Structure of the Atom

Dalton had called the atom indivisible. By the end of the century static electricity showed that the story was incomplete. On rubbing, an object becomes charged. The charge does not arrive as new matter from outside. It comes from particles inside the atom.

By 1900 one sub-atomic particle had been identified, the electron. The identification belongs to J. J. Thomson. The symbol is e−. The charge is taken as minus one and the mass as negligible.

Goldstein saw canal rays in 1886. They were positively charged radiations. The proton opened from them. The proton charge equals the electron charge with the opposite sign. The mass is about 2000 times the electron mass. The symbol is p+. The mass is taken as one unit and the charge as plus one. An atom with one proton and one electron is neutral.

Thomson was the first to give a model of the structure of the atom. Electrons are embedded in a sphere of positive charge, like currants in a Christmas pudding. The watermelon picture says the same thing. The red part is the positive charge and the seeds are the electrons.

Two points sit in the model. The atom is a positive sphere and the electrons are sunk in it. The negative and positive charges are equal, so the whole atom is neutral.

There is no nucleus in this model. Neutrality is explained. The later scattering experiment is not. It is not a Thomson statement that the nucleus holds only nucleons.

Rutherford wanted to know how the electrons sit in the atom. Fast alpha particles were sent at a thin gold foil. The foil was about 1000 atoms thick, and still as thin as possible. Alpha particles are doubly charged helium ions. The mass is 4 u, so a fast particle carries a good deal of energy. Because they are heavier than protons, large deflections were not expected.

The results went the other way. Most particles went straight through. Some bent by small angles. About one in every 12000 bounced back. Rutherford found that rebound astonishing.

There are three conclusions. Most of the space is empty, because most particles did not bend. The positive charge occupies a very small space, because few particles bent. All the positive charge and the mass sit in a very small volume, because very few particles turned by about 180° and came back. The radius of the nucleus came out about 10⁵ times smaller than the radius of the atom.

The nuclear model: there is a positive nucleus and nearly all the mass is there. Electrons revolve in circular paths. The nucleus is very small compared with the atom. A revolving electron can lose energy and fall into the nucleus, so the model does not explain the stability of the atom.

To remove the drawback of Rutherford, Niels Bohr stated two points. Only certain special orbits are allowed inside the atom. These are called discrete orbits. While revolving in these orbits the electron does not lose energy.

These orbits are also called energy levels. They are named K, L, M, N or n = 1, 2, 3, 4. When the shells are complete the atom is more stable and less reactive. That point opens later in valency.

In 1932 J. Chadwick found another particle. It has no charge and its mass is nearly equal to the proton mass. The name is neutron. The symbol is n. Every atom except hydrogen has neutrons in the nucleus. The mass of an atom is the sum of the masses of the protons and the neutrons. Both sit in the nucleus, so they are also called nucleons.

There are three particles: the electron negative, the proton positive, the neutron uncharged. A neutron is not made by sticking an electron and a proton together.

Bohr and Bury showed how to share electrons into orbits.

First rule: the maximum number of electrons in a shell is 2n². n is the orbit number.

Second rule: the outermost shell does not hold more than 8 electrons. Third rule: electrons are not put in an outer shell until the inner shells are filled. The filling is stepwise.

The table of the first eighteen elements stops here. Argon is 2, 8, 8. The 2n² maximum of M is 18, but in argon M is the outer shell, so it holds 8. The counts 2, 8, 8 in the picture are the outer filling. The full capacity of M is 18. Helium has 2 electrons in its outer shell and valency 0. The octet of 8 belongs to the later inert elements.

The electrons of the outer shell are the valence electrons. If the outer shell is completely full, the valency is zero. Helium has 2 outer electrons. The later inert elements have 8. An outer shell of 8 electrons is an octet.

An atom loses, gains or shares electrons to complete the octet. The number of electrons used in that work is the valency. Hydrogen, lithium and sodium have one electron in the outer shell, so the valency is 1. Magnesium is 2, aluminium is 3.

If the outer electrons are close to a full shell, the count turns around. Fluorine has 7 in the outer shell. Losing seven is hard and gaining one is easy. The valency is 8 − 7 = 1, not 7. Oxygen has 6, so the valency is 2. Chlorine is 1, sulphur is 2. Silicon has 4 in the outer shell, so the valency is 4. The table gives phosphorus both 3 and 5.

The atomic number Z is the number of protons in the nucleus. Every atom of an element has the same Z. An element is identified by its protons. For hydrogen Z = 1. For carbon Z = 6. In a neutral atom the electrons also equal Z.

Almost the whole mass belongs to protons and neutrons. Both are in the nucleus and are called nucleons. Mass number = number of protons + number of neutrons. Carbon has 6 + 6 = 12, so the mass is 12 u. Aluminium has 13 protons and 14 neutrons, mass number 27.

The way to write it: mass number above, atomic number below, symbol in the middle. Nitrogen is ¹⁴₇N. This is not a chemical equation. The mass number of oxygen is 8 + 8 = 16. Sulphur is 16 + 16 = 32.

Some atoms of an element have the same atomic number and different mass numbers. These are isotopes. Hydrogen has three forms. Protium has 1 proton and 0 neutrons. Deuterium has 1 proton and 1 neutron. Tritium has 1 proton and 2 neutrons. ¹²₆C and ¹⁴₆C of carbon are isotopes. So are 35 and 37 of chlorine.

Each isotope is a pure substance. Chemical properties stay alike and physical properties change. The average mass comes from the percentages. In nature chlorine is 35 u and 37 u in the ratio 3:1. No single atom has a mass of 35.5 u.

The uses differ. One isotope of uranium is a fuel in nuclear reactors. One isotope of cobalt is used in the treatment of cancer. One isotope of iodine is used in the treatment of goitre. Tincture of iodine is not that isotope.

Isobars are atoms of different elements that have the same mass number and different atomic numbers. Calcium has atomic number 20 and argon has 18. Both have mass number 40. Isotopes share Z. Isobars share A.