Question

In going across a row of the periodic table, electrons are added and ionization energy generally increases. In going down a column of the periodic table, electrons are also being added but ionization energy generally decreases. Explain.

Short answer

When we go across a row, several protons increases which hold the valence electrons very tightly so the ionization energy increases. As we go down the column, new shells are added to the atoms. So, it is easier to remove the electron due to the weak force of attraction between the nucleus and the valence electron. Hence, the ionization energy decreases.

Step-by-step solution

Step 1:Explain why going across a row of the periodic table, electrons are added and ionization energy generally increases.    

When we go across a row, not only more electrons are added to the table but also more protons are added. Several core-shell electrons remain the same. So, the effective nuclear charge increases. Hence, the valence electrons are held more tightly. So, the ionization energy increases.

Explain why in going down a column of the periodic table, electrons are also being added but ionization energy generally decreases.

As we go down the column of the periodic table, more and more shells are added to the table. Each shell when added increases the atomic radii. The distance of the nucleus from the outermost shell increases so, the strength of attraction on electrons by the nucleus decreases. So, the ionization energy decreases.