The atomic numbers of the three elements X, Y and Z are 2, 6 and 10 respectively.
(i) Which two elements belong to the same group?
(ii) Which two elements belong to the same period?
Give reasons for your choice.
In order to determine the group of the elements, let us first write down the electronic configuration of the elements from their atomic numbers.
Element Atomic Number Electronic Configuration
Shells: K L
X 2 2
Y 6 2 4
Z 10 2 8
In this case, X and Z are in the same group. Generally, elements which contain the same number of valence electrons belong to the same group of the periodic table. However, here, the element X (2) fills its outermost shell with two electrons and Z (10), with 8 electrons. Both belong to group 18 (noble gas).
(ii) The elements which have the same valence shell belong to the same period in the periodic table. We can see from the electronic configurations of Y and Z , that they have the same valence shell (L). Therefore, elements Y and Z belong to the same period in the periodic table.