Which element has 5 outer electrons in its 3rd shell?

Z

Explanation:

Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on.

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List of elements with electrons per shell.

Z 5

Element Boron

No. of electrons/shell 2, 3

Group 13

Phosphorus

Explanation:

Phosphorus has 15 total electrons. Its first shell can hold 2 electrons. Its second shell can hold 8 electrons. The remaining 5 electrons are in its third shell (the third shell can hold up to 18 electrons).

2 electrons (1st shell) + 8 electrons (2nd shell) + 5 electrons (3rd shell) = 15 total electrons (the number of electrons P has).