Periodicity, Group 2 and Group 7(17) GapFill

In the Periodic Table, elements are arranged in order of increasing   electronegativityatomic radiusrelative atomic massatomic number. The Periodic Table is sometimes divided into the s, p, d and f  setsblocksclassessections, each of which contains the elements whose highest-energy electron lies in the corresponding orbital type.

Moving down a group, the atomic radius of the elements increases because the number of   neutronsisotopesnucleielectron shells per atom increases. Within a period, the number is the same for all elements, but the  nuclear chargereactivitypolarityvalency increases. This leads the atomic radius of the elements to decrease across the period, and for their first ionisation energy to (generally) increase.

The elements in Group 2 are called the  acidic earthalkaline earthalkalibasic earth metals, while those in Group 7(17) are known as the  noble gaseschromogenshalogensantigens. Their highest-energy electrons lie in  p and ds and pd and pp and s subshells, respectively. The chemistry of the Group 2 elements is dominated by  -21+2+-2 ions, and the chemistry of Group 7(17) elements by   1+1-2-2+ ions and simple covalent compounds.