The size of an atom affects how much energy it takes to remove an electron. This energy is called ionization energy.

Here’s how it works:

1. Moving Across a Period:

   • As we go from left to right on the periodic table, the size of the atom gets smaller.
   • This happens because more positive charges in the nucleus pull on the electrons more strongly.
   • As a result, it's harder to remove an electron, so the ionization energy goes up.
   • For example, when we look at sodium (Na) and chlorine (Cl), chlorine has a higher ionization energy. This is because Cl's electrons are closer to the nucleus, making them harder to remove.

2. Moving Down a Group:

   • As we go down a group in the periodic table, the size of the atom gets bigger.
   • This is because new electron shells are added, which means the outer electrons are further away from the nucleus.
   • Because those outer electrons are farther away, it’s easier to remove them, so the ionization energy goes down.
   • For instance, potassium (K) has a lower ionization energy than lithium (Li) because K’s outer electrons are much further from the nucleus and feel less pull from it.

Understanding these changes helps us predict how elements will act!