How does electronegativity vary on moving across the row and down the columns of the periodic table?

What is electronegativity:

Electronegativity is a tendency. It shows us how much an element's atom attracts electrons towards itself. The more electronegative an element is, the more will it attract electrons towards itself. This means that an electronegative element is a non metallic element. It tends to gain electrons in order to form negative ions, and if not so, then it tries to attract the shared pair of electrons in a covalent bond towards itself.

Factors on which it is dependent

Electronegativity is dependent on the atomic number and the atomic size of an atom.

• Atomic number: Increased atomic number increases electronegativity.
  Reason: Electronegativity is the tendency of an atom to attract electrons towards itself. The more the atomic number will be, the more will be the positive charge in the nucleus, and therefore the more will the atom attract electrons towards itself.
• Atomic size: Lesser the atomic size, the more is the electronegativity
  Reason: Less atomic size means lesser distance between the nucleus and the outermost electron orbit. Thus the attractive force on the outermost shell will increase since it is closer to the positively charged protons in the nucleus. Hence the atom will attract electrons to come into its outermost orbit with more force, and this means that the electronegativity of the atom is more. Therefore the lesser the atomic size, the more electronegative an element is.

Variation of electronegativity

• Across rows in the periodic table: Electronegativity increases as we move from left to right in the periodic table.
  Reason: when one moves from left to right in a row of the periodic table, the atomic size of the elements decreases (this is explained in the post of atomic size), and the atomic number increases. Therefore the atoms of elements are able to attract electrons with more force. Therefore electronegativity increases.
• Down the columns of the periodic table: Electronegativity decreases as we move from top to bottom in the periodic table.
  Reason: When one goes from top to bottom in a group of the periodic table, the atomic size of elements increases but the atomic number also increases. The increase in atomic size is effective to a greater extent than the increase in atomic number, and hence the electronegativity of elements decreases down a group.