A period 3 element is an element found on the third period (row) of the periodic table. The row has eight elements sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, and argon.

Period 3 elements have electrons in the first three electron shells. The first and second electron shells are always full. The third electron shell (valence electrons) fills when atomic number gets bigger. Two electrons join the 3s orbital, and six electrons join the 3p orbitals.

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