New chemistry

New chemistry

Antoine Lavoisier revolutionized chemistry. He named the elements carbon, hydrogen and oxygen; discovered oxygen’s role in combustion and respiration; established that water is a compound of hydrogen and oxygen; discovered that sulfur is an element, and helped continue the transformation of chemistry from a qualitative science into a quantitative one.

Lavoisier announced a new fundamental law of nature; the law of conservation of mass:

matter is conserved in chemical reactions

In 1789 Lavoisier published his groundbreaking Elementary Treatise on Chemistry. It contained a list of chemical elements. The list included oxygen, nitrogen, hydrogen, sulfur, phosphorus, carbon, antimony, cobalt, copper, gold, iron, manganese, molybdenum, nickel, platinum, silver, tin, tungsten, and zinc.

Dalton's fascination with gases gradually led him to formally assert that every form of matter (whether solid, liquid or gas) was also made up of small individual particles called Atoms.

The main points of Dalton's atomic theory, as it eventually developed, are:

• Elements are made of extremely small particles called atoms.
• Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties.
• Atoms cannot be subdivided, created or destroyed.
• Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
• In chemical reactions, atoms are combined, separated or rearranged.

In an article he wrote for the Manchester Literary and Philosophical Society in 1803, Dalton created the first chart of atomic weights.

In 1808,In A New System of Chemical Philosophy, Dalton introduced his belief that atoms of different elements could be universally distinguished based on their varying atomic weights. In so doing, he became the first scientist to explain the behavior of atoms in terms of the measurement of weight. He also uncovered the fact that atoms couldn't be created or destroyed.

Following the Karlsruhe meeting in 1860, values of about 1 for hydrogen, 12 for carbon, 16 for oxygen, and so forth were adopted. This was based on a recognition that certain elements, such as hydrogen, nitrogen, and oxygen, were composed of diatomic molecules and not individual atoms.

An important long-term result of the Karlsruhe Congress was the adoption of the now-familiar atomic weights (actually, atomic masses) of approximately 1 for hydrogen, 12 for carbon, 16 for oxygen, Cl 35.5, K39, Ca 40, Br 80, Rb 85, Sr 88, I 127, Cs 133, Ba 137 and so forth.

On March 6, 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both weight and valence. This presentation stated that:

1. The atomic mass, exhibit an apparent periodicity of properties.
2. Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
4. The elements which are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of iodine (126.9).
8. Certain characteristic properties of elements can be foretold from their atomic weights.

In 1900 Electron was discovered as a subatomic particle.

Electron was discovered by J. J. Thomson in Cathode Ray Tube (CRT) experiment.

1. Electrons are negatively charged particles with charge-to-mass ratio −1.76×10⁸ C/gm
2. The charge of an electron was measured by R. Millikan in Oil drop experiment.
3. Charge of an electron is −1.60×10⁻¹⁹ C
4. Mass of an electron is 9.1×10⁻²⁸ gram.
5. Electron is approximately 2000 times lighter than hydrogen.

Rutherford proposed the following structural features of an atom:

1.Most of the atom’s mass and its entire positive charge are confined in a small core, called nucleus. The positively charged particle is called proton.

2.Most of the volume of an atom is empty space.
3.The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.

But scientists soon realized that the atomic model offered by Rutherford is not complete. Various experiments showed that mass of the nucleus is approximately twice than the number of proton. What is the origin of this additional mass?

In 1930, W. Bothe and H. Becker found an electrically neutral radiation when they bombarded beryllium with alpha particle. They thought it was photons with high energy (gamma rays).

• In 1932, Irène and Frédéric Joliot-Curie showed that this ray can eject protons when it hits paraffin or H-containing compounds.
• The question arose that how mass less photon could eject protons which are 1836 times heavier than electrons. So the ejected rays in bombardment of beryllium with alpha particles cannot be photon.
• In 1932, James Chadwick performed the same experiment as Irène and Frédéric Joliot-Curie but he used many different target of bombardment besides paraffin. By analyzing the energies of different targets after bombardment he discovered the existence of a new particle which is charge-less and has similar mass to proton. This particle is called neutron. Beryllium undergoes the following reaction when it is bombarded with alpha particle: Be⁹ + ᾳ⁴ → C¹² + C¹³ + ꞑ