Why do the elements at the bottom of the periodic table have lower ionization energies than their group/family partners at the top of the periodic table?

Elements at the bottom of the periodic table have lower ionization energies compared to their group or family partners at the top of the periodic table because, they have more energy levels.  

Ionization energy decreases down the group as less energy is required to remove outer most electrons as energy levels increases.

Further Explanation Ionization energy Ionization energy is the energy required to remove outermost electrons from the outermost energy level. Energy is required to remove an electron from an atom. The closer an electron is to the nucleus the more energy is required, since the electron is more tightly bound to the atom thus making it more difficult to remove, hence higher ionization energy. Ionization energy increases across the periods and decreases down the group from top to bottom.  Additionally, the ionization energy increases with subsequent removal of a second or a third electron. First ionization energy  This is the energy required to remove the first electron from the outermost energy level of an atom. Energy needed to remove the second electron to form a divalent cation is called the second ionization energy. Trend in ionization energy  1. Down the group (top to bottom)Ionization energy decreases down the groups in the periodic table from top to bottom. It is because as you move down the group the number of energy levels increases making the outermost electrons get further from the nucleus reducing the strength of attraction to the nucleus. This means less energy will be required compared to an atoms of elements at the top of the groups. 2. Across the period  (left to right)Ionization energy increases across the period from left to right. This can be explained by an increase in nuclear energy as extra protons are added to the nucleus across the period increasing the strength of attraction of electrons to the nucleus.Consequently, more energy is needed to remove electrons from the nucleus.

Keywords: Ionization energy, periodic table, energy levels, electrons

Learn more about Ionization energy: Chemical families: Trend in ionization energy: First ionization energy:

Level: High school  

Subject: Chemistry  

Topic: Periodic table and chemical families  

Sub-topic: Ionization energy