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“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
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 Combining power of 3
Ammonium ion, NH4+
Hydrogen carbonate ion, HCO3-
Hydroxide ion, OH-
Nitrate ion, NO3-
Carbonate ion, CO32-
Sulfate ion, SO42-
Phosphate ion, PO43-
Chemical 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.
112Cn
Copernicium
9F
19.00Fluorine
17Cl
35.45Chlorine
35Br
79.90Bromine
53I
126.9Iodine
85At
Astatine
7N
14.01Nitrogen
15P
30.97Phosphorus
33As
74.92Arsenic
51Sb
121.8Antimony
83Bi
209.0Bismuth
5B
10.81Boron
13Al
31Ga
69.72Gallium
49In
114.8Indium
81Tl
204.4Thallium
107Bh
Bohrium
108Hs
Hassium
109Mt
Meitnerium
110Ds
Darmstadtium
111Rg
Roentgenium
87Fr
Francium
88Ra
Radium
89–103
Actinoids
104Rf
Rutherfordium
105Db
Dubnium
106Sg
Seaborgium
1H
1.008Hydrogen KEY
2He
4.003Helium
3Li
6.941Lithium
4Be
9.012Beryllium
Atomic Number
Symbol
Standard Atomic WeightName
197.0Gold
6C
12.01Carbon
8O
16.00Oxygen
10Ne
20.18Neon
11Na
22.99Sodium
12Mg
24.31Magnesium
14Si
28.09Silicon
16S
32.07Sulfur
18Ar
39.95Argon
19K
39.10Potassium
20Ca
40.08Calcium
21Sc
44.96Scandium
22Ti
47.87Titanium
23V
50.94Vanadium
24Cr
52.00Chromium
25Mn
54.94Manganese
26Fe
55.85Iron
27Co
58.93Cobalt
28Ni
58.69Nickel
29Cu
63.55Copper
30Zn
65.38Zinc
32Ge
72.64Germanium
34Se
78.96Selenium
36Kr
83.80Krypton
37Rb
85.47Rubidium
38Sr
87.61Strontium
39Y
88.91Yttrium
40Zr
91.22Zirconium
41Nb
92.91Niobium
42Mo
95.96Molybdenum
43Tc
Technetium
44Ru
101.1Ruthenium
45Rh
102.9Rhodium
46Pd
106.4Palladium
47Ag
107.9Silver
48Cd
112.4Cadmium
50Sn
118.7Tin
52Te
127.6Tellurium
54Xe
131.3Xenon
55Cs
56Ba
137.3Barium
57–71
Lanthanoids
72Hf
178.5Hafnium
73Ta
180.9Tantalum
74W
183.9Tungsten
75Re
186.2Rhenium
76Os
190.2Osmium
77Ir
192.2Iridium
78Pt
195.1Platinum
79Au
197.0Gold
80Hg
200.6Mercury
82Pb
207.2Lead
84Po
Polonium
86Rn
Radon
79Au
The Combining Power or Valencies of the Transition Metals Vary
The Combining Power or Valencies of the Elements of the Periodic Table
132.9Cesium
26.98Aluminum
“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
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 Chemical formula
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
“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 Chemical formula
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
“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