The idea of shielding plays a big role in how atomic size and ionization energy change in the periodic table.

1. Atomic Size:

• Shielding happens when the inner electrons push against the outer electrons. This makes the outer electrons feel less pull from the nucleus.
• For example, if we look at Group 1 (like Lithium to Cesium), when we go down the group, more electron layers are added. This extra shielding makes the atoms get bigger.

2. Ionization Energy:

• Ionization energy is the amount of energy needed to take away an electron.
• More shielding means that the outer electron feels less attraction to the nucleus.
• For example, in Group 2, Beryllium has a higher ionization energy than Barium. This is because Barium has more shielding, making it easier to take away an electron.

Summary:

As you go down a group, more shielding leads to bigger atomic sizes and lower ionization energies.