Isotopes, Atomic Numbers, and
Mass Numbers
• Atomic number (Z) = number of protons in the
nucleus.
• Mass number (A) = total number of nucleons
in the nucleus (i.e., protons and neutrons).
• By convention, for element X, we write ZAX.
• Isotopes have the same Z but different A.
The Atomic Mass Scale
• 1H weighs 1.6735 x 10-24 g
and 16O 2.6560 x 10-23 g.
• We define: mass of 12C = exactly 12 amu.
• Using atomic mass units:
• 1 amu = 1.66054 x 10-24 g
• 1 g = 6.02214 x 1023 amu
Isotope Calculation Review
• The atomic masses listed on the periodic table
are average atomic masses
• They are determined by calculating the
weighted mean.
• Average atomic mass =
Σ (isotope mass)( relative abundance)
Isotope Calculations Example 1
Isotope
Atomic Mass
Relative Abundance
28Si
27.976
92.2297%
29Si
28.976
4.6832%
30Si
29.973
3.0872%
• Using the isotope
information for Silicon.
Find the average atomic
mass.
Isotope Calculations Example 2
Silver consists of two isotopes 107Ag and 109Ag. Its
average atomic mass is 107.87. Calculate the
percentage of each isotope in naturally occurring
silver. (Assume that the masses are 107.00 and
109.00 respectively.)
Naming & Formula Writing
Background: Periodic Table
• Some of the groups in the periodic table are given special
names.
• These names indicate the similarities between group
members:
Group 1: Alkali metals.
Group 2: Alkaline earth metals.
Group 16: Chalcogens.
Group 17: Halogens.
Group 18: Noble gases.
Background: Molecules
• Definition: a group of two or more atoms held
together by a covalent chemical bond.
• Typically a covalent bond is between two non-metals
(This is a general rule of thumb.)
• Examples: Water (H2O), Bromine (Br2), ammonia
(NH3), Vinegar (HC2H3O)
Background: Ions
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Definition: An atom or group of atoms that
have an overall positive or negative charge
Monatomic ions: atoms that have lost or gained
electrons.
Charge related to position on Periodic Table for
monoatomic ions
Cation: Positive ion (Typically metal)
Anion: Negative ion (Typically non-metal)
Background: Formulas
• Empirical formula: shows the lowest
whole number ratio of the atoms in the
compound.
• Molecular formula: shows the exact
number of each kind of atom in the
compound.
• Structural formula: shows how the
atoms in the molecule are bonded
together
Background: Formula of Molecular
Compounds
Background Practice: Benzene
H
C
H
?
H
C
C
C
C
?
H
C
H
Structural Formula
H
Molecular Formula
Empirical Formula
Background: Predicting Charges
I. Ionic Compounds
• Are formed because of the strong electrostatic
attraction between cations and anions.
• Binary ionic compounds are always between a
metal and a non-metal.
• Other ionic compounds must contain a
polyatomic ion
• Examples: Table salt (NaCl), baking soda
(NaHCO3), Epsom salts (MgSO4)
Common Cation Charges
Naming Ionic Compounds
• Simply write the name of the cation first
– Group 1,2 elements, Al3+, Zn2+, Ag1+, Ga3+, In3+ are simply
named
– Polyatomic cations are also simply named
– Other metals can have more than one charge, so the name
must indicate the charge with a roman numeral.
• Cu1+ is copper(I)
• Cu2+ is copper(II)
• Then write the name of the anion
– Polyatomic anions are simply named
– Remember the name of a monatomic anion ends in –ide.
• oxygen forms the anion oxide (O2-)
• nitrogen forms the anion nitride (N3-)
Lots of examples
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KCl
Mg 3N2
Na2SO4
(NH4)2CO3
CuO
Cu2O
FePO4
Potassium chloride
Magnesium nitride
Sodium sulfate
Ammonium carbonate
Copper(II)oxide
Copper(I)oxide
Iron(III)phosphate
Formula Writing
Ionic Compounds
• Identify the compound as ionic
• Find the formula and charge of the cation and
the anion.
• Use subscripts to indicate the number of each
ion needed to have an overall neutral charge.
“Drop and Swap”
• Reduce the subscripts to the lowest whole
number ratio.
Ionic Naming Examples
• Sodium fluoride
NaF
• Calcium nitride
Ca3N2
• Barium nitrite
Ba(NO2)2
• Lead(II)hydroxide
Pb(OH)2
• Manganese (IV) Sulfide
Mn2S4  MnS2
Na1+ F1Ca2+ N3Ba2+ NO21Pb2+ OH1Mn4+ S2-
Now You Try
Name
Formula
Silver chloride
Zinc nitrate
Ammonium hydroxide
Tin(II)sulfite
Al2O3
Fe2(SO4)3
MnO
NaNO3
Hydrates
• Hydrates are compounds that contain discrete
water molecules as part of the crystal lattice
structure.
• CuSO4•5H2O is called copper(II)sulfate
pentahydrate.
• You will use the Greek prefixes to indicate the
number of water molecules in the compound.
Prefixes for Hydrates
Number
1
2
3
4
5
6
7
8
9
10
Prefix
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
Naming Polyatomic Ions With Oxygen
Example
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Selenate is SeO42What is selenite?
Answer:
Bromate is BrO3What is hypobromite?
Answer:
Oxygen and Hydrogen Containing
Polyatomic Compounds
• Polyatomic anions containing oxygen with additional
hydrogens are named by adding hydrogen or bi- (one
H+), dihydrogen (two H+), etc., to the name as follows:
CO32- is the carbonate anion
HCO3- is the hydrogen carbonate (or bicarbonate) anion.
H2PO4- is the dihydrogen phosphate anion.
II. Naming Acids
II. Naming Acids
• A helpful mnemonic for naming oxyacids
I don’t feel well because I “ate” something
“ic”ky!
For example carbonate (CO32-) makes carbonic
acid
Practice With Acids
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HCN
HNO3
HNO2
HClO4
HClO3
H2SO3
HCl
HBr
HI
III. Binary Molecular
Diatomic Elements
More about the Elements
• Allotropes: Different forms of the same
element
• Some well know allotropes are:
Carbon: Graphite, Diamond, “Bucky Balls”
Oxygen: Oxygen gas, Ozone
Tin: White (metallic tin), Gray Tin
• Diatomic Elements: Elements that exist as
molecules with two atoms.
Formula
• Empirical formula: shows the lowest
whole number ratio of the atoms in the
compound.
• Molecular formula: shows the exact
number of each kind of atom in the
compound.
• Structural formula: shows how the
atoms in the molecule are bonded
together
Formula Writing and Naming
Binary Molecular Compounds
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Identify the molecular compound because there are two nonmetals.
The most metallic element is usually written first (i.e., the one
to the farthest left on the periodic table). Exception: NH3.
If both elements are in the same group, the lower one is
written first.
Use prefixes to indicate the number of a particular atom in
the compound.
mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca.
Truncate the name of the last element and then add –ide
Example: NCl3 is nitrogen trichloride
Now you Try
Name
Formula
XeF6
P2O3
Tetraphosphorus decoxide
Nitrogen triiodide