How many valence electrons does magnesium have in magnesium hydride depends on whether magnesium is treated as a neutral atom (Mg) or as the ion present in the compound (Mg²⁺). In General Chemistry, magnesium hydride (MgH₂) is described predominantly as an ionic solid composed of Mg²⁺ and H⁻ (hydride) ions, so the neutral-atom valence electrons explain what magnesium contributes, while the ionic picture explains where those electrons end up.
Valence electrons of neutral magnesium
Magnesium is a Group 2 (s-block) element. Its outermost occupied shell is the n = 3 shell, and the electron configuration ends in 3s². Those two 3s electrons are the valence electrons of neutral magnesium.
Magnesium has 2 valence electrons as a neutral atom because its highest principal energy level contains 3s².
Electron transfer picture in magnesium hydride (MgH₂)
In MgH₂, magnesium is commonly assigned an oxidation state of +2, and hydrogen is assigned −1 (hydride). This reflects electron transfer: magnesium loses its two 3s valence electrons, and each hydrogen gains one electron to complete its 1s shell.
The ionic description is strongest for the solid-state lattice (electrostatic attraction between Mg²⁺ and H⁻). In more advanced treatments, partial covalent character can be discussed, but the valence-electron accounting remains the same: magnesium contributes two electrons in total.
Lewis electron-dot interpretation
Lewis electron-dot diagrams track valence electrons and the stable noble-gas-like outcomes. Neutral Mg is shown with two dots (two valence electrons). Each H atom is shown with one dot. In MgH₂, Mg becomes Mg²⁺ (no valence dots shown on magnesium because the 3s shell is emptied), and each hydrogen becomes H⁻ with a pair of electrons (a filled 1s² shell).
Numerical summary in a compact electron-count table
| Species | Outer-shell description | Electrons in the outer shell (valence-shell view) | Interpretation in MgH₂ |
|---|---|---|---|
| Mg (neutral) | 3s² in the n = 3 shell | 2 | Two electrons available to be transferred/used in bonding |
| Mg²⁺ | n = 3 shell emptied; configuration ends at 2p⁶ | 0 (outermost shell has no electrons) | Magnesium cation in the ionic picture |
| H (neutral) | 1s¹ | 1 | Each hydrogen gains one electron |
| H⁻ (hydride) | 1s² (filled shell) | 2 | Hydride anion in MgH₂ |
Common pitfalls in “valence electrons in MgH₂” wording
A frequent ambiguity is mixing “valence electrons of the element magnesium” with “electrons in the outermost shell of Mg²⁺.” The element magnesium has 2 valence electrons, but once magnesium forms Mg²⁺, those two electrons are no longer on magnesium; they have been transferred to hydrogen in the simplest Lewis model for magnesium hydride.
Another common mistake is counting all electrons of magnesium (12) instead of valence electrons. Valence-electron counting uses only the electrons in the highest occupied principal level for the neutral atom, which for Mg is the n = 3 level containing 3s².
Direct conclusion
Magnesium has 2 valence electrons as a neutral atom, and magnesium hydride (MgH₂) is explained by transfer of those two electrons to hydrogen, giving Mg²⁺ and 2 H⁻ in the standard General Chemistry Lewis-dot and oxidation-state framework.