O-Level Chemical Bonding and Structure: worked solution

Question

When sodium is burned in air it produces sodium oxide, as illustrated in the figure. Once the reaction is complete the product is dissolved in universal indicator solution. Draw a dot-and-cross diagram showing how the outer-shell electrons are arranged in the ions of sodium oxide, Na2O. [2]

Na2O is ionic, formed by transfer of electrons from two sodium atoms to one oxygen atom. Each sodium (2,8,1) loses its single outer electron to become Na+, shown as [Na]+ with an empty outer shell (no dots or crosses drawn on the outer shell). The oxygen (2,6) gains the two electrons (one from each sodium) to complete its outer shell to 8, shown as [O with 8 outer-shell electrons: 6 of its own plus 2 transferred, drawn as dots and crosses]2-. Overall: two [Na]+ ions and one [O ..8 electrons..]2-, giving the formula Na2O.