Unit 4: Chemical Bonding

-How Elements Get Together-

3 types of bonding

What is Chemical Bonding?• The joining of atoms to form new

substances• The properties of these “substances”

are completely different from the properties of the original elements

• An interaction that holds 2 atoms together is called a chemical bond

Why revisit the Bohr Model?• Scientists and their

theories are best explained with models

• Remember: we use models to explain what we can not see or directly observe

• Based on theories

Do Atoms Bond Only through their Electrons?

• Yes and where are the electrons?• The electrons are located around the

nucleus in various energy levels• Remember:

– The atomic number is the number of protons in an atom

– the atomic number also represents the number of electrons in the atom.

What are the energy levels of an atom? p.311

What are the outer Electrons called?p.311

• Valence Electrons are the outer most orbital or “ring” of an atom

• ALL bonding occurs at this energy level • This energy level is the highest

How do you know how many Valance Electrons an atom has?

p.312

3 4 5 6 78

2

What is the “Octet Rule”?• The octet rule refers to the tendency of

atoms to prefer to have eight electrons in the valence shell.

• When atoms have fewer than eight electrons, they tend to react and form more stable compounds

• The exceptions are with Hydrogen (H) & Helium (He) that need only 2 in their outer shell.

Do All Atoms Bond?• Not necessarily • Group 18 atoms VERY

rarely form bonds• The Noble Gases have a

full outer electron shell - a full octet

• 8 valence electrons = stable; no bonding

How do Atoms fill their Outermost level?

• All Atoms are LAZY!• Some atoms accept or

take electrons – If they have 4 or more

valence they accept more to reach 8

• Some atoms give up electrons– If they have 1, 2 or 3

they give them up and drop down to their lower energy level

Sulfur

Magnesium

What are the Rules for determining what type of bond is

occurring?• Ionic Bonds

– ONLY with groups 1, 2 & sometimes 3 bonding with any nonmetal (groups 15-17)

• Covalent Bonds– ONLY between nonmetals

• Metallic Bonds– ONLY between metals

What are Ionic Bonds?• A bond that forms when electrons are

transferred from one atom to another atom.

• What is an Ion?– Charged particles that form when atoms

gain or lose electrons.– Can be a positive ion (+) or negative ion

(-)– Positive ion = more protons (+) than

electrons (-)– Negative ion = more electrons (-) than

protons (+)

How are Positive Ions Formed?

• Metal atoms tend to lose their valence electrons

• Sodium loses its 1 valence electron and drops to the second energy level

• Groups 1 & 2 lose their electrons very easily

Before After

How are Negative Ions Formed?

• Nonmetal atoms tend to accept electrons easily

• Chlorine accepts a single electron to complete its octet or outer energy level

• Group 17 atoms give off the most energy when they bond

Before After

Sodium + Chlorine = Salt

What are Salts?

• An ionic compound formed from the positive ion of Groups 1 or 2 and the negative ion of Groups 15 - 17.

• Not necessarily NaCl (table salt).

What are some uses of Salts?

• NaCl – sodium chloride or table salt

• NaSO4 – sodium hydroxide or lye

• CaSO4

– calcium sulfate or wallboard

• NaNO3 – sodium nitrate or food preservative

Elements That FormIonic Bonds

Groups 1,2 or 3 with Groups 15 - 17

What are Properties of Ionic Compounds?

• The + or - charges that are exchanged results in a neutral compound

• They form a crystal lattice– A 3-dimensional pattern

• Brittle, with high melting & boiling points

What are Lewis Dot Structures?

• Models that focus only on valence electrons• The element symbol is centered among the

dots representing the element’s valence electrons as shown below

Na Cl

What are Lewis Dot Structures?

• Models that focus only on valence electrons• The element symbol is centered among the

dots representing the element’s valence electrons as shown below

Na Cl

Ne

1. Draw the element symbol

2. Determine the number of valence electrons

3. Arrange the “dots” around the symbol as follows…

O

Be

A Quick Assignment• You need the small dry-erase board

at your table & a dry-erase marker• Turn to your book’s Periodic table• Draw the Lewis-dot structures for

the following Elements:• Hold your board up when finished

PotassiumStrontium

PhosphorousTin

GalliumSelenium

XenonSilicon

BerylliumLead

What are Covalent Bonds?

• When atoms share electrons • Mostly happen between nonmetals

on the periodic table• Properties include

– Low melting & boiling points– Brittle only in a solid state– Examples: water, sugar, oxygen, wood,

etc…

What are Covalent Bonds?p.318

H2

What does a Covalent bond of water look like?

The Lewis Dot model of water looks like….

The Lewis Dot Model looks like….

What are the Diatomic Molecules?

• The simplest molecule• Made up of two

bonded atoms• Elements found in

nature as diatomic molecules are called diatomic elements

• H2, O2, N2, & the Halogens - F2, Cl2, Br2 & I2 are diatomic elements

F2

What are the More-Complex Molecules?

• Typically have Carbon involved in some way

• Each carbon atom needs to make 4 covalent bonds to reach the octet (8 valence electrons)

• Hydrocarbons are good examples Molecule of Propane

What are Metallic Bonds?

• A bond formed by the attraction between positively charged metal ions and the electrons in the metal

• When metal atoms are close enough their valence electrons overlap

• The result is a solid (mostly) with electrons that flow freely around the material

What are the properties of Metallic

Bonds?• It is these bonds

that give all metals their particular characteristics

• Electrical conductivity– Electricity means

negative charges (electrons) moving through the metal

• Malleability & ductility

Malleability & Ductility?• Because the

electrons (-) move freely atoms can easily be rearranged

• Bent, hammered into sheets = malleability

• Pulled into wire = ductility

Bending without breaking?

• Because the electrons (-) move about fixed positive ions (+) – The compounds

can bend instead of break

• All metals have this property