atomic number

/əˌtɑmɪk ˌnʌmbər/

Other forms: atomic numbers

Scientists use the term atomic number to describe the number of protons in one atom of an element. For example, the atomic number of hydrogen is 1.

If you've ever taken a chemistry class, you've seen a chart called "the periodic table of elements." It lists every chemical element and each element's atomic number. Just by looking at the table, you can see that the atomic number of oxygen is 8 — this means that there are eight protons in one atom of oxygen. In chemistry and physics, the atomic number is represented by the letter Z.

Definitions of atomic number

1. noun

   the order of an element in Mendeleyev's table of the elements; equal to the number of protons in the nucleus or electrons in the neutral state of an atom of an element
   types:

   magic number

   the atomic number of an extra stable strongly bound atomic nucleus: 2, 8, 20, 28, 50, 82 or 126

   type of:

   number

   a concept of quantity involving zero and units