The table shows the atomic radii of the elements belonging to third period. Elements of third period Na Mg Al Si P S Cl Atomic radii (pm) 186 160 143 117 110 104 99 How does the atomic radius change as you go from left to right in a period?

Atomic radius defines the size of an atom which is the distance between the centre of the nucleus and the outermost shell of an atom. As we move left to right in the period atomic radius decreases. Sodium is the left most element in the third period has the largest atomic radius while the rightmost element of the third group chlorine has the smallest atomic radius. Elements of third period Na Mg Al Si P S Cl Atomic radii (pm) 186 160 143 117 110 104 99

Look at the table and tell: Second period Li Be B C N O F Atomic number 3 4 5 6 7 8 9 Which elements will form negative ions and which will form positive ions?

Atoms which have the tendency to lose electrons forms the positive ions. These ions are called cations. Electronic configuration of Li 3 = 2 , 1 Electronic configuration of Be 4 = 2 , 2 Electronic configuration of B 5 = 2 , 3 These elements have 1 , 2 , 3 electrons in their valence shell. These atoms can easily lose these valence electrons on little energy and forms positive ions Li + , Be + + , B + + + respectively. Atoms which have the tendency to gain electrons forms the negative ions. These ions are called anions. Electronic configuration of C 6 = 2 , 4 Electronic configuration of N 7 = 2 , 5 Electronic configuration of O 8 = 2 , 6 Electronic configuration of F 9 = 2 , 7 Carbon has four electrons in its valence shell which requires huge energy to lose or gain electrons. It generally shares its electrons. N , O and F have 5 , 6 , 7 electrons in their valence shell. These atoms can easily gain 3 , 2 , 1 electrons respectively to complete their octet and forms negative ions N - - - , O - - , F - respectively.