What is the oxidation state of each element of the following compounds?
1) CH₂Cl₂
2) (NH₄)₂ CrO₄
3) CuCO₃
4) BaO
5) K₂Cr₂O₇

• A. 1) C = 0; H = +1; Cl = –12) N = –3; H = +1; Cr = +6; O = –23) Cu = +2; C = +4; O = –24) Ba = +2; O = –25) K = +1; Cr = +6; O = –2
• B. 1) C = 0; H = +1; Cl = –12) N = +3; H = –1; Cr = +6; O = –23) Cu = +2; C = +4; O = –24) Ba = +2; O = –25) K = +1; Cr = –6; O = +2
• C. 1) C = +2; H = –1; Cl = +12) N = –3; H = +1; Cr = +6; O = –23) Cu = +2; C = +4; O = –24) Ba = +2; O = –25) K = +1; Cr = +6; O = –2
• D. 1) C = +2; H = –1; Cl = +12) N = +3; H = –1; Cr = +6; O = –23) Cu = +2; C = +4; O = –24) Ba = –2; O = +25) K = –1; Cr = –6; O = +2

Answer: (A)