(a) 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) The horizontal rows of elements in a periodic table are called periods. As we move from left to right in a period, the atomic number of elements increases which means that the no. of protons and electrons in the atom increases. Due to large positive charge on the nucleus, the electrons are pulled in more close to the nucleus and the size of the atom decreases.

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

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.

(ii) On moving from left to right in a period, the basic nature of oxides decreases and the acidic nature of oxides increases.

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 in nature.