Consider the group 1A element sodium (atomic number 11), the group 3A element aluminum (atomic number 13), and the group 7A element chlorine (atomic number 17). These elements are in period 3. How are the first ionization energies of these elements related?

Ionization energy is the energy needed to remove a valence electrón from a gaseous atom.

The energy needed to remove the first electron is the first ionization energy.

The stronger the atom atracts its electrons the higher the ionization energy.

A low ionization energy means that the atom may loose one electron easily to form a positive ion (cation)

In the period 3, sodium has the lowest ionization energy (it forms sodium ion easily), aluminium has a higher ionization energy (meaning that it is more difficult to form a positive ion) and Chlorine will never lose an electron to form a positive ion (on the contrary Chlorine is willing to accept one electron from other atom to form a negative ion, i. e. an anion).

The growing of the ionization energy inside a period is due to the size of the growing positive charge (number of protons) in the nucleous, which implies a stronger atraction to the electrons of valence.