An atom of a chemical element contains 15 electrons. how many of them are on the external energy level?

There are 5 electrons on the outer energy level. To find the number of such electrons, there are 2 options: either look at the periodic table (the number of external electrons is equal to the group number), or calculate (at the first energy level there are always 2 electrons, on the next 8, we are given 15 electrons, => from 15 we subtract 2 and 8, we get 5)

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