What is a period in a periodic table? How do atomic structures (electron arrangements) change in a period with increase in atomic numbers from left to right? (b) How do the following change on going from left to right in a period of the periodic table? (i) Chemical reactivity of elements (ii) Nature of oxides of elements Give examples in support of your answer.

(a) A period is a horizontal row of elements in aperiodic table. As the atomic number increases from left to right, the size of the atom increases.

(b)(i) On moving from left to right in a period of the periodic table the chemical reactivity first decreases and then increases.

For example:In the 3rd period of elements, sodium is a very reactive element, magnesium is less reactive whereas aluminium is still less reactive. Silicon is the least reactive in the third period. Now, phosphorus is quite reactive, sulphur is still more reactive whereas chlorine is very reactive.

• The basic nature of oxides decreases and the acidic nature of oxides increases from left to right in a period

For example:In the 3rd period of the periodic table, sodium oxide is highly basic in nature and magnesium oxide is comparatively less basic. The aluminium and silicon oxides are amphoteric in nature. Phosphorus oxides are acidic, sulphur oxides are more acidic whereas chlorine oxides are highly acidic.