Why did it require two sodium atoms to complete the na2o formula unit? na has +2 charge and o has -1 charge. na has +1 charge and o has -2 charge. na has -2 charge and o has +1 charge.

Na₂O is an ionic compound. Na is in group 1 which means it has one electron in the outer shell. O is in group 6 so it has 6 electrons in the outer shell.
To become stable atoms need to either lose the electrons in the outer shell or gain electrons to gain a complete outer shell configuration. 
To become stable Na loses its outer electron and becomes positively charged. It only loses one electron, so net charge is +1. O to become stable gains 2 electrons to complete its outer shell and becomes negatively charged. since it gains 2 electrons, the net charge is -2. 
cation - Na⁺
anion - O²⁻
O gains 2 electrons but Na can give only one electron, therefore 2 Na⁺ ions are required. 
the compound can be written by exchanging the charges
  ions         Na⁺       O²⁻
charge       +1       -2
exchange   Na₂O
Correct answer is Na has +1 charge and O has -2 charge

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