Transcript Chemical Formulas

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
Capital letter each element NaCl
Subscript # atoms
CH3Cl
Molecular compound the
number of atoms in a
molecule
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
OHPO43- NH4+
Quiz yourself
How many oxygen atoms in the
following?
–CaCO3
–Al2(SO4)3
How many ions in the following?
–CaCl2
–NaOH
–Al2(SO4)3
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
The positive side of the formula must = the
negative side of the element
Examples on board
1. Sodium chloride Na+ ClNaCl
(no charges written in the formula)
2. Calcium cholride Ca2+ ClCaCl2
3. Potassium sulfide K+ S2K2S
4. Aluminum chloride
Al3+ ClAlCl3
5. Barium phosphate
Ba2+ PO43Ba3(PO4)2
Cross over method - the charges become
the subscripts of the other ion
Beware subscripts must be simplest ratios
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
Formula: ZnS
Compound Name: Zinc Sulfide
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
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
Molecular Compounds- made of neutral
atoms sharing electrons & form neutral
molecules
2 types of formulas
molecular formula- represents a molecule
C6H6
empirical formulas - simplest ratio of
atoms (formula units are always empirical
formulas)
CH
diatomic molecule - 2 atoms of the
same element covalently bonded
together
There are 7
H2
N2 O2 F2
Cl2
Br2
I2
diatomic elements
and they form the
shape of a 7 on
the periodic table
except for H
which is always
an oddball
Only diatomic when pure element not
necessarily when forming a compound
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
Rules for molecular compounds
1. The less electronegative element is
given 1st and only given a prefix if it
has a subscript >1 (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
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-
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.
Acids
endings change
“-ate” to “-ic”
H2SO4
“-ite” to “-ous” H2SO3
sulfuric
sulfurous
Important acids to know:
HCl
hydrochloric
HNO3
nitric
H3PO4
phosphoric
HC2H3O2
acetic (also CH3COOH)
H2CO3
carbonic