Why 1st period has 2 element and 2nd period has 8 element?

Answer 1

The factor that determines the number of elements in a period is the number of electrons that can be fitted into the orbitals (shells, orbits). The first two elements (Hydrogen and Helium) have one and two electrons, respectively. They fit into the first orbital which is closer to the nucleus than other orbitals.

Because this orbital is so small, if you tried to put more than 2 electrons in it, the electromagnetic repulsions between electrons would push the extra electrons out of the first orbital. Only two can ever fit and so only elements can be in period 1.

The second orbital is further from the nucleus and so bigger. Therefore, the electrons in it are not as close to each other. The repulsive force is not as great and 8 electrons can be accomodated. Therefore, the next 8 elements ( Lithium, Berylium, Boron, Carbon, Nitrogen, Oxygen, Fluorine, Neon) all fit into period 2.

This answer is adequate for most year 11 students. For Year 12/13, You need to consider the shape of the orbitals. Period one has only one s-orbital which accepts two electrons of different "spin". Period 2 has an s-orbital and three p-orbitals at right angle to each other. These can accept two electrons each, of opposite "spin", giving a total of 8.

Answer 2

The number of elements in a period is based on the number of electron shells each element has. The first period has 2 elements because atoms in this period have only one electron shell. The second period has 8 elements because atoms in this period have two electron shells.