Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

Atomic radius: Smaller the atomic radius, the higher is the atomic energy. This is because the Coulombic attractive forces on the negatively charged valence electrons by the positively charged nucleus is much higher in the case of smaller atom-the force of attraction decreases with the increase in atomic radius, i.e separation between the electrons and the nucleus in accordance with the inverse square law.

Nuclear charge: the higher the positive charge of the nucleus, stronger is it's attraction for the electrons, and hence the harder it is to remove the electrons.

Orbital penetration: It's easier to remove electrons from p orbitals than from s orbitals, because the s orbitals penetrate towards the nucleus more closely than the p orbitals, thus making the electrons in the s orbital feel greater nuclear attraction.

Electron pairing: within a subshell, paired electrons are easier to remove than unpaired ones. This is because repulsion between electrons in the same orbital is higher than repulsion between electrons in different orbitals.

Shielding or screening effect of the inner orbitals: Due to the electrons present in the inner orbitals, the electrons in the outermost orbitals feel lesser attraction for the nucleus than expected. In other words, the inner electron orbitals effectively screen or shield the outermost electrons from the nucleus, due to which the ionization energy decreases.