Thus valencies get exchanged while writing formula.
1.The 1 of Sodium goes to Oxygen
2.The 2 of Oxygen goes to Sodium.
NOTE:Na is Sodium element; O is Oxygen element
Not to confuse Atomicity with Valency. Atomicity is the number of atoms present in one molecule/atom of a compound/element respectively. E.g.- Atomicity of
is 2 and that of