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Electronegativityis defined as the property of an atom/molecule to attract the electron of the other atom/molecule. Every atom has an electronegativity property which depends on the size of the atom and also the electron cloud.

Ionization energy is defined as the amount of energy required to eject the electron from the loosely held electron from the valence shell of the atom.

Electronegativity is a qualitative property thatdepends on the electron cloud and size of an atom.For example, the most electronegative atom in the periodic table is Fluorine.

Ionization energyis a quantitative property thatdepends on the amount of energy. As the amount of energy provided is greater, the faster the electron eject from the valence shell of the atom.

For example, the sodium atomrequires less energy to remove electrons from the valence shell and the fluorine atom requires a huge amount of I.E.

Hence, both, electronegativity (EN) and first ionization energy (${\text{1E}}_1$) increase across the period. Itis because while moving across the period, the number of valence electrons increases, and therefore, they more easily attract electrons towards themselves for completing their octet.

As the elements on the right side of the periodic table have a greater tendency to attract electrons, therefore they do not donate easily.Because of the huge difficulty in removing electrons needs more ionization energy.

Hence, both the tendency to attract electrons (EN) and the energy required to remove electron ( ${\text{1E}}_1$) increases across the period.