What causes variation in electronegativity across the periodic table?

Electronegativity variation across the periodic table is caused by atomic size and the number of protons in the nucleus.

Electronegativity is a measure of the ability of an atom to attract a bonding pair of electrons. It is influenced by two main factors: the atomic size (or atomic radius) and the number of protons in the nucleus (atomic number).

The atomic size decreases as you move from left to right across a period in the periodic table. This is because the number of protons in the nucleus increases, pulling the electrons closer and reducing the atomic radius. As the atomic size decreases, the electrons are closer to the nucleus and more strongly attracted to it. This increases the atom's electronegativity.

On the other hand, as you move down a group in the periodic table, the atomic size increases. This is due to the addition of electron shells. The increased distance between the nucleus and the outer electrons reduces the attraction, leading to a decrease in electronegativity.

The number of protons in the nucleus also plays a significant role. More protons mean a stronger positive charge in the nucleus, which can attract electrons more effectively. Therefore, atoms with more protons tend to have higher electronegativity.

However, it's important to note that these trends are not absolute. There are exceptions, particularly among the transition metals and the noble gases. Transition metals have similar electronegativities due to their similar electron configurations, while noble gases have low electronegativities as they have full electron shells and do not tend to form bonds.

In summary, the variation in electronegativity across the periodic table is primarily due to changes in atomic size and the number of protons in the nucleus. These factors affect how strongly an atom can attract a bonding pair of electrons.