Chapter 5Atoms and Bonding

Valence ElectronsAre those electrons that have the

highest energy level and are held most loosely.

These electrons are the electrons involved in chemical reactions.

The number of valence electrons in an atom of an element determines many properties of that element.

Valence ElectronsEach element has a specific

number of valence electrons ranging from 1 to 8.

Electron Dot Diagram- is a representation of the valence electrons in an atom, using dots.

Every element wants to fill its outer energy level. Thus, most elements want 8 valence electron.

Electron Dot Diagram

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Chemical ReactionElements with 8 valence electron

are less reactive/more stable then elements that want to gain valence electrons.

Reactions happen in one of two ways:◦An element gains electrons to make

itself more stable.◦An element gives up electrons to

make itself more stable.

Chemical BondsA chemical bond is the force of

attraction that holds two atoms together as a result of the arrangement of electrons between them.

During atom bonding, electrons may be transferred or shared between the atoms.

The bonding causes chemical reactions resulting in the form of new substances.

Reading the Periodic TablePeriods

◦Valence electrons increase from left to right on the periodic table.

◦Valence electrons of an atom in the next period are of a higher energy level.

Columns◦Valence electron count is the same◦Elements have similar properties

(except elements in groups 3-14 follow a slightly different pattern)

Reading the Periodic TableNoble Gases

◦All have 8 valence electrons◦Exception is helium with 2◦Very stable◦Do not react easily

Halogens◦Group 17◦7 electrons◦Very reactive with atoms that can

give up or share electrons

Reading the Periodic TableAlkali Metals

◦Group 1 ◦Have 1 valence electron◦Very reactive ◦Want to lose their one electron

Reading the Periodic TableOther metals

◦Groups 2 through 12 ◦Most have 1,2, or 3 valence electrons◦React by losing electrons (especially with

halogens)◦How reactive a metal is depends on how

easily it loses electrons.◦Usually reactivity of metals decrease

from left to right on the periodic table.◦ In group 1 and 2, reactivity increases

from top to bottom.

Reading the Periodic TableNonmetals

◦Most are gases at room temperature. ◦5 of the nonmetals are solids and

one is a liquid.◦They become stable by gaining or

sharing electrons (except for group 18)

◦Nonmetals usually combined with metals by gaining electrons

◦Nonmetals usually combined with other nonmetals by sharing electrons.

Reading the Periodic TableMetalloids

◦Lie on a zigzag line between the metals and nonmetals.

◦3 to 6 valence electrons◦Can either lose or share electrons

Hydrogen◦Above group 1 ◦1 valence electron◦Nonmetals◦Reactive ◦Quiet different from alkali metals

IonsAn ion is an atom or groups of

atoms that has an electric charge.

When an atom loses an electron, it loses a negative charge and becomes a positive ion.

When an atom gains an electron it gains a negative charge and becomes a negative ion.

Polyatomic ions Polyatomic ions are ions that are

made of more than one atom◦They act as a single unit◦They have overall positive or

negative charges.

Ionic BondsAn ionic bond is the attraction

between two oppositely charged ions.◦They form as a result of the

attraction between positive and negative ions.

◦They usually form when a metal combines with a nonmetal

◦A compound consisting of positive and negative ions is called ionic compound.

Chemical FormulasChemical formulas- combinations

of symbols that shows the ratio of elements.

When ionic compounds form, the ions come together in a way that balances out the charges on the ions.

The chemical formula for the compound reflects this balance.

SubscriptsSubscripts tells you the ratio of

elements in a compound.◦No subscript is an understood 1◦Polyatomic ions are written similarly

Naming Ionic CompoundsFor an ionic compound

◦The name of the positive ion comes first followed by the negative ion

◦The positive ion is usually a metal Exceptions-some positive polyatomic ions

exist

◦If negative item is a single element, the ending of its name changes to –ide

◦If the negative item is a polyatomic ion, its name usually ends in –ate or –ite.

Properties of Ionic Compounds

Characteristics of ionic compounds◦Hard solids◦Brittle crystals◦High melting points◦Conduct electricity when dissolved in

water

Ionic Crystals Ions that form and orderly, three-

dimensional arrangement are called crystals.

Every ion is attracted to ions of opposite charges that surround it.

The crystals are hard and brittle because of the attraction among all ions.

High Melting PointIonic bonds are very strong so it

takes a lot of energy to break the bonds.

Thus, ionic bonds have high melting points.

Electrical ConductivityWhen dissolved in water, ions are

free to move around and are conductive.

Solid ionic compounds are not conductive because ions cannot move so a current can not occur.

Covalent BondsThe chemical bond formed when

two atoms share electrons.These types of bonds usually

form between atoms of nonmetals.

Electron SharingThe force that holds atoms

together in a covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons.

A molecule is a neutral group of atoms joined by covalent bonds.

How Many Bonds?The amount of total covalent

bonds that a nonmetal can form is equal to the amount of valence electrons an atom needs to fill its highest energy level.

Double and Triple BondingA double bond is a chemical bond

formed when atoms share two pairs of electrons.

A triple bond is a chemical bond formed when atoms share three pairs of electrons.

Molecular CompoundsA compound that is composed of

moleculesCharacteristics-

◦Lower melting points◦Lower boiling points ◦Do not conduct electricity when

dissolved in water◦Have a weaker bond

Unequal Sharing of ElectronsAtoms of some elements pull

more strongly on shared electrons than do atoms of other elements.

As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges.

Polar and Nonpolar BondsThe unequal sharing of electrons

can cause one atom in a bond to be positive and the other to be negative.

Polar bond-a covalent bond in which electrons are shared unequally

Nonpolar bond- a covalent bond in which electrons are shared equally.

Polar Bonds in MoleculesMolecules with nonpolar bonds

are nonpolar.Molecules with polar bonds can

be polar or nonpolar.A nonpolar molecule with polar

bonds only occurs if the molecule’s atoms pull with the same amount of force in opposite directions.

Attraction Between Molecules

The properties of polar and nonpolar compounds differ because of differences in attractions between their molecules.

Polar molecules are strongly attracted to polar molecules.

This is why oil (nonpolar) and water (polar) do not mix.

Metals and AlloyCommon characteristics of metals

◦Most are solids◦Can be hammered flat or drawn into a wire

(malleability).◦Conducts electricity well◦A lot of every day metals are a form of an

alloy. An alloy is a mixture made of two or more

elements that has the properties of a metal. One element is always a metal.

Alloys are generally stronger and less likely to react with air or water than the pure metals from which they are made.

Physical and Chemical PropertiesPhysical Properties

◦Metal alloys are used in many cases over the original metal because the alloys are a stronger than the original metal but still withholds many of the original elements properties.

Chemical Properties◦Many times strong metals are

alloyed with other elements so that the object will not react with water (rust).

Metallic Bonding Most metals have 1,2, or 3 valence electrons.These valence electrons are usually lost when

bonding with other elements living the metal with a positive charge.

Metals exist in crystals which the valence electrons are free to move between the positively charged ions.

Metallic bonds- an attraction between a positive metal ion and the electrons surrounding it.

A metal or metal alloy consists of positively charged metal ions embedded in a “sea of valence electrons.

Metallic PropertiesMetallic properties

◦Easily change shape (malleability)◦Easily conduct electric current◦Luster ◦Easily conduct heat◦The “sea of electrons” model of solid

metals explains these properties.

Important LinksWatch “Physical Science Series:

Chemical Bonding” video on ionic bonds. http://player.discoveryeducation.com/index.cfm?guidAssetId=547c7d47-0d64-4969-8eff-ac94d18f1061

CreditsFrank, D. V., Jones, G. T., & Little, J.

G. (2006). Science Explorer. Boston: Pearson Prentice Hall.

Learning, U. (Director). (1998). Physical Science Series: Chemical Bonds [Motion Picture].

The Alkane Series: Building Them From Scratch (2009). Retrieved 1/31/09 from www.green-planet-solar-energy.com/alkane.html