When more magnesium is added to the HCl, more heat is created. I know why more heat is created with the bonds breaking, but I don't know where the extra heat comes from. I assume the extra heat is coming from the surrounding air and the test tube but what is actually happening within the elements to actually be absorbing more heat?

"I assume the extra heat is coming from the surrounding air "

No.

It's coming from the reaction of the acid with the metal.

 

It is then given to the air- it warms the room up very slightly.

I assume the extra heat is coming from the surrounding air and the test tube

 

If during chemical reaction heat would be taken from test tube, flask, or from air, they would be cooled down.

Use for instance sodium bicarbonate with acetic acid, and you should notice that temperature of flask has decreased.

Look into the bonds of the chemicals involved. The extra energy is from the unstable bond oh h nd cl which keeps the two bonded but immediately gets released as heat energy from bond dissociation.

[...] unstable bond [...]gets released as heat energy from bond dissociation.

 

No: bonds are attractive, that's why molecules exist. Breaking them absorbs energy.

But creation of the new strong bonds, between Mg and Cl, release energy.

 

As a secondary subtlety, a few electrons can be on antibonding molecular orbitals, provided that other electrons on bonding orbitals hold the atoms together, for instance in O₂ or in optically excited ethylene. If not, the molecule splits quickly: this happens for instance in excimer lamps, where the excited electronic state holds the atoms together (like Xe₂) but the non-excited state, after light emission, lets the atoms repel an other.