Chemical Formulas - Formula and Compound Terms

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another name for a negative ion


another name for a positive ion

binary compound

contains two elements

ternary compound

contains three or more elements

ionic compound

made of a positive & a negative ion

molecular compound

atoms share electrons...not ionic

covalent compound

same as molecular compound

chemical formula

shows # & kind of atoms (includes
molecular, empirical and structural)

molecular formula

shows # & kind of atoms in molecule

empirical formula

simplest whole # ratio of atoms

structural formula

show how atoms are connected


one atom

diatomic & Polyatomic

two atoms (di) or more (poly)

Chemical Formulas
Memorization Tips - Negative Ions

all negative ions (anions) end in "-ide", "-ate", or "-ite"
• these are single atoms
• exceptions: hydroxide (OH-) and cyanide (CN-)
• you can tell charge from position on the periodic table.
• Family VII (F, Cl, Br, I ) all form 1- ions.
• Family VI (O, S) all form 2- ions.
• these contain several oxygen atoms. You just have to
memorize them... there is no rule about how many oxygens.
"-ites" contain one less O than the -ates... same charge.

Chemical Formulas
Memorization Tips - Positive Ions

metals form + ions "-ous" ions < "-ic" ions
• Family I (Li, Na, K, etc.) all form 1+ ions
• Family II (Be, Mg, Ca, Sr, Ba, Ra) all form 2+ ions
• Family III (Al) forms a 3+ ion

mercury(I) Hg2^2+ mercury(II) Hg^2+
copper(I) Cu+ copper(II) Cu2+
tin(II) Sn^2+ tin(IV) Sn^4+
iron(II) Fe^2+ iron(III) Fe^3+

Chemical Formulas
Formula Conventions

used to show the charges on ions
Mg^2+ the 2 means a 2+ charge (lost 2 electrons)

used to show numbers of atoms in a formula unit
H2SO4 two H's, one S, and 4 O's

used to show the number of formula units
2Br- the 2 means two individual bromide ions

Hydrates CuSO4 • 5 H2O
some compounds have water molecules included

Chemical Formulas
How Ions Form

Positive ions form by LOSING one or more electrons.
Negative ions form by GAINING one or more electrons

PROTONS are not gained or lost from the nucleus except innuclear reactions that require MUCH more energy than isusually available.

Metals become POSITIVE ions.
Non-Metals become NEGATIVE ions.
Semi-Metals sometimes become ions and sometimes shareelectrons as molecular compounds.

Chemical Formulas
Writing Ionic Formulas

The positve ion is written first.
The total positive charge must match the total negative
charge in the compound.

Use parentheses when you need several polyatomic ions...
Al2(SO4)3 is correct Al2(Cl)3 is incorrect

Be careful of OH- ions...
Ba(OH)2 is correct BaOH2 is incorrect

Reduce subscripts in final formula except with Hg2^2+
SnS2 is correct Sn2S4 is incorrect
mercurous chloride, Hg2Cl2 is correct

Chemical Formulas
Other Ions (beyond the 40)
and how they relate to oxidation numbers

bicarbonate, HCO3- bisulfate, HSO4-
bisulfide, HS- biphosphate, HPO4^2-

perchlorate ClO4- oxidation number of Cl = +7
chlorate ClO3- oxidation number of Cl = +5
chlorite ClO2- oxidation number of Cl = +3
hypochlorite ClO- oxidation number of Cl = +1

chlorine Cl2 oxidation number of Cl = 0
chloride Cl- oxidation number of Cl = -1
chlorate ClO3-
bromate BrO3- similar to chlorate

Chemical Formulas
Oxidation Numbers
What they are and how you find them

The oxidation number is the "apparent charge" on an atom.
The oxidation number of any substance in its elemental form is defined as 0.
Example: the oxidation number of H in H2 is 0
the oxidation number of H in H2O is +1
The oxidation numbers of each of the atoms in a substance
add up to the charge on the substance...
CO3^2- C + O + O + O = -2
x + -2 + -2 + -2 = -2 \ x = +4
CH4 C + H + H + H + H = 0
x + 4(+1) = 0 \ x = -4

Chemical Formulas
Writing and Naming Acids

Acids are ionic formulas in which the positive ion is H+.
Use as many H+ ions as the charge on the negative ion.
Three rules for naming:
if the anion ends with: the acid is named:
-ite ********ous acid
-ate ********ic acid
-ide hydro********ic acid
• Acids from sulfide, sulfite, and sulfate include a "ur"
H2S is hydrosulfuric acid, not hydrosulfic acid
• Acids from phosphate and phosphite include a "or"
H3PO4 is phosphoric acid, not phosphic acid

Chemical Formulas
Stock Names vs. Traditional Names

The Stock System of naming compounds is used...
• when a positive ion has more than one possible charge
(i.e. cuprous, cupric, etc.)
Traditional: mercurous, Hg22+ mercuric, Hg2+
Stock: mercury(I) mercury(II)
Traditional: cuprous, Cu+ cupric, Cu2+
Stock: copper(I) copper(II)
• for molecular compounds where the elements have many
different oxidation numbers (i.e. N in NO2, NO, N2O, etc.)

Stock Name: Traditional Name:
NO2 nitrogen(IV) oxide nitrogen dioxide
NO nitrogen (II) oxide nitrogen monoxide

Chemical Formulas
Naming Molecular Compounds
(Traditional Method)

The first element is named using the name of the element.
The second element always end in "-ide."
Indicate the number of atoms using the prefix...
1 mono- 6 hexa-
2 di- 7 hepta-
3 tri- 8 octa-
4 tetra 9 nona-
5 penta 10 deca-
If the first element has only one atom, don't use the mono-
If the second element is oxygen, drop the vowel...
monoxide, not monooxide
tetroxide, not tetraoxide

Chemical Formulas
Determining Ions from Formulas

Given the formula of an ionic compound, you can determine the original ions.
Most of these problems are obvious if you have the ions
NaCl Na+ Cl-
K2SO4 K+ SO4^2-

Some need a little "detective work"...
CuS Cu+ or Cu2+?
since S is ^2- (memorized) Cu must be 2+.
You can also figure out ions you've never seen before...
Ga(NO3)3 must be Ga^3+ since NO3- (memorized)

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