Which element has more metallic character - aluminium or magnesium?
I know that metallic character decreases along a period (from left to right) and increases down a group.
Aluminium comes after Magnesium in the same period (period 3) in the periodic table. So, I thought Magnesium would have greater metallic character.
However, I read in a book that Aluminium has greater metallic character. Is this true? If this is true, why?
Answer
Yes, you are right that $\ce{Al}$ is more metallic than $\ce{Mg}$.
It is because, $\ce{Mg}$ have fully filled $3\text{s}$ subshell. Due to this, the first ionisation energy$^{[1]}$ of $\ce{Al}$ (578 kJ/mol) is less than that of $\ce{Mg}$ (738 kJ/mol).
Filled and half-filled subshells show a small increase in stability in the same way that filled shells show increased stability. So, when trying to remove an electron from one of these filled or half-filled subshells, a slightly higher ionization energy is found.$^{[2]}$
You can also say that higher levels of subshells are further away from the nucleus which is why it is easier to remove an electron from $\ce{Al}$ than from $\ce{Mg}$.
References:
1. http://en.wikipedia.org/wiki/Ionization_energy
2. http://www.grandinetti.org/ionization-energy-trends
Edit: Updated reference(s)
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