How can atomic number and mass number be used to find the numbers of protons, electrons, and neutrons?

2 Answers
  • Atomic number is the number of protons
  • Atomic mass subtract atomic number is the number of neutrons
  • Atomic number is the number of electrons in a neutral atom


The atomic number is assigned based on the number of protons, so the atomic number is always the same as the number of protons

Atomic mass is the sum of protons and neutrons. Protons and neutrons both have masses of 1 amu, but the mass of electrons is negligible so it is left out. The atomic mass of an element is never a round number because it's a weighted average of all the isotopes of the element. Just round the number to the nearest integer get the most likely atomic mass.

In a neutral atom number of protons = number of electrons since there is no overall charge. In an ion, add an electron for every negative charge and subtract an electron for every positive charge (ex. #Mg^(2+)# has 10 electrons because its atomic number is 12 and you subtract two electrons for the 2+ charge)


The atomic number of O is 8 and the mass number is ~16

So it has

  • 8 protons
  • 8 neutrons (16-8)
  • 8 electrons (since we're talking about a neutral O atom)
Oct 15, 2016

Well the atomic number, #Z#, is the number of protons, massive positively charged, nuclear particles............


..........And for the neutral atom, #Z="the number of electrons."# Why should this be so?

The #"mass number"# #=# #"Number of neutrons"+Z#.

Note that #Z# defines the identity of the element. There should be many other answers here on this site that deal with these issues.