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Chemistry
Elements and Compounds
Atoms and Atomic Structure

What is the atomic number?

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May 16, 2016 12:54PM

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.
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 have have different numbers of neutrons in its nucleus. Two atoms of the same element with two different numbers of neutrons in the nucleus are isotopes. Also, note that if an atom is not charged (neutral), than 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 to the left of this answer for a periodic table. The number listed for each element is its atomic number.

The atomic number of an element is the number of protons and electrons an atom of it has. For example, Hydrogen has an atomic number of 1, and it has 1 proton and 1 neutron. Oxygen is number 8, and it has 8 protons and 8 electrons. The number of neutrons it has doesn't affect it.