

Contoh :

Reaksi reduksi oksidasi antara logam seng dengan timbal (II) oksida.

Zn_(s) + PbO_(s) → ZnO_(s) + Pb_(s)

Zn_(s) Menerima oksigen (reduksi) Pb_(s)

PbO_(s) Kehilangan oksigen (oksidasi) ZnO_(s)

Zn bertindak sebagai reduktor.

PbO bertindak sebagai oksidator.

Perkembangan Konsep Reduksi dan Oksidasi

2011-01-02T07:25:00.000-08:00

Reaksi kimia dapat digolongkan ke dalam reaksi redoks dan reaksi bukan redoks. Istilah redoks berkaitan dengan peristiwa reduksi dan oksidasi. Pengertian reduksi dan oksidasi itu sendiri telah mengalami perkembangan. Pada awalnya, peristiwa reduksi dan oksidasi dikaitkan dengan pelepasan dan pengikatan oksigen, sedangkan reduksi sebagai pelepasan oksigen. Pada perkembangan selanjutnya, oksidasi dan reduksi dikaitkan dengan penangkapan atau pelepasan elektron, dan kemudian dengan perubahan bilangan oksidasi. Hal ini dimaksudkan untuk memberikan cakupan yang lebih luas bagi jenis reaksi tersebut.

Reaksi redoks banyak terjadi dalam kehidupan kita sehari-hari. Beberapa contoh dapat kita sebutkan , yaitu perkaratan besi, reaksi-reaksi pembakaran, oksidasi makanan dalam sel, fotosintesis, dan peleburan biji logam. Aki , baterai, dan berbagai proses elektrolisis seperti penyepuhan, juga berjalan berdasarkan reaksi redoks.

Pengertian Kompetensi Guru

2011-01-02T07:09:00.000-08:00

Menurut (Soekamto, dkk., 2006) kompetensi merupakan kebulatan penguasaan pengetahuan, keterampilan dan sikap yang ditampilkan melalui unjuk kerja. Depdiknas (2004:7) merumuskan kompetensi guru sebagai pengetahuan, keterampilan, dan nilai-nilai dasar yang direfleksikan dalam kebiasaan berfikir dan bertindak.Menurut Syah (2000:203) kompetensi guru adalah kemampuan seorang guru dalam melaksanakan kewajiban-kewajibannya secara bertanggung jawab dan layak.

Muhaimin (2004:151) menjelaskan kompetensi guru adalah seperangkat tindakan intelegen penuh tanggung jawab yang harus dimiliki seorang guru sebagai syarat untuk dianggap mampu melaksankan tugas-tugas dalam bidang pekerjaannya. Mulyasa (2003:38) mengartikan kompetensi guru sebagai penguasaan terhadap suatu tugas, keterampilan, sikap, dan apresiasi yang diperlukan untuk menunjang keberhasilan. Majid (2005:6) menjelaskan kompetensi yang dimiliki oleh setiap guru akan menunjukkan kualitas guru dalam mengajar. Kompetensi tersebut akan terwujud dalam bentuk penguasaan pengetahuan dan profesional dalam menjalankan fungsinya sebagai guru. Berdasarkan uraian di atas kompetensi guru dapat didefinisikan sebagai penguasaan terhadap pengetahuan, keterampilan, nilai dan sikap yang direfleksikan dalam kebiasaan berpikir dan bertindak dalam menjalankan profesi sebagai guru.

Standar Nasional Pendidikan

2011-01-02T03:07:00.000-08:00

Standar nasional pendidikan Indonesia diatur dalam peraturan Pemerintah Nomor 19 Tahun 2005. Standar Nasional Pendidikan (SNP) adalah kriteria minimal tentang sistem pendidikan di seluruh wilayah hukum Negara Kesatuan Republik Indonesia. Lingkup Standar Nasional pendidikan meliputi:

Standar isi adalah ruang lingkup materi dan tingkat kompetensi yang dituangkan dalam kriteria tentang kompetensi tamatan, kompetensi bahan kajian, kompetensi mata pelajaran, dan silabus pembelajaran yang harus dipenuhi oleh pesera didik pada jenjang dan jenis pendidikan tertentu.

Standar proses adalah standar nasional pendidikan yang berkaitan dengan pelaksanaan pembelajaran pada satu satuan pendidikan untuk mencapai standar kompetensi lulusan.

Standar kompetensi lulusan adalah kualifikasi kemampuan lulusan yang mencakup sikap,pengetahuan, dan keterampilan.

Standar pendidik dan tenaga kependidikan adalah kriteria pendidikan prajabatan dan kelayakan fisik maupun mental, serta pendidikan dalam jabatan.

Standar sarana dan prasarana adalah standar nasional pendidikan yang berkaitan dengan kriteria minimal tentang ruang belajar, tempat berolah raga, tempat beribadah, perpustakaan, laboratorium, bengkel kerja, tempat bermain, tempat berkreasi dan bereaksi, serta sumber belajar lain, yang diperlukan untuk menunjang proses pembelajaran, termasuk penggunaan teknologi informasi dan komuikasi.

Standar pengelolaan adalah standar nasional pendidikan yang berkaitan dengan perencanaan, pelaksanaan, dan pengawasan kegiatan pendidikan pada tingkat satuanpendidikankabupaten/kota, provinsi, atau nasional agar tercapai efisiensi dan efektifitas penyelenggaraan pendidikan.

Standar pembiayaan adalah standar yang mengatur komponen-komponen dan besarnya biaya operasi satuan pendidikan yang berlaku selama satu tahun.

Standar penilaian pendidikan adalah standar nasional pendidikan yang berkaitan dengan mekanisme, prosedur, dan instrumen penilaian hasil belajar peserta didik.

Perkembangan Kurikulum Kimia di SMP

2011-01-02T02:57:00.000-08:00

Adanya pengembangan KBK menjadi KTSP menssyaratkan adanya mata pelajaran IPA terpadu (SAINS) dan IPS pada tingkat SMP. Pembelajaran terpadu yang terdiri atas beberapa mata pelajaran menyediakan lingkungan belajar sehingga peserta didik mendapat pengalaman belajar konsep-konsep dari berbagai sub mata pelajaran. Pengertian terpadu disini mengandung makna menghubungkan IPA dengan berbagai mata pelajaran .

Dengan karakteristik mata pelajaran Ilmu Pengetahuan Alam (IPA) SMP/MTS antara sebagai berikut.(www. Puskur. net)

Ilmu Pengetahuan Alam merupakan gabungan dari unsur-unsur fisika, kimia, biologi, serta bumi dan antariksa.
Kompetensi dasar IPA berasal dari struktur keilmuan fisika, kimia, biologi, serta bumi dan antariksa yang dikemas sedemikian rupa sehingga menjadi pokok bahasan atau topik tertentu.
Kompetensi dasar IPA juga menyangkut berbagai masalah yang dirumuskan dengan pendekatan interdisipliner dan multidisipliner.

Karakteristik mata pelajaran IPA SMP/MTS tersebut merupakan bagian dari kriteria struktur kurikulum SMP/MTS yang didasarkan kepada standard kompetensi mata pelajaran. Dalam pendistribusian mata pelajaran SMP tersebut kimia masih tergabung dalam kelompok mata pelajaran Sains. Hal ini mengakibatkan waktu belajar kimia relatif sedikit dibandingkan dengan pelajaran lain yang memiliki jam les yang sama.

Dalton, John (1766–1844)

2011-01-02T02:54:00.001-08:00

English chemist, one of the founders of atomic theory. Some of his proposals have since proved to be incorrect, but his chief contribution was that he channeled the thinking of contemporary scientists along the correct lines, particularly in his method of using established facts to explain a new phenomenon.

Dalton was born in the village of Eaglesfield near Cockermouth, Cumbria, on or about September 6, 1766. He was the third of six children of a weaver, who was a devout Quaker and did not register the date of his son's birth. Dalton attended the village Quaker school and by the age of 12 was running it. He later became headmaster of a school in Kendal, before taking up a post in 1793 to teach mathematics and natural philosophy in Manchester. Dalton was largely self-taught, his Quaker beliefs excluding him from attending Oxford or Cambridge universities (at that time open only to members of the Church of England).

Even before he moved to Manchester, a wealthy Quaker friend, the blind philosopher John Gough, had stimulated in Dalton an interest in meteorology and for 57 years (beginning in 1787) he kept a diary of observations about the weather. He gave lectures on this subject to the Manchester Literary and Philosophical Society, of which he became honorary secretary and later president. He determined that the density of water varies with temperature, reaching a maximum at 6.1°C/42.5°F; the modern value of this temperature is 4°C). He also lectured about color blindness, a condition he shared with his brother and which for a time was known as Daltonism. He resigned his lectureship in Manchester in 1799 in order to pursue his own researches, working as a private tutor to make a living. He did, however, remain as the society's secretary and was given accommodation in a house they bought for him. This house, still containing many of Dalton's records, was destroyed in a bombing raid in 1940 during World War II. He was awarded a government pension of £150 in 1833, which was doubled three years later. He died in Manchester on July 27, 1844.

From his interest in the weather, atmosphere, and gases in general, Dalton in 1803 proposed his law of partial pressures (which states that, in a mixture of gases, the total pressure is the sum of the pressures that each component would exert if it alone occupied the same volume). He also studied the variation of a gas's volume with temperature, concluding (independently of Joseph Gay-Lussac) that all gases have the same coefficient of thermal expansion. Gaseous diffusion and the solubility of gases in water were also the subjects of his experiments.

The work on the absorption of gases led Dalton to formulate his atomic theory—he considered that gases must be made up of particles that can somehow occupy spaces between the particles that make up water, and that in a mixture of gases the different particles must intermingle rather than separate into layers depending on their density. When presented in his book New System of Chemical Philosophy (1808), the idea that atoms of different elements have different weights was supported by a list of atomic weights (atomic masses) and his newly devised system of chemical symbols. Combinations of element symbols could be made to represent compounds.

Many of the atomic weights (confused with equivalent weights) were incorrect, for example oxygen's was 8 and carbon's 6, but a pattern had been established, introducing order to a science that was hitherto little more than a collection of facts.

Taking into account later work, Dalton's atomic theory may be summarized as follows:

(a) Matter cannot be subdivided indefinitely, because each element consists of indivisible particles called atoms.

(b) The atoms of the same element are alike in every respect, having the same weight (mass), volume, and chemical properties; atoms of different elements have different properties.

(c) In chemical combinations of different elements, atoms join together in simple definite numbers to form compound atoms (now called molecules).

His formula for water—one hydrogen atom combined with one oxygen atom—was wrong, although he was more fortunate with carbon monoxide (“carbonic oxide”) and carbon dioxide (“carbonic acid”). But nevertheless he did bring a sort of order to the existing chaos, and provided a foundation for several generations of scientists. Several years later Jöns Berzelius was to supersede Dalton's system with the chemical symbols and formulas still used today.

Throughout his life Dalton retained his Quaker habits and dress, and new acquaintances were often taken aback by his appearance. He continued to keep his diary, which eventually ran to 200,000 entries. He distrusted the results of other workers, preferring to rely on his own experiences. As he grew older he became almost a recluse, with few friends and deeply involved in his pursuit of knowledge. And although he shunned fame and glory, he became famous even outside the realms of science. When his coffin stood on public display in Manchester Town Hall, more than 40,000 people filed past to pay their respects.

Einstein, Albert (1879–1955)

2011-01-02T02:41:00.000-08:00

German-born U.S. theoretical physicist who revolutionized our understanding of matter, space, and time with his two theories of relativity. Einstein also established that light may have a particle nature and deduced the photoelectric law that governs the production of electricity from light-sensitive metals. For this achievement, he was awarded the 1921 Nobel Prize for Physics. Einstein also investigated Brownian motion and was able to explain it so that it not only confirmed the existence of atoms but could be used to determine their dimensions. He also proposed the equivalence of mass and energy, which enabled physicists to deepen their understanding of the nature of the atom and explained radioactivity and other nuclear processes. Einstein, with his extraordinary insight into the workings of nature, may be compared with Isaac Newton (whose achievements he extended greatly) as one of the greatest scientists ever to have lived.

Einstein was born in Ulm, Germany, on March 14, 1879. His father's business enterprises were not successful in that town and soon the family moved to Munich, where Einstein attended school. He was not regarded as a genius by his teachers; indeed there was some delay because of his poor mathematics before he could enter the Eidgenössosche Technische Hochschule in Zürich, Switzerland, when he was 17. As a student he was not outstanding and Hermann Minkowski, who was one of his mathematics professors, found it difficult in later years to believe that the famous scientist was the same person he had taught as a student.

Einstein graduated in 1900 and after spending some time as a teacher, he was appointed a year later to a technical post in the Swiss Patent Office in Berne. Also in 1901 he became a Swiss citizen and then in 1903 he married his first wife, Mileva Marié. This marriage ended in divorce in 1919. During his years with the Patent Office, Einstein worked on theoretical physics in his spare time and evolved the ideas that were to revolutionize physics. In 1905 he published three classic papers on Brownian motion, the photoelectric effect, and special relativity.

Einstein did not, however, find immediate recognition. When he applied to the University of Berne for an academic position, his work was returned with a rude remark. But by 1909 his discoveries were known and understood by a few people, and he was offered a junior professorship at the University of Zürich. As his reputation spread, Einstein became full professor or Ordinariat, first in Prague in 1911 and then in Zürich in 1912, and he was then appointed director of the Institute of Physics at the Kaiser Wilhelm Institute in Berlin in 1914, where he was free from teaching duties.

The year 1915 saw the publication of Einstein's general theory of relativity, as a result of which Einstein predicted that light rays are bent by gravity. Confirmation of this prediction by the solar eclipse of 1919 made Einstein world famous. In the same year he married his second wife, his cousin Elsa, and then traveled widely to lecture on his discoveries. One of his many trips was to the California Institute of Technology during the winter of 1932. In 1933 Adolf Hitler came to power and Einstein, who was a Jew, did not return to Germany but accepted a position at the Princeton Institute for Advanced Study, where he spent the rest of his life. During these later years, he attempted to explain gravitational, electromagnetic, and nuclear forces by one unified field theory. Although he expended much time and effort in this pursuit, success was to elude him.

In 1939, Einstein used his reputation to draw the attention of the U.S. president to the possibility that Germany might be developing the atomic bomb. This prompted U.S. efforts to produce the bomb, though Einstein did not take part in them. In 1940 Einstein became a citizen of the United States. In 1952, the state of Israel paid him the highest honor it could by offering him the presidency, which he did not accept because he felt that he did not have the personality for such an office. Einstein was a devoted scientist who disliked publicity and preferred to live quietly, but after World War II he was actively involved in the movement to abolish nuclear weapons. He died at Princeton on April 18, 1955.

Einstein's first major achievement concerned Brownian motion, the random movement of fine particles that can be seen through a microscope and was first observed in 1827 by Robert Brown when studying a suspension of pollen grains in water. The motion of the particles increases when the temperature increases but decreases if larger particles are used. Einstein explained this phenomenon as being the effect of large numbers of molecules bombarding the particles. He was able to make predictions of the movement and size of the particles, which were later verified experimentally by the French physicist Jean Perrin. Experiments based on this work was used to obtain an accurate value of the Avogadro number, which is the number of atoms in one mole of a substance, and the first accurate values of atomic size. Einstein's explanation of Brownian motion and its subsequent experimental confirmation was one of the most important pieces of evidence for the hypothesis that matter is composed of atoms.

Einstein's work on photoelectricity began with an explanation of the radiation law proposed in 1901 by Max Planck. This is E = hν, where E is the energy, h is a number known as Planck's constant, and ν is the frequency of radiation. Planck had confined himself to black-body radiation, and Einstein suggested that packets of light energy are capable of behaving as particles called “light quanta” (later called photons). Einstein used this hypothesis to explain the photoelectric effect, proposing that light particles striking the surface of certain metals cause electrons to be emitted. It had been found experimentally that electrons are not emitted by light of less than a certain frequency ν0; that when electrons are emitted, their energy increases with an increase in the frequency of the light; and that an increase in light intensity produces more electrons but does not increase their energy. Einstein suggested that the kinetic energy of each electron, mv2, is equal to the difference in the incident light energy hν and the light energy needed to overcome the threshold of emission hν0. This can be written mathematically as:

mv2 = hν − hν0

and this equation has become known as Einstein's photoelectric law. Its discovery earned Einstein the 1921 Nobel Prize for Physics.

Einstein's most revolutionary paper of 1905 contained the idea that was to make him famous, relativity. Up to this time, there had been a steady accumulation of knowledge that suggested that light and other electromagnetic radiation do not behave as predicted by classical physics. For example, no method had been found to determine the velocity of light in a single direction. All the known methods involved a reflection of light rays back along their original path. It had also proved impossible to measure the expected changes in the speed of light relative to the motion of the earth. The Michelson–Morley experiment had demonstrated conclusively in 1881 and again in 1887 that the velocity of light is constant and does not vary with the motion of either the source or the observer. To account for this, Hendrik Lorentz and George FitzGerald independently suggested that all lengths contract in the direction of motion by a factor of (1 − v2/c2 ), where v is the velocity of the moving body and c is the speed of light.

The results of the Michelson–Morley experiment confirmed that no “ether” can exist in the universe as a medium to carry light waves, as was required by classical physics. This did not worry Einstein, who viewed light as behaving like particles, and it enabled him to suggest that the lack of an ether removes any frame of reference against which absolute motion can be measured. All motion can only be measured as motion relative to the observer. This idea of relative motion is central to relativity, and is one of the two postulates of the special theory, which considers uniform relative motion. The other is that the velocity of light is constant and does not depend on the motion of the observer. From these two notions and little more than school algebra, Einstein derived that in a system in motion relative to an observer, length would be observed to decrease by the amount postulated by Lorentz and FitzGerald. Furthermore, he found that time would slow by this amount and that mass would increase. The magnitude of these effects is negligible at ordinary velocities and Newton's laws still held good. But at velocities approaching that of light, they become substantial. If a system were to move at the velocity of light, to an observer its length would be zero, time would be at a stop, and its mass would be infinite. Einstein therefore concluded that no system can move at a velocity equal to or greater than the velocity of light.

Einstein's conclusions regarding time dilation and mass increase were later verified with observations of fast-moving subatomic particles and cosmic rays. Length contraction follows from these observations, and no velocity greater than light has ever been detected. Einstein went on to show in 1907 that mass is related to energy by the famous equation

E = mc2

This indicates the enormous amount of energy that is stored as mass, some of which is released in radioactivity and nuclear reactions—for example, in the sun. One of its many implications concerns the atomic weights of the elements, which are not quite what would be expected by the proportions of their isotopes. These are slightly decreased by the mass equivalent of the binding energy that holds their molecules together. This decrease can be explained by, and calculated from, the famous Einstein formula.

Minkowski, Einstein's former teacher, saw that relativity totally revised accepted ideas of space and time and expressed Einstein's conclusions in a geometric form in 1908. He considered that everything exists in a four-dimensional space–time continuum made up of three dimensions of space and one of time. This interpretation of relativity expressed its conclusions very clearly and helped to make relativity acceptable to most physicists.

Einstein now sought to make the theory of relativity generally applicable by considering systems that are not in uniform motion but under acceleration. He introduced the notion that it is not possible to distinguish being in a uniform gravitational field from moving under constant acceleration without gravitation. Therefore, in a general view of relativity, gravitation must be taken into account. To extend the special theory, he investigated the effect of gravitation on light and in 1911 concluded that light rays would be bent in a gravitational field. He developed these ideas into his general theory of relativity, which was published in 1915. According to this theory, masses distort the structure of space–time.

Einstein was able to show that Newton's theory of gravitation is a close approximation of his more exact general theory of relativity. He was immediately successful in using the general theory to account for an anomaly in the orbit of the planet Mercury that could not be explained by Newtonian mechanics. Furthermore, the general theory made two predictions concerning light and gravitation. The first was that a red shift is produced if light passes through an intense gravitational field, and this was subsequently detected in astronomical observations in 1925. The second was a prediction that the apparent positions of stars would shift when they are seen near the sun because the sun's intense gravity would bend the light rays from the stars as they pass the sun. Einstein was triumphantly vindicated when observations of a solar eclipse in 1919 showed apparent shifts of exactly the amount he had predicted.

Einstein later returned to the quantum theory. In 1909, he had expressed the need for a theory to reconcile both the particle and wave nature of light. In 1923, Louis de Broglie used Einstein's mass–energy equation and Planck's quantum theory to achieve an expression describing the wave nature of a particle. Einstein's support for de Broglie inspired Erwin Schrödinger to establish wave mechanics. The development of this system into one involving indeterminacy did not meet with Einstein's approval, however, because it was expressed in terms of probabilities and not definite values. Einstein could not accept that the fundamental structure of matter could rest on chance events, making the famous remark “God does not play dice.” Nevertheless, the theory remains valid.

Albert Einstein towers above all other scientists of the 20th century. In changing our view of the nature of the universe, he has extended existing laws and discovered new ones, all of which have stood up to the test of experimental verification with ever-increasing precision. The development of science in the future is likely to continue to produce discoveries that accord with Einstein's ideas. In particular, it is possible that relativity will enable us to make fundamental advances in our understanding of the origin, structure, and future of the universe.

Atomic number

2011-01-01T11:52:00.000-08:00

The number of elementary positive charges (protons) contained within the nucleus of an atom. It is denoted by the letter Z. Correspondingly, it is also the number of planetary electrons in the neutral atom.

Relation to x-ray wavelengths

The concept of atomic number emerged from the work of G. Moseley, done in 1913–1914. Using a crystal of potassium ferrocyanide in the Bragg x-ray spectrometer, he measured the wavelengths of the most energetic rays (K and L lines) produced by using the elements calcium to zinc as targets in an x-ray tube. The square root of the frequency, √v of these x-rays increased by a constant amount in passing from one target to the next. These data, when extended, gave a linear plot of atomic number versus √v for all elements studied, using 13 as the atomic number for aluminum and 79 for that of gold. Further, when extrapolated, this plot predicted the atomic number of hydrogen to be 1 and that of uranium to be 92.

Moseley's atomic numbers were quickly recognized as providing an accurate sequence of the elements, which the chemical atomic weights had sometimes failed to do. For example, the elemental pairs argon-potassium, cobalt-nickel, tellurium-iron, which had appeared in the periodic table in the reverse order of their atomic weights, were shown in their correct order by their atomic numbers. Additionally, the atomic number sequence indicated the positions of elements that had not yet been discovered. Two examples were Z = 43 (technetium) and Z = 61 (promethium); both elements are unstable and were not correctly identified until they had been produced in nuclear reactions.

The atomic number not only identifies the chemical properties of an element but facilitates the description of other aspects of atoms and nuclei. Thus, atoms with the same atomic number are isotopes and belong to the same element, while nuclear reactions may alter the atomic number: in alpha decay, Z → Z − 2; in beta emission, Z → Z + 1; and in positron emission or K capture, Z → Z − 1.

Atomic and nuclear properties

When specifically written, the atomic number is placed as a subscript preceding the symbol of the element, while the mass number (A) precedes as a superscript, for example, ²⁷₁₃Al, ²³⁸₉₂U.

Teori Atom Dalton

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Tahukah Anda bahwa di dunia ilmu kimia ini patut dikenang satu nama sebagai pencetus teori atom modern yang asli. Dia adalah seorang guru dan ahli kimia berkebangsaan Inggris bernama John Dalton (1776 – 1844). Sumbangan Dalton merupakan keunikan dari teorinya yang meliputi dua hal:

Dia adalah orang pertama yang melibatkan kejadian kimiawi seperti halnya kejadian fisis dalam merumuskan gagasannya tentang atom.
Dia mendasarkan asumsinya pada data kuantitatif, tidak menggunakan pengamatan kualitatif atau untung-untungan. Teori atom Dalton dikemukakan berdasarkan dua hukum, yaitu hokum kekekalan massa dan hukum perbandingan tetap.

Teori atom Dalton dikembangkan selama periode 1803-1808 dan didasarkan atas tiga asumsi pokok, yaitu:

Setiap unsur kimia tersusun oleh partikel-partikel kecil yang tidak dapat dihancurkan dan dipisahkan yang disebut atom. Selama mengalami perubahan kimia, atom tidak bisa diciptakan dan dimusnahkan. Semua atom dari suatu unsur mempunyai massa dan sifat yang sama, tetapi atom-atom dari suatu unsur berbeda dengan atom-atom dari unsure yang lain, baik massa maupun sifat-sifatnya yang berlainan.
Dalam senyawa kimiawi, atom-atom dari unsur yang berlainan melakukan ikatan dengan perbandingan angka sederhana.

Pengenalan Atom

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Atom merupakan partikel paling kecil yang masih mempunyai sifat unsur. Menurut para ahli fisika, jari-jari suatu atom sekitar 3 – 15 nm (1 nm = 10-9 meter). Sampai sekarang belum ada alat yang dapat memperbesar atom sehingga dapat diamati secara jelas. Walaupun atom tidak dapat dilihat dengan jelas, para ahli dapat membuat perkiraan gambaran mengenai atom berdasarkan data eksperimen dan kajian teoretis yang dilakukannya. Perkiraan tentang gambaran atom tersebut dinamakan model atom. Itulah sebabnya mengapa model atom telah beberapa kali mengalami perubahan sesuai dengan perkembangan ilmu pengetahuan.

Konsep atom pertama kali dikemukakan oleh Democritus. Atom berasal dari kata atomos (dalam bahasa Yunani a = tidak, tomos = dibagi), jadi atom merupakan partikel yang sudah tidak dapat dibagi lagi. Menurut Dalton konsep atom Democritus ini tidak bertentangan dengan Hukum Kekekalan Massa dan Hukum Kekekalan Energi, sehingga Dalton membuat teori tentang atom yang salah satunya adalah materi tersusun atas partikel-partikel terkecil yang tidak dapat dibagi lagi. Tetapi konsep atom Dalton belum memuaskan para ilmuwan pada masa itu. Ditemukannya elektron, proton, neutron, dan radioaktivitas menyebabkan timbulnya teori baru tentang atom. Mulai dari teori atom Thomson, Rutherford, Bohr, dan Mekanika Kuantum.

Teori atom pertama kali dikemukakan oleh John Dalton pada tahun 1803, yaitu atom merupakan partikel terkecil yang tidak dapat dibagi lagi. Kemudian diketahui bahwa atom ternyata terdiri atas partikelpartikel yang lebih kecil lagi yaitu proton, elektron, dan neutron. Partikel penyusun atom itu disebut partikel subatom atau partikel dasar atom. Proton merupakan partikel subatom yang bermuatan positif, ditemukan oleh Eugen Goldstein pada tahun 1886. Elektron merupakan partikel subatom yang bermuatan negatif, ditemukan oleh Joseph John Thomson pada tahun 1897. Neutron merupakan partikel subatom yang tidak bermuatan, ditemukan oleh James Chadwick pada tahun 1932. Model atom terus berkembang mulai dari model atom Dalton, Thomson, Rutherford, Bohr, sampai dengan model atom modern yang kita gunakan sekarang.