Properties of atoms and nuclear reactions

The periodic table arranges elements in order of increasing atomic number, Z, which is the number of protons in the nucleus. In a neutral atom the number of electrons equals Z. In the shell model used in this course, the nucleus is surrounded by electrons in distinct energy levels, or shells. The electrons are held near the nucleus by the electrostatic attraction between the positive nucleus and the negative electrons.

For elements 1 to 20 the shells fill in a simple pattern: the first shell holds at most 2 electrons, the second holds 8, and the third holds 8 before the fourth shell begins. No s, p, d or f orbital theory is needed.

The layout of the table follows directly from this pattern. The period number equals the number of occupied shells, so sodium (2,8,1) is in period 3. The group is linked to the number of outer-shell (valence) electrons: group 1 atoms have one, group 2 atoms have two, group 17 atoms have seven and group 18 atoms have a full outer shell. Elements in the same group therefore have similar chemical behaviour and show gradual changes in physical properties.

Atomic radius. Across a period the nuclear charge increases while electrons are added to the same shell, so the outer electrons are pulled closer and the radius decreases: a sodium atom is larger than a chlorine atom. Down a group each new period adds another occupied shell, so the radius increases: lithium is smaller than sodium, which is smaller than potassium. Valency is the number of electrons an atom typically loses, gains or shares when it bonds.

Across periods 2 and 3 it rises from 1 (group 1) to 4 (group 14), then falls to 3, 2, 1 and 0 for groups 15 to 18.

Ions. Atoms with one to three outer electrons tend to lose them and form positive ions; atoms with five to seven outer electrons tend to gain electrons and form negative ions. The charge on a stable ion equals the number of electrons lost or gained to reach a full outer shell: group 1 forms 1+ ions, group 2 forms 2+ ions, group 16 forms 2− ions and group 17 forms 1− ions.

Comparing groups. Group 1 metals are soft, have low densities and have melting points that decrease down the group. Group 2 metals are harder and denser, with higher melting points. Group 17 elements exist as diatomic molecules whose melting and boiling points increase down the group: fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature.

Group 18 elements are monatomic gases that are very unreactive because their outer shells are full, and their boiling points also rise down the group.