Atomic Number And Mass Number


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 mass number of an atom is its total number of protons and neutrons. Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).

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

Atomic number and mass number worksheet

So it has

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

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


Atomic number 33 periodic table. .....And for the neutral atom, #Z='the number of electrons.'# Why should this be so?

FindAtomic Number And Mass Number

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.

Periodic Table Mass Number

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