# Why is the hydride ion negative?

Recently, I saw a video about sodium hydride and it saw it was made up with $\ce{Na+}$ ions and $\ce{H-}$ ions. My problem is: aren't all hydrogen ions positive? And in what conditions can $\ce{H-}$ occur?

• Sodium and most other metals have a lower electronegativity than hydrogen. Thus, in metal hydrides the metal transfers an electron to hydrogen leading to a negatively charged hydride ion, $\ce{H-}$, and a positively charged metal ion. – Philipp Dec 6 '14 at 1:29
• It's negative simply because it has more electrons (2) than protons (1), so the main question is how it can occur. – David Richerby Dec 6 '14 at 11:49
• It seems likely that you'd be interested in this; there are actually negative ions of the alkali metals as well, alkalide ions. – Jason Patterson Jan 3 '15 at 5:04

And in what conditions can $\ce{H^{−}}$ occur?
In any phase. The hydride ion however doesn't stick around for long in water since it's such a strong base. First, hydrogen isn't that electronegative to start with, so tacking on an extra electron isn't helping with its stability. Second, combination with an hydrogen proton, $\ce{H+}$, is highly entropically favorable because the product, $\ce{H2}$, is a gas.
The $pK_a$ of $\ce{H2}$ has been estimated to be ~35 so that speaks a lot to the strength of the hydride ion as a base.