Electronegativity

Electronegativity measures an atom's strength to attract and form bonds with electrons. This property exists due to the electronic configuration of atoms. Most atoms prefer to fulfilling the octet rule (having the valence, or outer, shell comprise of 8 electrons). Since elements on the left side of the periodic table have less than a half-full valence shell, the energy required to gain electrons is significantly higher compared to the energy required to lose electron

✌️As you move to the right across a period of elements, electronegativity increases. When the valence shell of an atom is less than half full, it requires less energy to lose an electron than gain one and thus, it is easier to lose an electron. Conversely, when the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.
✌️As you move down a group, electronegativity decreases. This is because the atomic number increases down a group and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.