Molecules and Compounds

Chapter 19

What we will Learn• Relate chemical behavior of an element, including bonding, to its placement on the periodic table

•Explain how elements form chemical bonds and identify the role of electrons in bonding

•Predict chemical formulas of compounds made up of two different elements

•Write chemical formulas for compounds made up of many different elements

•Calculate the formula mass of a compound and compare different compounds based on their formula masses

The Octet RuleWhen an atom has eight valence electrons, it is said to have an octet of electrons.

Atoms form chemical bonds with other atoms by either sharing electrons, or transferring them in order to complete their octet and move to a lower energy state.

This is known as the octet rule.

Predicting Atomic Behavior

Looses 1 electron to satisfy Octet Rule

Octet Rule Satisfied

Looses 1 electron to satisfy Octet Rule

Types of BondsChemical Bonds fall into two categories, depending on whether the valence electrons are transferred or shared

Ionic Bonds: When electron(s) are transferred from one atom to the other.

Covalent Bonds: When electron(s) are shared between atoms.

Ionic BondsAtoms that either gain or lose an electron become ions. Ions may have either positive or negative electric charge.

The positive and negative ions are attracted to each other, creating the bond. Ionic bonds tend to form between more than one pair of atoms at a time. The bond between sodium (Na) and chlorine (Cl) in sodium chloride (salt) is a good example of an ionic bond.

Ionic Bonds

If you put an alkali (Na) with a halogen (Cl), you get an ionic bond because one atom strongly wants to lose an electron and the other strongly wants to gain one.

Alkali Metals

Halogens

Ionic BondsWhen sodium, with its +1 charge, comes into contact with chlorine, with its -1 charge, they become electrically neutral as longas they are together. This is because +1 and -1 cancel each other out.

This also explains why sodium and chlorine combine in a 1:1 ratio to make sodium chloride

IONIC BONDINGPROPERTIES OF IONIC COMPOUNDS

• Form between a metal and a nonmetal

• Brittle/crystalline• High melting/boiling points

• Dissolve (ions come apart) in water

IONIC BONDINGNAMING IONIC COMPOUNDS

• The metal (positive ion) goes first in the formula

• The nonmetal (negative ion) goes last in the formula

• The name of the metal stays the same, and the name of the nonmetal is changed so it ends in –ide.

For example: Oxygen becomes Oxide

NAMING IONIC COMPOUNDS

• NaCl

• HF

• MgI2

• KBr

• Sodium Chloride

• Hydrogen Fluoride

• Magnesium Iodide

• Potassium Bromide

This is called Binary Nomenclature

Oxidation NumbersA sodium atom always ionizes to become Na+ (a charge of +1) when it combineswith other atoms to make a compound. Therefore, we say that sodium has anoxidation number of 1+. An oxidation number indicates how many electrons arelost, gained, or shared when bonding occurs.

Oxidation Numbers

Writing a Chemical Formula Using Oxidation Numbers1. Find the oxidation numbers of each

element in the compound2. Determine the ratios of each

element and then write the chemical formula

Oxidation Numbers

Some elements have more than one oxidation number. In this case, roman numerals are used to distinguish the oxidation number.

Element Oxidation NumberCopper (I) 1+Copper (II) 2+Iron (II) 2+Iron (III) 3+Chromium (II) 2+Chromium (III) 3+Chromium (VI) 6+Lead (II) 2+Lead (IV) 4+

Polyatomic Ions

Name Formula Oxidation #Ammonium NH4

+ 1+Acetate C2H3O2

- 1-Hydroxide OH- 1-Phosphate PO4

3- 3-Nitrate NO3

- 1-Hydronium H3O+ 1+Sulfate SO4

2- 2-

Polyatomic Ions: contains more than one type of atom (poly means many)

Writing Chemical formulas with polyatomic Ions

1. Find the formula and charge of the positive Ion2. Find the formula and charge of the negative ion (for poly atomic ions you will not have to memorize them they will be given)3. Determine how many of each ion are needed so the charges are equal to zero and write the chemical formula (Hint: Least common multiple)

Example: Al2(SO4)3

Writing Chemical formulas with polyatomic Ions

Example: Write the formula for Aluminum Sulfate

1. Find Aluminum’s Oxidation Number (it is 3+)2. Sulfate’s oxidation number will be given (it is 2-)

Al3+ SO42-

3. How many of each do we need to satisfy the charges? The common denominator between 3 and 2 is 6. 6/3 = 2 Aluminums 6/2 = 3 Sulfates

Al2(SO4)3

Covalent BondsIn a covalent bond the electrons are shared between atoms.

An important differencebetween covalent and ionic bonds is that covalent bonds act only between the atoms in a single molecule, while ionic bonds act between all adjacent atoms

Covalent BondsElements that have two to six valence electrons tend to form covalent bonds with each other since the tendency to take or receive electrons is more matched.

Covalent bonds can form between two different types of atoms, or between two or more atoms of the same type. The gases nitrogen (N2), oxygen (O2) and hydrogen (H2) are a few examples. We call these covalently bonded atoms of the same type diatomic molecules.

• This is how you DRAW a SINGLE COVALENT BOND

• A single line means each atom shared one electron each to get full.

Covalent Bonds

• Some atoms need more than one electron to become stable.

• What type of element is oxygen?

• What kind of bond will two oxygen atoms make?

Covalent Bonds

Covalent Bonds– Draw the Electron Dot Diagram for

Oxygen.

– How many electrons does each oxygen need to become stable?

– How many TOTAL electrons are shared?

– FOUR

• How would you draw this?

• This is a DOUBLE COVALENT BOND. • 2 pairs of electrons are shared

O O

Covalent Bonds

– What kind of bond would 2 nitrogen atoms form?

– COVALENT– How many electrons does each atom

need?– Three–How many total electrons are

shared?– Six, 3 pairs of electrons

Covalent Bonds

– How would you draw this?– This is a TRIPLE COVALENT BOND 3

pairs shared

Covalent Bonds

– Sometimes, one atom is not enough to fill all the “empty” spots another atom has.

– How many electrons does Carbon need to be stable?

– How many electrons does Hydrogen need to be stable?

Covalent Bonds

Covalent BondsHow do you write the Chemical

Formula for a MOLECULE?

H2O2

F2 N2

CH4

• Naming Covalent Molecules– The element with the lowest ionization

energy goes first (farthest from Fluorine).– Each element gets a prefix to identify how

many there are.– Exception: If there is only one of the first

element, no prefix is used. – The second element changes so that it ends

with –ide.

Covalent Bonds

Number Prefix Number Prefix

1 Mono- 5 Penta-

2 Di- 6 Hexa-

3 Tri- 7 Septa-

4 Tetra- 8 Octa-

Prefixes for covalent bonding nomenclature

Covalent Bonds

Name the following molecules• H2O• CO2

• SF6

• B2Si• SO3

P2O5

• Dihydrogen monoxide• Carbon dioxide• Sulfur hexafluoride• Diboron monosilicide• Sulfur trioxide• Diphosphorus pentoxide

Covalent Bonds

Covalent Bonds

Molecular Weight

Does a molecule of water (H2O) have the same mass as a group of atoms (also called a formula unit) that make up the ionic compound calcium carbonate (CaCO3)?

Carbon atoms are used as a standard for determining the atomic mass units for theother elements on the periodic table. One carbon atom is equivalent to 12.01 atomic mass units.

Molecular Weight

A chemical Formula does 3 things for us:

1. Tells us the types of atoms in the compound2. Lets you know if polyatomic ions are present3. Allows you to calculate the Molecular Weight (MW) or Formula Mass

Molecular Weight

Lets go back to our first example: Water vs Calcium Carbonate.

Water has a chemical formula of H2O

Hydrogen has an atomic mass of 1.01 amu Oxygen has an atomic mass of 16.00 amu

So one molecule of water has:

2(1.01 amu) + 1(16.00 amu) = 18.02 amu

Molecular Weight

Lets go back to our first example: Water vs Calcium Carbonate.

Calcium Carbonate has a chemical Formula Of CaCO3

Calcium has an atomic mass of 40.08 amu Oxygen has an atomic mass of 16.00 amu

So one molecule of Calcium Carbonate has:

1(40.08 amu) + 1(12.01 amu) + 3(16.00 amu) = 100.09 amu

Carbon has an atomic mass of 12.01 amu

Molecular Weight

The molecular weight comparison:

18.02 amu for Water vs. 100.09 amu for Calcium Carbonate

Molecular Weight

Molecular Weight

Rules for finding Molecular Weight:

1. Write the chemical formula for the compound.2. List the atoms, number of each atom, and the atomic mass of each atom3. Add up the values for each type of atom

Molecular Weight

IsomersIsomers: Compounds that have the same molecular formula but different structural formulas.

IsomersExample: C3H6O

How many ways can we bond these atoms? (hint there are 7 ways minimum)