An element X belongs to the 2nd period and 17th group. If 3 more electrons are added to the element, then which of the following statements is true about the element X ?

The element is fluorine because it belongs to 2nd period containing 2 shells and contains 7 electrons in the outermost valence shell. When 3 more electrons will be added, the last 2 electrons will enter the M shell. Hence it will be Magnesium of 3rd period and 2nd group.