Magnesium

Magnesium is atom number 12 — every Magnesium atom has exactly 12 protons in its center, and that number is what makes it Magnesium and nothing else. Its symbol is Mg. At room temperature it is a solid.

Magnesium is a chemical element with symbol Mg and atomic number 12. It is a shiny gray solid which bears a close physical resemblance to the other five elements in the second column (Group 2, or alkaline earth metals) of the periodic table:they each have the same electron configuration in their outer electron shell producing a similar crystal structure. Magnesium is the ninth most abundant element in the universe.

Summary from the source dataset, derived from Wikipedia (CC BY-SA 3.0) · original article

Magnesium sits in period 3, group 2 of the s-block. Its 12 electrons fill shells of 2, 8, 2 — written as [Ne] 3s2. Its electronegativity is 1.31 on the Pauling scale, and pulling the first electron off takes 737.7 kJ/mol. The outermost shell holds 2 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2
Ionization energies (kJ/mol): 737.7, 1450.7, 7732.7, 10542.5, 13630, 18020, 21711, 25661 … (12 published)
Electron affinity: -40 kJ/mol · Electronegativity (Pauling): 1.31
Density: 1.738 g/cm³ · melts 923 K · boils 1363 K

Values exactly as published — see where the data comes from.