Why does atomic size decrease as we move from left to right along a period in the periodic table?

Give reasons for the following:
$(a)$ Lithium atom is smaller than Sodium atom.
$(b)$ Chlorine (Atomic Number $17$) is more electronegative than Sulphur (Atomic Number $16$).

The scientist who showed that the atomic number of an element is a more fundamental property than its atomic mass is:

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

Describe the basic character of the oxides of third period elements across the period from left to right.

$(i)$ The elements of the second period along with their atomic numbers in parentheses are given below: $B(5)$,$Be(4)$,$O(8)$,$N(7)$,$Li(3)$,$C(6)$,$F(9)$.
$(a)$ Arrange them in the same order as they appear in the periodic table.
$(b)$ Which element has the largest and smallest atom?
$(ii)$ Why does the atomic radius change as we move from left to right in a period?