What is the atomic number and what does it tell you about the number of protons in an atom?
The atomic number is equal to the number of protons in the nucleus, and for neutral atoms, it is the same as the number of electrons. In an attempt to keep atoms and ions from getting confused with one another, it is best to think of atoms as always being electrically neutral and ions should be considered charged positively or negatively depending on whether electrons are gained or lost by an atom during the chemical combination of atoms in forming substances such as water, salt and rust found on metals. Protons in atoms and ions are always the same for a specific atom.
Every element has a unique atomic number, and so each element's identity is determined by the number of protons in its nucleus.
However, the same element can have different numbers of neutrons in its nucleus. Two atoms of the same element with two different numbers of neutrons in the nucleus are called isotopes. Also, note that if an atom is not charged (neutral), then the number of protons must equal the number of electrons, and so the atomic number is also equal to the number of electrons in a neutral atom of each element.
However, if the atom is charged (and therefore called an ion), it will have a different number of electrons orbiting it. If the number of electrons is less than the number of protons, it will be a positively charged ion, and the charge will equal the difference between the number of electrons and protons. The same is true if it has more electrons than protons, except now the charge will be negative.
Look under the Web Links below this answer for a periodic table. The number listed for each element is its atomic number. Also see the Related Questions links for more information about the number of protons, neutrons, and electrons in an atom.