Chemical BondingChapter 17

Adapted from “Conceptual Physical Science” Hewitt

Chemical Bonding

General theme

How can the periodic table can be used to determine chemical bonding?

Electron Affinity

Electron Affinity

The ability of an atom to attract additional electrons

 

How many electrons an atom attracts is dependent on its valence shell.

Electron Affinity continuedOctet Rule

Atoms desire to have a full valence shell.

Generally, this means they want 8 electrons in their outer shell.

Atoms will gain or lose electrons in order to achieve this stability.

Electron Affinity continued

Periodic Table

Elements in the upper right corner have the greatest electron affinity.

Elements in the lower left corner have the smallest electron affinity.

Electron Affinity continued

Atoms are electrically neutral…

…until they gain or lose electrons.

Ions

How can an atom become an ion?

An atom becomes an ion by gaining or losing electrons

Each electron has the same quantity of negative charge and the same mass

Ion

Ion

A charged atom

Atoms are electrically neutral, but if an atom becomes charged by gaining or losing an electron it becomes an ion.

Cation

Cation

Positive ion (+)

Loses electrons

Metals commonly do this.

Anion

Anion

Negative ion (-)

Gains electrons

Non-metals commonly do this.

Cations and anionsCation

Any atom or group of atoms that has a positive charge

 AnionAny atom or group of atoms that has a negative charge

 When bonding, atoms want 8 valence electrons

Metals

Metals

Valence electrons of most metals are weakly held to the nucleus

Metals have a very _______ electron affinity.LOW

Metallic BondMetallic Bond

A chemical bond in which metal atoms are held together by their attraction to a common pool of electrons

 Metal ions are held together by freely flowing electrons.

This electron mobility accounts for a metal’s high heat and electrical conductivity

AlloyAlloyA mixture of 2 or more metallic elements

Example: White Gold Au and Pd

Sterling silver 92.5% Ag 7.5% Cu

Steel 97% Fe 2% Mn 1% C

 Aluminum cans are alloys of Al, Zn and Mn

Ionic bondIonic Bond

Metal atoms lose electrons to become cations (+)

Non-metal atoms gain electrons to become anions (-)

 

Cations and anions are pulled together by an electrical force

Ionic Bond

Ionic Bond

An electrical force of attraction between two oppositely charged ions

 

Remember atoms want 8 electrons (in general) in outer shell.

Ionic Bond

Sodium wants to get rid of an electron…

Na

Chlorine wants to get an electron…

Cl

+1

-1

Positive Na is attracted to negative Cl…

Ionic CompoundsIonic Compound

A chemical compound made with ionic bonds

NaCl, CaF2, KI, LiCl, MgCl2

Ionic compounds typically consist of elements found on opposite sides of the periodic table Fig 17.1, p. 405 (metal ion and non-metal ion)

IonsNa+, Li+, K+ each lost 1 electron

Mg2+, Ca2+, Ba2+ each lost 2 electrons

O2-, S2- each gained 2 electrons

F-, Cl-, Br- each gained 1 electron

 

Numeral 1 is optional in super script notation

Ionic compounds

For all ionic compounds, positive and negative charges must balance

 

Determine the compound created between Fluorine and Calcium

Ionic compounds

Ca2+ F-

Calcium needs to get rid of two electrons…

Fluorine can only accept one electron…

WHAT DO WE DO?

Bring in another Fluorine!

F-

Ionic compounds

F- Ca2+ F-

Calcium will bond with two Fluorine atoms…

CaF2

Ionic compounds

Aluminum Oxide

Al O+3 -2

Al2O3

Electron Dot Structures

Valence electrons are usually the only electrons used in chemical bonds, therefore; only valence electrons are shown in electron dot structures.

 

Octet Rule

Octet Rule

In forming compounds, atoms tend to achieve the electron configuration of a noble gas.

All noble gases, except He, have 8 valence electrons

Covalent CompoundsCovalent bonds primarily involve non-metals.

(these are elements with high electron affinities)

Noble gases are exception - their outer electron shells are full, therefore, they have no electron affinity and do not form covalent bonds

Covalent Bonds

Covalent bond

A chemical bond where atoms are held together by their mutual attraction for the 2 electrons they share

 

Co- means sharing

Covalent CompoundsIn a covalent bond —there are no ions involved

but the force holding the two atoms together is still electrical.

Covalent compound- any group of atoms held together by one or more covalent bonds

Ex: H2, O2, N2, CO2, H2O

Covalent Compounds

Example: how / why bonds form between Hydrogen and Fluorine.

Covalent CompoundsOxygen has affinity for 2 extra electrons

Oxygen finds 2 extra electrons when it bonds with 2 H atoms to form water.

Each H atom gets access to an additional electron by bonding to O.

Win-Win situation for O and H 

Electronegativity

Electronegativity

The ability of an atom to attract a bonding pair of electrons to itself when bonded to another atom.

Electronegativity

-+

This chemical compound is slightly polar

Polar

Polar

Compounds which have a different in charge on each end.

Similar to North and South poles of a magnet.

Polar

When the electronegativity of the 2 atoms differ, the bond is polar (ex: H-F)

The greater the electronegativity, the greater the polarity

See figure 17.23 page 414

Polarity

To determine bond polarity, simply find the difference in the electronegativity of the bonded elements.

Polarity

H-F

H = 2.2 F = 3.98

Polarity

3.98-2.2 = 1.78

toward Fluorine

DipoleDipole

Difference of charge which creates two poles

 

F has a greater electro negativity

Slightly (+) H – F Slightly (-)

 

H :F

To Polar or not to polar?

A Covalent bond is either polar or non-polar

When the 2 atoms have the same electro negativity, no dipole is formed and the bond is non-polar (ex: H2, N2)

Covalent Compounds

The number of covalent bonds an atom is able to form is equal to the number of additional electrons it can attract.

There may be more than 1 covalent bond between 2 atoms

Double? Triple? Covalent bondsO2 two O atoms connected by 2 covalent

bonds called a double covalent bond (4 electrons are shared, 2 from each atom)

 O O

 N2 triple covalent bond, 6 electrons are

shared, 3 from each atom

N N

Single covalent bond dots

Single covalent bond

Pairs of electrons not shared between atoms are called unshared pairs

Molecules

Molecule

A group of atoms held tightly together by covalent bonds

 

A diamond is a single large molecule made of many carbon atoms connected by numerous covalent bonds

Compare and ContrastMetallic bond network of positively charged metal

ions are held together within a fluid of freely moving electrons

Ionic bond each ion is attracted to all the oppositely charged ions surrounding it

 Covalent bond involves only the two atoms

sharing electrons (like a marriage)