Chapter 20

Chapter 20Chemical Bonds

Sec. 1: Stability in Bonding

Atoms from different elements can combine to form compounds.

When atoms combine, the compound has different properties than the elements.Ex. Sodium Chloride (table salt) is made of

sodium & chlorine. Sodium is a soft metal that reacts violently

with water.Chlorine is a poisonous greenish-yellow gas.

FormulasChemical Formula—tells what elements

a compound contains and the exact number of the atoms of each element.Ex. NaCl has 1 atom of sodium & 1 atom of

chlorineEx. H2O has 2 atoms of hydrogen and 1 atom

of oxygen.A subscript (small # written below) is

written after a symbol to tell how many atoms of that element are in the compound.

Atomic StabilityAtoms combine to form a compound that

is more stable than the separate atoms.Noble gases are already stable and do not

form compounds.Their outer energy level is full of electrons.

Atoms can gain, lose, or share electrons to get a full valence electron shell.

Chemical Bond—the force that holds atoms together in a compound.

Sodium & ChlorineSodium has 1

valence electron.Chlorine has 7, so

it needs 1 more electron to be stable.

If sodium give its 1 valence e- to chlorine, they are both stable with 8 valence e-.

Water MoleculeIn water, each

hydrogen atom needs 1 e- to fill its outer shell.

Oxygen needs 2 e- to fill its outer shell.

To be stable, the atoms share electrons.

Types of BondsIonic Bonds

Contain a metal & a nonmetal (at least 1 of each)

Electrons are gained or lostEx. Salt, NaCl

Covalent BondsContain 2 or more nonmetals or

hydrogenElectrons are sharedEx. Water, H2O

IonsWhen atoms gain or lose electrons in

an ionic bond, they form ions.Ion—an atom with a positive or

negative charge.You can tell the charge of the ion that

an element will form based on its position on the periodic table.

Polyatomic Ions—ions made of more than one atom.

Write the charges on you periodic table!!!1+

2+ 3+ 3- 2- 1-

Types of IonsThere are 2 types of ions: cations &

anions.Cations—ions that have a positive

chargeThey form by atoms losing electronsMetals form cations

Anions—ions that have a negative chargeThey form by atoms gaining electronsNonmetals form anions

Ionic BondingIn an ionic bond, one atoms gives

electrons to another atom.When atoms combine this way, it is

called a formula unit.Ex. NaCl is a formula unit

The charge of a formula unit is always 0.

The positive and negative charges must always balance each other.

Oxidation number—the charge of an ion

1+

2+ 3+ 3- 2- 1-

Multiple Oxidation Numbers

Some metals (other metals & transition metals) can have more than 1 oxidation number.

If an element can have more than 1 oxidation number, the charge of the ion is written as a roman numeral in parentheses.Ex. Copper (I) ion is Cu1+

Copper (II) ion is Cu2+

Writing Formulas for Ionic Compounds

Write the symbol for the cation (either the metal element symbol or polyatomic ion) firstEx. Mg2+ or NH4

+

Write the symbol for the anion (either the element symbol or polyatomic ion) secondEx. Cl- O2-

Use subscripts to balance the charges of the ions.Ex. KCl, (NH4)2O, MgO, MgCl2

Balancing Chemical Formulas Practice

http://www.chemfiles.com/flash/formulas.html Write the name of the chemical formula, ions

and charges, and the chemical formulas when you get them balanced for 10 problems!!!!!

Don’t forget subscripts and parenthesis!

Naming Ionic Compounds

Write the name of the cation first (remember cations are metals)Ex. Ca2+ is calcium

Some elements can have more than 1 charge (see table 2 p. 588)If it does put the charge of the ion

using roman numerals and put it in parentheses.

Ex. Cu2+ Copper (II)

Ionic Naming ContinuedWrite the name of the anion second, but

change the ending to –ide.Ex. Chlorine becomes Chloride

If the anion is a polyatomic ion DO NOT CHANGE THE ENDING.Ex. NO3

- is still named Nitrate

Put the cation & anion together to get the full name.KI is Potassium IodideCuSO4 is Copper (II) Sulfate

Covalent BondsCovalent bonds share electronsComposed of 2 or more nonmetals (and H)Covalent compounds are called moleculesMolecules can be polar or nonpolarPolar molecule—slightly positive and negative

at parts—but overall neutrale- are unevenly shared

Nonpolar molecule—electrons are shared equally—completely neutral

Naming Covalent Compounds

Use prefixes to tell how many of each element you have.This is done because you can have

different covalent compounds containing the same elements.

Change the ending of the 2nd element to -ideEx. N2O is dinitrogen monoxide.

You can leave off the prefix mono- on the first element.Ex. NO2 is nitrogen dioxide

Not mononitrogen dioxide

Mono—1Di—2Tri—3Tetra—4Penta—5Hexa—6Hepta—7Octa—8Nona—9Deca—10

Formulas for Covalent CompoundsUse the prefixes as the subscript

in the formula.Ex. Phosphorus Trichloride

PCl3Dinitrogen Tetrafluoride

N2F4