<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> A period 3 element is one of the chemical elements in the third row (or period) of the periodic table of the chemical elements . The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when the periodic table skips a row and a chemical behaviour begins to repeat, meaning that elements with similar behavior fall into the same vertical columns . The third period contains eight elements: sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, and argon . The first two, sodium and magnesium, are members of the s - block of the periodic table, while the others are members of the p - block . Note that there is a 3d subshell, but it is not filled until period 4, such giving the period table its characteristic shape of "two rows at a time". All of the period 3 elements occur in nature and have at least one stable isotope . </P> <P> As the atomic number of elements in Period 3 increases, the atomic radius decreases . </P> <P> As the atomic mass of elements in Period 3 increases, the electronegativity increases . </P>

In the third period as the atomic number increases from group 1 to group 18 the atomic radius