List 5 numbers with a mean of 5

One way is to make 5 the 3rd number in a series of equally spaced numbers:

3,4,5,6,7 = mean of 5

1, 3,  5,  7,  9 = mean of five

4,  4.5,  5,  5.5,  6 = mean of five

4.8,  4.9,  5.0, 5.1, 5.2  = mean of five
Do you get the idea?

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table