When an atom has equal numbers of positive and negative particles, it has a neutral charge. But if an atom has extra electrons or is missing electrons, it is known as an ion, and it may bear a positive or negative charge. In simple terms, if there are no electrons, the atom has a positive charge. If the atom contains electrons, it has a negative charge.
TL;DR (Too Long; Didn't Read)
An easy way to work out the charge of an atom is to look at the periodic table. Elements on the left side of the table usually become positively charged ions and elements on the right side of the table typically have a negative charge. However, you can use a scientific formula to determine the formal charge of an atom.
Properties of Atoms
Often referred to as the "building blocks" of everything in the world, atoms represent the smallest particles of a chemical element that can exist; A chemical element is a substance that is made entirely from one type of atom. Atoms can connect with one another to make molecules, which then make up the objects, known as matter, around you. Atoms consist of particles called protons, electrons and neutrons. Protons have a positive electrical charge, electrons have a negative electrical charge and neutrons have no electrical charge. The protons and neutrons clump together in the center of the atom known as the nucleus and the electrons circuit the nucleus. A specific atom will have an equal number of protons and electrons and most atoms have as many or more neutrons as protons.
Atomic Number of an Element
The atomic number of an element, also called a proton number, reveals the number of protons or positive particles in an atom. A normal atom with equal numbers of positive and negative particles has a neutral charge. In other words, the number of electrons is equal to the atomic number. Ions are atoms with additional electrons resulting in a negative charge or missing electrons, giving the atom a positive charge.
Determine Charge of an Atom
If you look at the periodic table -- a table of the chemical elements arranged in order of atomic number -- you'll see that elements on the left side usually have a positive charge and elements on the right side have a negative charge. To work out the formal charge of an atom, the formula is:
FC = GN - UE - 1/2 BE
Where FC = formal charge, GN = periodic table group number, or the number of valence electrons in free, non-bonded atom, UE = number of unshared electrons and BE = number of electrons shared in covalent bonds.
For example, if you want to work out the charge of hydrogen H, found at the top left corner of the periodic table, it has one valence electron GN = 1, no unshared electrons UE = 0, and two shared electrons in the oxygen-hydrogen covalent bond, so BE = 2.
The calculation is:
1 - 0 - (2 ÷ 2)
which means the formal charge on a hydrogen atom is 0.