Atomic Structure — Notes

An atom has a small, dense nucleus holding protons and neutrons, with electrons moving round it.

• Atomic number (Z) = number of protons. It decides the element. In a neutral atom, electrons = protons.
• Mass number (A) = protons + neutrons. Neutrons = A − Z.
• Symbol: $\ce{^{A}_{Z}X}$, for example $\ce{^{31}_{15}P}$ is phosphorus with 15 protons, 16 neutrons and 15 electrons.
• An ion has gained or lost electrons: $\ce{Ca^2+}$ (Z = 20) has 20 protons, 20 neutrons (A = 40) and 18 electrons. The nucleus is unchanged.

The electron was identified by J. J. Thomson, the proton by Goldstein and the neutron by Chadwick. The mass of an electron is so small (about one eighteen-hundred-and-fortieth of a proton) that the mass of an atom is taken as the mass of its nucleus.

Rutherford and his co-workers directed fast, positively charged alpha particles at a very thin gold foil, with a fluorescent screen round it.

Rutherford's model: an atom has a tiny, positively charged nucleus at the centre, and electrons revolve round it, much as planets go round the sun.

Merits: it showed that an atom has a nucleus and that most of it is empty space.

Limitation (demerit): a charged electron moving in a circle should lose energy continuously and spiral into the nucleus, so the atom should collapse; but atoms are stable. The model also gave no account of how the electrons are arranged.

Bohr removed the limitation of Rutherford's model with these postulates:

1. Electrons revolve round the nucleus only in certain fixed circular paths called orbits (shells or energy levels), named K, L, M, N outwards from the nucleus.
2. Each orbit has a fixed energy, which increases with distance from the nucleus.
3. An electron does not lose energy while it stays in an orbit, so it does not fall into the nucleus and the atom is stable.
4. An electron absorbs energy when it jumps to a higher orbit and gives out energy when it falls to a lower orbit.

Only these ideas are required at this level; Bohr's mathematical expressions are not part of the syllabus.

The maximum number of electrons in the shell number n is 2n²: K 2, L 8, M 18, N 32.

Rules for the first 20 elements:

• Fill the K shell first (2), then the L shell (8), then the M shell.
• The outermost shell holds at most 8 electrons (octet); the K shell holds at most 2 (duplet).
• After 2, 8, 8 the next electrons go into the N shell (potassium 2, 8, 8, 1; calcium 2, 8, 8, 2).

Chemical activity. A full outer shell (duplet or octet) is stable, so helium, neon and argon are inert. Atoms with an incomplete outer shell react by losing, gaining or sharing electrons to reach the nearest stable arrangement: sodium (2, 8, 1) loses one electron, chlorine (2, 8, 7) gains one and oxygen (2, 6) gains two. The number of electrons lost, gained or shared is the valency.

Isotopes are atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). They have the same chemical properties because they have the same number of electrons.

Written as $\ce{^{1}_{1}H}$, $\ce{^{2}_{1}H}$, $\ce{^{3}_{1}H}$; $\ce{^{12}_{6}C}$, $\ce{^{13}_{6}C}$, $\ce{^{14}_{6}C}$; $\ce{^{35}_{17}Cl}$, $\ce{^{37}_{17}Cl}$.

Chlorine's mass number in tables is 35.5 because natural chlorine is a mixture of chlorine-35 and chlorine-37 in the ratio about 3 : 1: (3 × 35 + 1 × 37) ÷ 4 = 35.5. Books give the ratio as 3 : 1 or as about 75 percent and 25 percent; the values are approximate.