Electrons are positively charged and located in the nucleus together with the neutrons. Electrons have a charge of +1 and a mass of approximately 1 atomic mass unit (amu). Every element has a unique Element Symbol and a unique Atomic Number which can be accessed via the Periodic Table with Atomic Mass.

The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons. The sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom.

uranium element

Every atom of a particular element contains the same number of protons. Each element has a unique atomic number, or a unique number of protons in its nucleus.

We know that the atomic number of sodium is 11. This tells us that sodium has 11 protons and because it is neutral it has 11 electrons. Since sodium has 11 protons, the number of neutrons must be 23 – 11 = 12 neutrons.

The mass number of an atom is equal to the number of protons plus the number of neutrons that it contains. In other words, the number of neutrons in any atom is its mass number minus its atomic number. Number of neutrons = mass number – atomic number.

11 protons

19 electrons

There are 10 electrons present in Na+. The atom of sodium has 11 electrons, 11 protons along with 12 neutrons, but Na+ contains one less electron, 11 protons along with 12 neutrons, as the ion has lost 1 electron.

20 neutrons