Just Ask Antoine!
Atoms & ions
Energy & change
The quantum theory
Electrons in atoms
The periodic table
names and formulas of polyatomic ions
- Explain the mole concept, and convert between grams, moles, and atoms and molecules.
- Determine mass percent composition of a sample from experimental data.
- Determine mass percent composition of a compound from its formula.
- Determine empirical formula of a compound from its mass percent composition.
Before you start...
Ionic and molecular compounds
What holds atoms together in a molecule?
- Molecular compounds are made of molecules.
- each molecule contains anywhere from two atoms (diatomic molecules) to thousands (biological molecules).
- each molecule has the same element composition and properties as the compound.
- synonym: covalent compound
Common molecular compounds
- examples: H2O, CO2, C6H12O6, NH3, CH4
- Ionic compounds are made of positive ions (cations) and negative ions (anions).
- cations combine with anions in just the right numbers to give an electrically neutral compound.
- metals form cations easily, and nonmetals form anions, so metal/nonmetal compounds are often ionic
- cations and anions pack into orderly arrays in solids; they become mobile when the compound melts
- individual molecules don't normally exist!
an ion pair
an ion cluster
an ion crystal
- examples: NaCl, KBr, Na2S, MgBr2
- synonym: salts
classifying compounds as ionic or covalent
A more detailed comparison of ionic and molecular compounds.
Table: Comparing ionic and molecular compounds.
||close on the periodic table
||widely separated on the periodic table
||good, when melted or dissolved
|state at room temperature
||solid, liquid, or gas
Can a compound be classified as ionic or covalent from its formula alone?
Formulas for molecular compounds
- A molecular formula shows the type and number of atoms in a molecule
- type of atom indicated by element symbol
- number of atoms per molecule indicated by subscripts (if greater than one)
- H2O contains 2 hydrogen atoms and one oxygen atom per molecule.
- Carbon tetrafluoride, CF4, contains four fluorine atoms and one carbon atom per molecule.
- atoms in formulas are sometimes grouped to show how they're connected in the molecule
- Methanol is usually written as CH3OH to show that 3 hydrogens are bound to the carbon and another hydrogen is bound to the oxygen.
- Acetic acid can be written as CH3COOH or as HC2H3O2 or C2H4O2. The first formula shows how the molecule is put together; the second formula emphasizes that one hydrogen is different from the others; the third formula is the least informative because it shows only the numbers and types of atoms in the molecule.
- groups that appear more than once in the molecule are enclosed in parentheses
- CH3(CH2)3CH3 could be written as C5H12, but all information about the structure of the molecule would be lost.
- (CH3CH2)4P2O7 molecules contain 8 carbons, 20 hydrogens, two phosphoruses, and seven oxygens.
- molecular weight = sum of weights of atoms in the molecule
- The molecular weight of CH3OH is approximately 12 + 4*1 + 16 = 32 since the carbon, hydrogen, and oxygen have atomic weights of 12, 1, and 16 u, respectively.
Formulas for ionic compounds
- empirical formula gives the elemental composition of a compound
- writing ionic empirical formulas
- write the cation formulas, including charge
- write the anion formulas, including charge
- combine enough cations with enough anions to give a total charge of zero
- trick: swap charges as subscripts
- don't write charges when the ions are combined
- use the simplest (lowest) cation-to-anion ratio possible
- list cations first, anions last
Potassium ions (K+) and chloride ions (Cl-) combine to give potassium chloride, KCl
Calcium ions (Ca2+) and bromide ions (Br-) combine to give calcium bromide, CaBr2
Aluminum ions (Al3+) and sulfide ions (S2-) combine to give aluminum sulfide, Al2S3
- naming ionic compounds from formulas
- name the anions
- name the cations
- recall that the names of transition metal and main group cations must include their charge as a Roman numeral.
- the name of compound is cation name followed by anion name
Na2S contains sodium ion and sulfide ion. The compound is sodium sulfide.
SnCl4 contains a tin cation and four chloride ions. Each chloride carries a -1 charge, so the tin must have a +4 charge. The compound is tin(IV) chloride.
- the formula weight is the sum of atomic weights for atoms in the formula
+ 16.0 u
+ 1.0 u
+ 12.0 u
+ 3×16.0 u
2× 14.0 u
+ 2×4×1.0 u
+ 32.0 u
+ 4×16.0 u
More about polyatomic ions
- definition: ions formed from more than one atom
- examples: ammonium (NH4+), hydroxide (OH-), nitrate (NO3-), sulfate (SO4-)
- polyatomic ions retain their identity within ionic compounds, and in many reactions
common polyatomic ions by family
- names, formulas and charges of common polyatomic ions should be memorized!
- formulas for ionic compounds containing polyatomic ions are written as usual, except:
- put parentheses around polyatomic ions whenever there are more than one
- don't break up polyatomic ions (write Ca3(PO4)2, not Ca3P2O8)
names and formulas of polyatomic ions
- definition: a map that shows how atoms are bonded within a molecule
- structural formulas of polyatomic ions are bracketed, with the charge indicated by a superscript
- Sticks indicate shared electron pairs. There can be more than one pair shared, as in the C=O group in this acetic acid molecule.
Finding chemical formulas experimentally
- to obtain a molecular formula from an empirical formula:
- you must know the molecular weight
- compute the ratio of molecular weight to formula weight
- multiply all subscripts in the empirical formula by this ratio
Give the molecular formula for a compound with empirical formula CH and molecular weight 78.11 daltons.
Give the molecular formula for a compound with empirical formula CH2O and molecular weight 180 daltons.
- release hydrogen ions when dissolved
- leading H's in formula are acidic hydrogens
- guidelines for naming acids
- name the anion within the acid
- change the anion ending to one of the following:
||in acid, replace with:
||hydrochloric acid, HCl
||nitrous acid, HNO2
||sulfuric acid, H2SO4
Binary Covalent Compounds
- naming binary covalent compounds
- write the name of the first nonmetal
- write the name of the second nonmetal with the ending changed to -ide
- insert prefixes into the name to reflect subscripts in the formula:
|"Pentoxide" sounds a little better than "pentaoxide". Drop the "a" before an "o".||
- never start a name with mono-
common names should be used for the following compounds:
- contain several compounds packed in regular way into a crystal
- formulas of addition compounds
- added compounds are separated by a dot
- prefix compound with the number of times it occurs per formula unit
- naming addition compounds
- translate numbers after a dot into Greek prefixes
- name each compound in order
- cross out first redundant ion names
- water in addition compounds is called hydrate
- hydration waters can be driven off by strong heating
- compound with all hydration waters driven off is called anhydrous
- anhydrous salts usually absorb water from air to become hydrates again
|Na2SO4 · 10H2O
||sodium sulfate decahydrate
|CuSO4 · 5H2O
||copper(II) sulfate pentahydrate
|CaCO3 · MgCO3
||calcium magnesium carbonate
||carbonate is named only once
|CaHPO4 · 2H2O
||calcium hydrogen phosphate dihydrate
|Ga2(SO4)3 · 18H2O
||gallium(III) sulfate 18-hydrate
||Use numbers instead of prefixes when the number is larger than 12
|ZnSO4 · (NH4)2SO4 ·6H2O
||zinc ammonium sulfate hexahydrate
||zinc is always +2, so zinc(II) is not necessary
|Sign up for a free monthly|
newsletter describing updates,
new features, and changes
on this site.
General Chemistry Online! Compounds
Copyright © 1997-2005 by Fred Senese
Comments & questions to email@example.com
Last Revised 12/14/21.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/index.shtml