What Is the Atomic Number?

The Significance of the Atomic Number in Chemistry

Each element has its own unique atomic number, which is the number of protons in its atom.
Each element has its own unique atomic number, which is the number of protons in its atom. Steven Hunt, Getty Images

Each element on the periodic table has its own atomic number. In fact, this number is how you can distinguish one element from another. The atomic number is simply the number of protons in an atom. For this reason, it's sometimes called the proton number. In calculations, it is denoted by the capital letter Z. The symbol Z comes from the German word zahl, which means number of numeral, or atomzahl, a more modern word which means atomic number.

Because protons are units of matter, atomic numbers are always whole numbers. At present, they range from 1 (the atomic number of hydrogen) to 118 (the number of the heaviest known element). As more elements are discovered, the maximum number will go higher. Theoretically, there is no maximum number, but elements become unstable with more and more protons and neutrons, making them susceptible to radioactive decay. Decay may result in products with a smaller atomic number, while the process of nuclear fusion may produce atoms with a larger number.

In an electrically neutral atom, the atomic number (number of protons) is equal to the number of electrons.

Why the Atomic Number Is Important

The main reason the atomic number is important is because it's how you identify the element of an atom. Another big reason it matters is because the modern periodic table is organized according to increasing atomic number.

Atomic Number Examples

No matter how many neutrons or electrons it has, an atom with one proton is always atomic number 1 and always hydrogen. An atom the contains 6 protons is by definition an atom of carbon. An atom with 55 protons is always cesium.

Terms Related To Atomic Number

If the number of electrons in an atom varies, the element remains the same, but new ions are produced.

If the number of neutrons changes, new isotopes result.

Protons are found together with neutrons in the atomic nucleus. The total number of protons and neutrons in an atom is its atomic mass number (denoted by the letter A). The average sum of the number of protons and neutrons in a sample of an element is its atomic mass or atomic weight.