The formal charge is calculated by using the formula shown below. FC= Number of Valence electrons - [number‌of‌lone‌pair+
1
2
‌number‌of‌bond‌pair]
For first oxygen, formal charge is equal to 6−[6+1]=−1 For second oxygen, formal charge is equal to 6−[4+2]=0 For third oxygen, formal charge is equal to 6−[2+3]=1