Writing Chemical formula with polyatomic groups
1. Use the Periodic table to determine the combining powers of single elements.
Eg. Magnesium is in Group 2 and has a combining power of 2.
2. Use Table 1 to determine the valency of the major polyatomic groups.
Table 1: The Combining Power of Polyatomic Ions
Combining power of 1
Combining power of 2
Ammonium ion, NH4+
Carbonate ion, CO32-
Hydrogen carbonate ion, HCO3-
Sulfate ion, SO42-
Combining power of 3
Phosphate ion, PO43-
Hydroxide ion, OHNitrate ion, NO3Chemical Formula Help
Rule 1: The charge tells you the combining power. Do not write the charge in the chemical formula!
Eg. The chemical formula for magnesium nitrate is Mg(NO3)2 and not Mg2+(NO3-)2  or Mg(NO3-)2 
Rule 2: Brackets() are needed when a polyatomic group > 1 in a compound. Poly = ‘many’
The hydroxide ion, formula OH-, is a polyatomic group since it is made up of two different elements.
Eg. The chemical formula for strontium hydroxide is Sr(OH)2 and not SrOH2 
Eg. The formula for lithium hydroxide is LiOH and not Li(OH)  since there is only one OH group.
Rule 3: Do not use brackets when single elements are > 1 in a formula.
Eg. The chemical formula for lithium carbonate is Li2CO3 and not (Li)2CO3 
Rule 4. Roman numerals are used for metal valencies that vary. Eg. Iron(II) oxide, FeO
If in doubt use a combining power of 2. Eg. Copper sulfate, CuSO4 and lead chloride, PbCl2
Rule 5. The valency of silver is always 1. The valency of zinc is always 2.
“Aluminum” is a commonly used spelling alternative spelling for “aluminium” that is used in the US
May be freely copied for educational use. ©www.chemicalformula.org
1
H
1.008
KEY
Hydrogen
3
Li
6.941
4
Be
9.012
Lithium
Beryllium
11
Na
22.99
12
Mg
24.31
Atomic Number
Symbol
Standard Atomic Weight
Name
Helium
5
B
10.81
79
Au
197.0
Gold
Boron
13
Al
26.98
The Combining Power or Valencies of the Transition Metals Vary
Sodium
Magnesium
19
K
39.10
20
Ca
40.08
Calcium
Scandium
Titanium
Vanadium
37
Rb
85.47
Rubidium
38
Sr
87.61
Strontium
39
Y
88.91
Yttrium
40
Zr
91.22
Zirconium
41
Nb
92.91
Niobium
Molybdenum
Technetium
Ruthenium
55
Cs
132.9
56
Ba
137.3
57–71
Cesium
Barium
Lanthanoids
72
Hf
178.5
Hafnium
73
Ta
180.9
Tantalum
74
W
183.9
Tungsten
75
Re
186.2
Rhenium
76
Os
190.2
87
Fr
88
Ra
89–103
104
Rf
105
Db
106
Sg
107
Bh
Francium
Radium
Actinoids
Rutherfordium
Dubnium
Seaborgium
Bohrium
Potassium
2
He
4.003
The Combining Power or Valencies of the Elements of the Periodic Table
21
Sc
44.96
22
Ti
47.87
23
V
50.94
Carbon
7
N
14.01
Nitrogen
8
O
16.00
9
F
19.00
10
Ne
20.18
14
Si
28.09
15
P
30.97
16
S
32.07
Sulfur
17
Cl
35.45
Chlorine
18
Ar
39.95
34
Se
78.96
35
Br
79.90
36
Kr
83.80
Oxygen
Fluorine
Neon
Aluminum
Silicon
Phosphorus
32
Ge
72.64
33
As
74.92
Arsenic
Selenium
Bromine
51
Sb
121.8
52
Te
127.6
53
I
126.9
54
Xe
131.3
Argon
Chromium
25
Mn
54.94
Manganese
26
Fe
55.85
27
Co
58.93
28
Ni
58.69
29
Cu
63.55
30
Zn
65.38
31
Ga
69.72
Gallium
Germanium
42
Mo
95.96
43
Tc
44
Ru
101.1
45
Rh
102.9
Rhodium
46
Pd
106.4
Palladium
47
Ag
107.9
Silver
48
Cd
112.4
Cadmium
49
In
114.8
50
Sn
118.7
Tin
Antimony
Tellurium
77
Ir
192.2
80
Hg
200.6
81
Tl
204.4
82
Pb
207.2
83
Bi
209.0
85
At
86
Rn
Platinum
79
Au
197.0
84
Po
Iridium
78
Pt
195.1
Polonium
Astatine
Radon
108
Hs
109
Mt
110
Ds
111
Rg
112
Cn
Hassium
Meitnerium
24
Cr
52.00
Mg(OH)2
Al2(SO4)3
6
C
12.01
Iron
Osmium
Cobalt
Nickel
Copper
Gold
Darmstadtium Roentgenium
Zinc
Mercury
Indium
Thallium
Lead
Bismuth
Iodine
Krypton
Xenon
Copernicium
NH4Cl
Na2SO4
MgCO3
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Chemical Formula of Compounds with Polyatomic Groups – Sheet 1
A polyatomic group contains more than 1 element. Eg. The hydroxide group, OH- contains oxygen and hydrogen.
Polyatomic groups greater than 1 in a formula need brackets. eg. Mg(OH)2 and not MgOH2
No. Compound
1
silver nitrate
2
ammonium nitrate
3
zinc sulfate
4
aluminum hydroxide
5
sodium sulfate
6
potassium carbonate
7
magnesium hydroxide
8
calcium carbonate
9
potassium phosphate
10
sodium nitrate
11
potassium hydroxide
12
magnesium phosphate
13
barium hydroxide
14
ammonium sulfate
15
aluminum phosphate
16
sodium hydrogen carbonate
17
barium nitrate
18
iron(II) hydroxide
19
zinc phosphate
20
aluminum carbonate
21
silver phosphate
22
aluminum sulfate
23
potassium sulfate
24
silver sulfate
25
magnesium nitrate
Chemical formula
“Aluminum” is a commonly used spelling alternative spelling for “aluminium” that is used in the US
May be freely copied for educational use. ©www.chemicalformula.org

Chemical Formula of Compounds with Polyatomic Groups – Sheet 2
A polyatomic group contains more than 1 element. Eg. The hydroxide group, OH- contains oxygen and hydrogen.
Polyatomic groups greater than 1 in a formula need brackets. eg. Mg(OH)2 and not MgOH2
No.
Compound
26
sodium hydroxide
27
barium phosphate
28
calcium sulfate
29
potassium hydrogen carbonate
30
copper nitrate
31
calcium phosphate
32
sodium carbonate
33
lead(II) sulfate
34
silver hydroxide
35
calcium nitrate
36
calcium hydroxide
37
sodium phosphate
38
ammonium phosphate
39
aluminum nitrate
40
barium sulfate
41
magnesium sulfate
42
iron(II) sulfate
43
iron(III) hydroxide
44
lead(II) nitrate
45
copper sulfate
46
barium carbonate
47
copper hydroxide
48
copper carbonate
49
zinc nitrate
50
potassium nitrate
Chemical formula
“Aluminum” is a commonly used spelling alternative spelling for “aluminium” that is used in the US
May be freely copied for educational use. ©www.chemicalformula.org

Chemical Formula of Compounds with Polyatomic Groups – Sheet 1- ANSWERS
No. Compound
Chemical formula

1
silver nitrate
AgNO3

2
ammonium nitrate
NH4NO3

3
zinc sulfate
ZnSO4

4
aluminum hydroxide
Al(OH)3

5
sodium sulfate
Na2SO4

6
potassium carbonate
K2CO3

7
magnesium hydroxide
Mg(OH)2

8
calcium carbonate
CaCO3

9
potassium phosphate
K3PO4

10
sodium nitrate
NaNO3

11
potassium hydroxide
KOH

12
magnesium phosphate
Mg3(PO4)2

13
barium hydroxide
Ba(OH)2

14
ammonium sulfate
(NH4)2SO4

15
aluminum phosphate
AlPO4

16
sodium hydrogen carbonate
NaHCO3

17
barium nitrate
Ba(NO3)2

18
iron(II) hydroxide
Fe(OH)2

19
zinc phosphate
Zn3(PO4)2

20
aluminum carbonate
Al2(CO3)3

21
silver phosphate
Ag3PO4

22
aluminum sulfate
Al2(SO4)3

23
potassium sulfate
K2SO4

24
silver sulfate
Ag2SO4

25
magnesium nitrate
Mg(NO3)2

“Aluminum” is a commonly used spelling alternative spelling for “aluminium” that is used in the US
May be freely copied for educational use. ©www.chemicalformula.org
Chemical Formula of Compounds with Polyatomic Groups – Sheet 2 - ANSWERS
No.
Compound
Chemical formula

26
sodium hydroxide
NaOH

27
barium phosphate
Ba3(PO4)2

28
calcium sulfate
CaSO4

29
potassium hydrogen carbonate
KHCO3

30
copper nitrate
Cu(NO3)2

31
calcium phosphate
Ca3(PO4)2

32
sodium carbonate
Na2CO3

33
lead(II) sulfate
PbSO4

34
silver hydroxide
AgOH

35
calcium nitrate
Ca(NO3)2

36
calcium hydroxide
Ca(OH)2

37
sodium phosphate
Na3PO4

38
ammonium phosphate
(NH4)3PO4

39
aluminum nitrate
Al(NO3)3

40
barium sulfate
BaSO4

41
magnesium sulfate
MgSO4

42
iron(II) sulfate
FeSO4

43
iron(III) hydroxide
Fe(OH)3

44
lead(II) nitrate
Pb(NO3)2

45
copper sulfate
CuSO4

46
barium carbonate
BaCO3

47
copper hydroxide
Cu(OH)2

48
copper carbonate
CuCO3

49
zinc nitrate
Zn(NO3)2

50
potassium nitrate
KNO3

“Aluminum” is a commonly used spelling alternative spelling for “aluminium” that is used in the US
May be freely copied for educational use. ©www.chemicalformula.org