Sodium bicarbonate has the formula ` NaHCO_3` . It has one sodium ion per formula unit.
Sodium bicarbonate is an ionic compound with the stock name sodium hydrogen carbonate. Ionic compounds are always electrically neutral because they have equal amounts of positive and negative charge. Sodium, being in group 1A of the periodic table, forms the ion `Na^+. ` Hydrogen carbonate is a polyatomic ion with the formula `HCO_3^-. ` The formula `NaHCO_3` results in a neutral compound, with the +1 ion balancing out the -1 ion.
There's a compound with a similar name, sodium carbonate, which has the formula `Na_2CO_3` . It has two sodium ions per formula unit because the carbonate ion has the formula `CO_3^(2-)` . Two Na+ ions are needed to balance the two negative charges of the carbonate ion.
When trying to determine the formula of an ionic compound, you need to make sure that there's the same amount of positive and negative charge so that the overall charge is zero. An easy way to do this is the switch the charges and make them subscripts (without the signs, of course.) In the sodium carbonate example, the +1 on sodium becomes a subscript of 1 on carbonate and the -2 on carbonate becomes a subscript of 2 on sodium. Any time the magnitudes of the positive and negative charges are the same, such as in sodium bicarbonate, the subscripts are both 1.