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.
