Periodicity

As you move accross a period, atomic number increases by one. The number of electrons therefore increases by one, and is reflected in the electron configuration:

From Period 2:

• 4Be:  1s2 2s2
• 5B:    1s2 2s2 2p1
• 6C:    1s2 2s2 2p2

From Period 3:

• 12Mg:  1s2 2s2 2p6 3s2
• 13 Al:   1s2 2s2 2p6 3s2 3p1
• 14 Si:   1s2 2s2 2p6 3s2 3p2

• Atomic radius decreases across a period:

Because as nuclear charge increases, (i.e increasing number of protons) there is a freater attraction between electrons in the same outer shell and the nucleus.

• Atomic radius decreases down a group:

Because outer she;lls are further from the nucleus and increasingly shielded as more inner shells reduce the attraction between the electrons and the nucleus.

• The 1st I.E is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole o gaseous ions.
• Factors affecting I.E include: atomic radius, nuclear charge and sheilding
• There is a general increase in 1st I.E across Period 2 and 3 with increasing atomic number. This is due to: increasing strength of attraction between an increasing number of protons in the nucleus when electrons are being added to the same outer shell and the atomic radius is decresing. Consequently, more energy is required to remove an electron crossing a period due to an increasing attraction between nucleus and outer electrons.
•  Decrease in 1st IE down a group. This is due to: there is greater shielding of outer electrons from more inner shells which also leads to less attraction between them and the nucleus. Consequently, less energy is required to remove an electron going down a group due to a decreasing attraction between nucleus and outer electrons.