Chemical Formulas

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Chemical Formulas
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Chemical formulas indicate the relative number of
atoms of each kind element in a chemical compound
(ionic and molecular)
Ionic compound the number of atoms in a formula
units
Molecular compound the number of atoms in a
molecule
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Ionic Compounds
Monatomic ions - ions formed from a single
atom Na or S2- usually tell by column on
periodic table, some elements have more than one
oxidation number or charge
Binary compounds- only
2 elements in the compound Na2S
Polyatomic ions - ions formed from more
than one type of atom covalently bonded together
OH- PO43- NH4
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Quiz yourself How many oxygen atoms in the
following? CaCO3 Al2(SO4)3 How many ions in
the following? CaCl2 NaOH Al2(SO4)3
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Rules for writing formulas for ionic compounds
1.
when making the formula the cation (positive
ion) always goes 1st then the anion (negative
ion) 2. the
compound is neutral - charge 3.
subscripts added to make charges cancel
4.
When adding subscript to polyatomic ion it is
put in ( )s
5. Formula unit is always the simplest
ratio of ions must subscripts
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The positive side of the formula must the
negative side of the element
Examples on board
1. Sodium chloride Na Cl-
NaCl (no charges
written in the formula)
2. Calcium cholride Ca2 Cl-
CaCl2
3. Potassium sulfide K S2-
K2S
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4. Aluminum chloride Al3 Cl- AlCl3
5. Barium phosphate Ba2 PO43- Ba3(PO4)2
Cross over method - the charges become the
subscripts of the other ion Beware subscripts
must be simplest ratios
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Naming ionic compounds (stock system)

• 1. Binary compound - 2 elements
• when naming the compound the name of the first
  element(ion) stays the same
• the last element ends in ide
• example NaCl sodium chloride

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Formula ZnS
Compound Name Zinc Sulfide
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2. Polyatomic ions- use their own name (on
ion sheets). Example NH4Cl ammonium
chloride Ba3(PO4) barium phosphate Na
OH sodium hydroxide
http//www.chem4kids.com/files/atom_compounds.html
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Stock System 3. Elements with more than one
oxidation number (charge) -put the oxidation
number in Roman numerals in ( )s Example
Pb(NO3)2 lead (II) nitrate Usually transition
metals have more then one oxidation number also
lead and tin Check ion sheet
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Molecular Compounds- made of neutral atoms
sharing electrons form neutral molecules
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types of formulas
molecular formula- represents a molecule
C6H6 empirical formulas - simplest ratio
of atoms (formula units are always empirical
formulas)
CH
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diatomic molecule - 2 atoms of the same element
covalently bonded together
There are 7 diatomic elements and they form the
shape of a 7 on the periodic table except for H
which is always an oddball
H2 N2 O2 F2
Cl2 Br2
I2
Only diatomic when pure element not necessarily
when forming a compound
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Use prefixes to name molecular compounds
prefix subscript mono no
subscript (1 atom) di
2 tri 3
tetra 4
penta 5
hexa 6
hepta 7 octa 8
nona 9 deca 10

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Rules for molecular compounds 1. The less
electronegative element is given 1st and only
given a prefix if it has a subscript gt1 (never
start a name with mono)
2. Second element-
add a prefix and add ide (drop prefix ending if
first letter in the name of the element is a
vowel) P4O10 tetraphosphorous decoxide
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Oxidation Numbers

• In polar covalent bonds, the electrons are
  located closer to one atom than the other. This
  is reflected in a partial charge, ? and ?-, for
  each atom.
• The oxidation state or oxidation number of an
  atom is an indication of the amount of charge
  each atom carries. The one with the greater EN
  gets both e-

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Example We know that in HCl, H has a lower EN
and is ? and Cl has a greater EN is ?-. We say
H is in the 1 oxidation state and Cl is in the
-1 oxidation state.
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Acids
endings change
-ate to -ic H2SO4 sulfuric
-ite to -ous
H2SO3 sulfurous
Important acids to know
HCl hydrochloric
HNO3 nitric
H3PO4 phosphoric HC2H3O2 acetic
(also CH3COOH) H2CO3 carbonic