"how many even integers greater than two and less than twenty are equal in value to the sum of two prime numbers? "

The integers involved are 4 ,6, 8, 10, 12,14, 16 , 18

4  = 2 + 2 so 4 is one of the required integers.
6 = 3 + 3
8 = 3 + 5
10 = 3 + 7
12 = 5 + 7
14 = 3 + 11
16 = 3 + 13

They all are.
18 = 7 + 11

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