The lone electron pair on nitrogen also is available for making a covalent bond in which it supplies both of the electrons. An ammonia molecule can bind another proton (H+) and become an ammonium ion, NH₄+, by the reaction shown on the right. The positively charged H+ ion, or proton, is attracted to the lone pair at the negative end of the ammonia dipole. The covalent bond formed is then completely indistinguishable from the other three N-H bonds. The ammonium ion is a regular tetrahedron with 109.5 degrees H-N-H angles all around. Thus it has the same arrangement of atom centers and electrons as the methane molecule. If by some magic one could reach into the nitrogen nucleus in an NH₄+ ion and turn off the charge on one proton, the result would be a methane molecule. The sole but crucial difference is that the ammonium ion has one more proton in the nucleus of the central atom and hence an overall charge of +1. We will come back to the chemical significance of this after we have discussed oxygen and its hydrogen compound, water.