Question

Question

A helium nucleus 4He has 2 neutrons. How many protons are there?

The number of protons + neutrons A = 4.

If the number of neutrons N is 2 then there must be 4 - 2 = 2 protons.

There are 2 protons in the helium nucleus.

Ions

Normally, atoms are neutral.

They have the same number of protons in the nucleus as they have electrons orbiting in the energy levels around the nucleus.

Atoms can, however, lose or gain electrons due to collisions or other interactions such as rubbing a material with a cloth.

When they do, they form charged particles called ions:

  • if the atom loses one or more electrons, it will have more protons than electrons and so it becomes a positively-charged ion;
  • if the atom gains one or more electrons, it will have more electrons than protons and so it becomes a negatively-charged ion;
  • the atom will not normally gain or lose protons due to collisions or other interactions because they are held very tightly inside the nucleus. Electrons are much more loosely held.
Helium ion, two neutrons, two protons and two electrons.

A helium atom has two electrons in an energy level outside the nucleus.

The atom is neutral as it has two positive protons and two negative electrons.

A helium atom that has lost or gained an electron is a helium ion.

The ion has two positive protons but one electron so it is a positive ion:

Helium ion, two neutrons, two protons and one electron.

The ion has two positive protons but three electrons so it is a negative ion:

Helium ion, two neutrons, two protons and three electrons.