When hydrogen met fluorine… bang!

When hydrogen met fluorine a covalent compound was formed, hydrogen fluoride.

crack the code:
mm = alloy
mn = ionic
nn = covalent

When one non-metal reacts with another non-metal they react to form compounds with covalent bonds. A covalent bond is a shared pair of electrons.

We refer to these covalently bound compounds as molecules.

A theoretical model for covalent bond formation between a hydrogen atom and a fluorine atom is illustrated by the animation below (outer energy level electrons shown only):

Dot and cross diagram for hydrogen fluoride

Dot and cross diagram for hydrogen fluoride

By sharing electrons each of the atoms participating in the formation of the bond gains the electron arrangement of the nearest noble gas. This is because when counting the electrons around each of the atoms we count the shared as “belonging exclusively” to both of them, i.e. we count the shared pair twice.

In the case of hydrogen fluoride, the hydrogen counts the shared pair as its own and gains the electron arrangement of helium, whilst fluorine gains the electron arrangement of neon with eight electrons in its outer shell.

Of course, this model does not accurately account for what happens when hydrogen gas reacts with fluorine gas because neither of the substances exits as individual atoms, but as H2 and F2 molecules.

However, the two gases do react very readily upon mixing as shown in this movie clip from the Royal Society of Chemistry.

Reactions which give out energy, in this case in the form of a violent explosion, are called exothermic reactions. So why do H2 and F2 react so readily and why do they give out energy?