Because H2O has 2 H and 1O and 1 H is already in the equation, the answer should contain 1H and 1O, meaning it's either H O- or OH- however the proper way to write it is OH
An easy was to remember is by writing H2O down as HOH and when you split it up it becomes H+ + OH-
Due to it's electronic configuration w/c is 1s2 2s2 2ps 3s1 considering the last w/c is 3s1, sodium should be in row 3 period a1.