Ionic Bonding

Section 4.1

Introduction to Bonding

Chemical bond: an interaction between atoms or ions that results in a reduction of the potential energy of the system, thereby becoming more stable

Three types of bonds: ionic, metallic, and covalent

The bond type depends on the atom's electronegativitites

More

If the atoms have very different electronegativities, then ionic bonding occurs

If they both have high electronegativities, then covalent bonding occurs

If they both have low electronegativities, then metallic bonding occurs

Practice: What Kind of Bond?

Na and Cl Sr and O C and O Ni and Fe N and O Li and N Ti and Cr

Ionic

Ionic

Covalent

Metallic

Covalent

Ionic

metallic

Valence Electrons

Valence electrons are the electrons in the outermost energy level, which is the highest occupied energy level

They are the electrons responsible for the chemical properties of atoms

Core electrons – are those in the energy levels below.

Keeping Track of Electrons Atoms in the same group have

the same outer electronic structure and therefore the same number of valence electrons.

The number of valence electrons is easily determined. It is the group number for a short group element

Group 2: Be, Mg, Ca, etc. Each has 2 valence electrons

Electron Dot (Lewis Dot)diagrams A way of showing &

keeping track of valence electrons.

Write the symbol - it represents the nucleus and inner (core) electrons

Put one dot for each valence electron (8 maximum)

They don’t pair up until they have to (Hund’s rule)

X

The Electron Dot Diagram (Lewis Structure) for Nitrogen

Nitrogen has 5 valence electrons to show.

First we write the symbol. NThen add 1 electron at a time to each side.

• Now they are forced to pair up as one side has two electrons

The Octet Rule The noble gases are unreactive in

chemical reactions In 1916, Gilbert Lewis used this fact to

explain why atoms form certain kinds of ions and molecules

The Octet Rule: in forming compounds, atoms tend to achieve a noble gas structure; 8 in the outer level is stable

• Each noble gas (except He, which has 2) has 8 electrons in the outer level

Formation of CationsMetals lose electrons (are oxidized) to

attain a noble gas structure.They make positive ions (cations)If we look at the electronic structure, it

makes sense to lose electrons:Na: 1s22s22p63s1 shows 1 valence

electronNa1+: 1s22s22p6 This is a noble gas

structure with 8 electrons in the outer level.

Electron Dots For Cations

Metals have few valence electrons (usually 3 or less); calcium has only 2 valence electrons

Ca

Electron Dots For Cations

Metals will lose the valence electrons

Ca

Electron Dots For Cations

Form positive ions

Ca2+No dots are now shown for the cation.

This is named the “calcium ion”.

Electron Configurations: AnionsNonmetals gain electrons to

attain noble gas electronic structures.

They make negative ions (anions)S = 1s22s22p63s23p4 = 6 valence

electronsS2- = 1s22s22p63s23p6 = noble gas

structure. Halide ions are ions from chlorine

or other halogens that gain electrons

Electron Dots For Anions

Nonmetals will have many valence electrons (usually 5 or more)

They will gain electrons to fill outer shell.

P 3-(This is called the “phosphide ion”, and should show dots)

Stable Electron Configurations

All atoms react to try and achieve a noble gas structure.

Noble gases have 8 valence electrons and so are already stable

This is the octet rule (8 in the outer level is particularly stable).

Ar

Ionic BondingAnions and cations are held

together by opposite charges (+ and -)

Simplest ratio of elements in an ionic compound is called the formula unit (also called the empirical formula).

The bond is formed through the transfer of electrons (lose and gain)

Electrons are transferred to achieve noble gas structure.

Ionic Bonding

Na ClThe metal (sodium) tends to lose its one electron from the outer level.

The nonmetal (chlorine) needs to gain one more to fill its outer level, and will accept the one electron that sodium is going to lose.

Ionic Bonding

Na+ Cl -

Note: Remember that no dots are now shown for the cation

Ionic Bond

Negative charges are attracted to positive charges.

Negative anions are attracted to positive cations.

The result is an ionic bond.A three-dimensional crystal

lattice of anions and cations is formed.

Ionic Compounds

The ionic substance is held together by strong electrostatic attractions between all ions in all three dimensions

No molecules present An ionic lattice is formed This gives them distinct physical

properties

Preserve Electroneutrality

When ions combine, electroneutrality must be preserved.

In the formation of magnesium chloride,

2 Cl- ions must balance a Mg2+ ion:

Mg2+ + 2 Cl- → MgCl2

NaCl

CsCl

TiO2

Please go to the “Naming ions, compounds and molecules presentation.

Properites of Ionic Compounds Hard, brittle crystalline solids

Relatively high melting and boiling points

Do not conduct electricity when solid, but do when molten or in aqueous solution

Are more soluble in water than other solvents

- Page 198

The ions are free to move when they are molten (or in aqueous solution), and thus they are able to conduct the electric current.

Predicting Ionic ChargesGroup 1A: Lose 1 electron to form 1+ ions

H+ Li+ Na+ K+ Rb+

Predicting Ionic ChargesGroup 2: Loses 2 electrons to form 2+ ions

Be2+ Mg2+ Ca2+ Sr2+ Ba2+

Predicting Ionic ChargesGroup 3: Loses 3

electrons to form 3+ ions

B3+ Al3+ Ga3+

Predicting Ionic ChargesGroup 5: Gains 3

electrons to form 3- ions

N3-

P3-

As3-

Nitride

Phosphide

Arsenide

Predicting Ionic ChargesGroup 6: Gains 2

electrons to form 2- ions

O2-

S2-

Se2-

Oxide

Sulfide

Selenide

Predicting Ionic ChargesGroup 7: Gains 1 electron to form 1- ions

F-

Cl-

Br-Fluoride

Chloride

Bromide

I- Iodide

Predicting Ionic ChargesGroup 0: Stable

noble gases do not form ions!

Predicting Ionic Charges Many transition elements have more than one possible oxidation state.

Iron (II) = Fe2+

Iron (III) = Fe3+

Note the use of Roman numerals to show charges