What are the inner electrons?

Whenever we think of an atom, we imagine a nucleus made up of protons and neutrons, surrounded by electrons that revolve around it, similar to how planets revolve around the sun .

But not all electrons are the same, and two classes can be distinguished, depending on how close or far they are from the atomic nucleus.

Some are further from the nucleus and are called valence electrons. These are responsible for all the chemical reactions that we know of. On the other hand, those that are closest to the nucleus and are more strongly bound to it, are what we call internal electrons .

Characteristics of the internal electrons

They are the most stable electrons

As a general rule of thumb, the lower the energy level of an electron, the more stable it is. The inner electrons occupy the orbitals at the lowest energy levels of an atom, therefore they are also the most stable electrons.

The nucleus attracts electrons because they both have opposite charges: the nucleus is positive while the electrons are negative. This force increases as the distance decreases.

As a consequence of this, the internal electrons, which are those that are closest to the nucleus, are also those that are most strongly attached to it. For this same reason, they are also the most difficult electrons to remove.

They are not part of the chemical bonds

When one atom approaches another, the first thing to touch are its outermost electrons. For this reason, only valence electrons are involved in chemical reactions. The internal electrons do not participate in chemical bonds , nor do they participate in any chemical reaction.

They are very arresting

This means that the inner electrons get between the outer electrons and the nucleus, creating a kind of screen that reduces the force with which the nucleus attracts the valence electrons.

They always come in pairs

Only two electrons with opposite spins fit in each orbital, which are called paired electrons (which is another way of saying that they form a pair). Since the orbitals where the inner electrons meet are always completely filled, then these electrons are always paired.

Knowing how many internal electrons an atom has is very simple. You just need to know how many electrons in total the atom has, and how many of those electrons belong to the valence shell. All other electrons are the inner electrons.

But how do we find out these numbers? We explain it in the following sections.

The total number of electrons equals the atomic number

Since atoms are neutral (that is, they have no net electrical charge), they must have the same number of positive charges (protons) as negative charges (electrons). Since the atomic number indicates how many protons are in the nucleus, then it also indicates how many electrons an atom has around it when it is neutral.

In the case of ions, which unlike atoms do have an electric charge, the total number of electrons is obtained by subtracting the atomic number minus the charge of the ion, with everything and its sign.


  • The carbon atom (C) is element number 6 on the periodic table (that’s its atomic number). Therefore, the carbon atom has 6 electrons in total.
  • The sodium ion (I) (Na + ) has a positive charge of +1, and the atomic number of sodium is 11. So the total number of electrons in this ion is: 11 – (+1) = 10 electrons.
  • The chloride ion (Cl  ) has a negative charge of -1, and the atomic number of chlorine is 17. So the total number of electrons in this ion is: 17 – (- 1) = 17 + 1 = 18 electrons.

The number of valence electrons equals their group on the periodic table

The elements that belong to group 1A (the alkali metals) all have 1 valence electron. Those that belong to 2A (alkaline earths) have 2, those of 3A (boron group, aluminum, gallium, etc.) have 3 and so on. This does not apply to transition metals.

As before, ions have their charge subtracted by calculating their valence electrons.


  • Oxygen belongs to group 6A so it has 6 valence electrons.
  • The sulfide ion (S 2- ) also belongs to group 6A, but has a charge of -2, so it has: 6 – (–2) = 6 + 2 = 8 valence electrons.

Examples of different atoms and their internal electrons

Internal electrons of carbon (C)

As we saw a moment ago, carbon has 6 electrons in total. Also, it belongs to group 4A so it has 4 valence electrons, so it has, in total, 6 – 4 = 2 internal electrons.

Nitrogen internal electrons (N)

Nitrogen has 7 electrons in total. Also, it belongs to group 5A so it has 5 valence electrons, so it has, in total, 7 – 5 = 2 internal electrons.

Internal electrons of argon (Ar)

Argon is element 18 so it has 18 electrons in total. Also, it belongs to group 8A so it has 8 valence electrons, so it has, in total, 18 – 8 = 10 internal electrons.

Internal electrons of the chloride ion (Cl  )

As we saw a moment ago, chloride has 18 electrons in all. Also, it belongs to group 7A and has a -1 charge, so it has 7 – (- 1) = 8 valence electrons. So it has 18 – 8 = 10 internal electrons, just like argon.

Internal electrons of calcium ion +2 (Ca 2+ )

Calcium is element 20, and this ion has 2 positive charges, so it has a total of 20 – 2 = 18 electrons around the nucleus. This element belongs to group 2A and since its charge is +2, then it has no valence electrons left (2 – 2 = 0). For this reason, all of the 18 electrons that calcium has left are what were originally the inner electrons.

There are two elements that do not have internal electrons

The two lightest elements on the periodic table, which are hydrogen (H) and helium (He), only have 1 and 2 electrons respectively. Both electrons are in the only energy level that these atoms have, which is level 1.

For this reason, these electrons correspond to the valence electrons of these two elements. Since there are no lower energy levels, then there are no internal electrons either.

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