One might ask what is periodicity? The word periodicity has many meanings. Periodicity is defined as the occurrence of similar properties in elements having similar positions in the periodic table. Periodicity is found through certain periodic trends. There are four periodic trends found in the periodic table: atomic radius, ionization energy, electron affinity, and electronegativity.
The first periodic trend is atomic radius. Atomic radius is found in several ways. The radii of some atoms are determined by measuring the distance between like atoms that are joined together. There are other measurements that are taken from bond lengths of compound atoms. As you move down a group, a new electron shell is added to the atoms. This makes the atomic radius of an atom increase. Thus, when you move down a group the atomic radius increases. As you move from left to right, the radii decreases. You are adding more protons and electrons, but the electrons are in the same energy level so the protons are attracting the electron wave.
This takes up less room, and it makes the atom radius smaller.
Ionization Energy is the amount of energy needed to remove an electron from an atom in its ground state. Ionization energy decreases as you move down a group of elements, but it increases across a period. The farther you move down in a group, the farther away from the nucleus the last energy level is. More electrons lie between the outermost electrons and the nucleus. These outer electrons are "shielded", and they don't have as much hold to the atom so they can be easily removed. This requires less ionization energy, thus, making the ionization energy decrease as you move down the periodic table. As you move left to right, the atoms have more pull on their...