Normally we would expect an atom in its neutral, or "ground" state, to have an equal number of protons and electrons. Because the two particles have an equal charge, the charges will cancel out and give the atom an overall charge of 0.
So, if at atom has 35 protons in the nucleus, we could expect it to have 35 electrons orbiting that nucleus.
However, it's rare to find a free, unbound, neutral atom in the real world. Most atoms form some kind of bond or develop into an ion, and this will result in a difference in the number of electrons compared to protons.
The element with 35 protons is bromine, which tends to form -1 ions, meaning that we might expect to find it with 36 electrons instead of 35. Halogens, like bromine, also tend to form bonds with elements like hydrogen, which can provide extra electron in the form of a bond instead of an ion. This will alter our definition even further, in that the bond will involve two shared electrons, and the atom will then have 34 electrons orbiting itself and 2 electrons orbiting itself as well as another atom.
