How does the electronic configuration in atoms change(i) in a period from left to right?(ii) in a group from top to bottom?

(i) In a period from left to right:

1. The atomic number increases from left to right in a period. This means that the number of protons in the nucleus of the atom increases, which also means that the number of electrons in the atom increases.

2. As we move from left to right, electrons are added one at a time to the lowest energy orbital available within the same energy level.

3. The electrons are added to the same energy level rather than a new one because the energy levels are filled in a specific order, not necessarily in numerical order. For example, the 4s orbital is filled before the 3d orbital because it has a lower energy.

4. The increase in the number of protons causes an increase in the positive charge of the nucleus, which in turn increases the attraction between the nucleus and the electrons. This causes the atomic radius to decrease from left to right across a period.

(ii) In a group from top to bottom:

1. As we move down a group, the atomic number increases. This means that the number of protons in the nucleus of the atom increases, which also means that the number of electrons in the atom increases.

2. However, unlike in a period, as we move down a group, the electrons are added to a new energy level further from the nucleus.

3. The increase in the number of energy levels means that the atomic radius increases as we move down a group.

4. The increase in the number of energy levels also means that there is an increase in the shielding effect. The inner electrons shield the outer electrons from the positive charge of the nucleus, which decreases the attraction between the nucleus and the outer electrons.

5. Therefore, the atomic size increases from top to bottom in a group.