Question

A) What is the number of valence electrons in the atoms of the first element in a period?
(B) What is the usual number of valence electrons in the atoms of the last elements in the period?

Answer 1

We can predict the number of valence electrons. For neutral atoms, the number of valence electrons is equal to the atom's main group number. The main group number for an element can be found from its column on the periodic table. For example, carbon is in group four and has four valence electrons. Oxygen is in group six and has six valence electrons.

Complete answer:
-$1$ is the number of valence electrons in the atoms of the first element in a period. A valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.
-The usual number of valence electrons in the atoms of the last elements in a period is $8$. As a general rule, a main-group element (except hydrogen or helium) tends to react to form ${s^2}{p^6}$ electron configuration. This tendency is called the octet rule, because each bonded atom has $8$ valence electrons including shared electrons.

Note:
The relationship between group number and valence electrons is that the number of valence electrons is equal to the group number of the representative elements. The relation between the group number and valence electrons is that "group number is equal to the number of valence electrons".