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

• Using the isotope information for Silicon. Find the average atomic mass.

Isotope Atomic Mass Relative Abundance

28Si 27.976 92.2297%29Si 28.976 4.6832%30Si 29.973 3.0872%

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.)

Isotope Calculations Example 2

Naming & Formula Writing

• 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: Periodic Table

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

• 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

C

C C

C

CC

H

H

H H

H

H

Structural Formula

?

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

• KCl Potassium chloride• Mg 3N2 Magnesium nitride

• Na2SO4 Sodium sulfate

• (NH4)2CO3 Ammonium carbonate

• CuO Copper(II)oxide• Cu2O Copper(I)oxide

• FePO4 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 Na1+ F1-

NaF• Calcium nitride Ca2+ N3-

Ca3N2

• Barium nitrite Ba2+ NO21-

Ba(NO2)2

• Lead(II)hydroxide Pb2+ OH1-

Pb(OH)2

• Manganese (IV) Sulfide Mn4+ S2-

Mn2S4 MnS2

Now You TryName 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 Prefix1 mono2 di3 tri4 tetra5 penta6 hexa7 hepta8 octa9 nona

10 deca

Naming Polyatomic Ions With Oxygen

Example

• Selenate is SeO42-

• What is selenite?• Answer:• Bromate is BrO3

-

• What is hypobromite?• Answer:

• 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.

Oxygen and Hydrogen Containing Polyatomic Compounds

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• HCN• HNO3

• HNO2

• HClO4

• HClO3

• H2SO3

• HCl• HBr• HI

III. Binary MolecularDiatomic 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, OzoneTin: 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 NamingBinary Molecular Compounds

• Identify the molecular compound because there are two non-metals.

• 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