How is the number of electrons in an element determined?

The number of electrons in an atom depends on its neutrality, cation or anion.

Neutral atom:

  • All atoms of elements found in the periodic table have neutral atoms.
  • A neutral atom always has the same number of protons as the number electrons.
  • Number of protons in an Atom = Atomic number for the atom
  • Also, the number protons can be used to determine the number electrons.
  • Number of protons = Number electrons
  • Ex: Sodium (Z = 11)

So, 11 = Number of protons + Number of electrons

Cation (positively charged ion)

  • When a neutral atom loses an electron, it forms cations. The number of lost electrons determines the amount of positive charge.
  • If the ion’s value is positive, it means that there are more protons than electrons.
  • Add the number positive charges to the atomic number to find the number electrons.
  • Ex: Na+ ion forms from Sodium (Z =11) when it loses an electron.

The charge for the Na + =+1 is shown here

Number of electrons in Na+ Ion = Z – number positive charges

= 11 – 1 =10 electrons

Ex: Manganese (Z = 25) loses 2 electrons and forms the Mg+2 Ion.

The charge for Mg+2 is +2

Therefore, the number of electrons in Mg+2 Ion = Z – 2.

= 25 – 2 = 23 Electrons

Anion (a negatively charged ion)

  • An electron is created when it is added to a neutral atom. The number of electrons that have been added will determine the number of negative charges.
  • An anion can be formed if the number of electrons in the ion exceeds that of protons.
  • To find the number electrons in an anion add the number negative charges to the atomic numbers.
  • Ex: Fluorine (Z = 9) gains an electron, and forms F-ion (Z = 9)

The charge on the F ion is =-1

Thus, the total number of electrons within F-ion = Z + total positive charges

= 9 + 1 = 10 electrons

Ex: Oxygen (Z = 8) gains 2 electrons and forms O-2.

The total O-2 charges are shown below = -2

Thus, the total number of electrons in O-2 = Z+ 2

= 8 + 2 = 10 electrons

We hope you find this helpful.


Please enter your comment!
Please enter your name here