Bonding Ionic Covalent

8/8/2019 Bonding Ionic Covalent

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BondingBonding



Review: bond types, bond energy We have been talking about atomic structure,

now we are going to focus on molecules There are 2 types of bonding: ionic, covalent From 3A: ionic = stealing of electrons to form

+ve and ±ve ions. +ve and ±ve then attract

Covalent = sharing of electrons We will see that there is no clear dividing line.

What causes atoms to form molecules? Basically, all things that happen spontaneously

are energetically favorable (a book dropping) You never see a book spontaneously rise Something must be energetically favorable

about atoms coming together as molecules We will look at ionic first and then covalent



Ionic bondingIonic bonding involves 3 steps (3 energies)

1) loss of an electron(s) by one element,2) gain of electron(s) by a second element,

3) attraction between positive and negative

+Na e ± Na+Ionization energy

Cl e ±

+ Cl ±

Electron affinity

Lattice energy+ Cl ± Na+Cl ± Na+



Ionic bonding: energiesIonic bonding: energies Let¶s keep track of numerical energy values

By convention, a requirement for energy is

given a + sign (we have to put energy in) and

is called endothermic, a release of energy is

given a ± sign and is called exothermic.



Ionic bondingIonic bonding involves 3 steps (3 energies)

1) loss of an electron(s) by one element,2) gain of electron(s) by a second element,

3) attraction between positive and negative

+Na e ± Na+Ionization energy

Cl e ±

+ Cl ±

Electron affinity

Lattice energy+ Cl ± Na+Cl ± Na+

+ 496

± 349

± 766



Ionic bonding: energiesIonic bonding: energies Let¶s keep track of numerical energy values

By convention, a requirement for energy is

given a + sign (we have to put energy in) and

is called endothermic, a release of energy is

given a ± sign and is called exothermic. Problem: the sum is +147. A spontaneous

change must involve a net lowering of energy

Solution: the lattice energy provides theenergy needed

Note that although we represent this as a

three step process it actual occurs all at once



7.1 questions (pg. 2257.1 questions (pg. 225 ± ± 8)8)1. A metal + non-metal gives what kind of bond?

2. What is necessary for any stable compound to formfrom its elements?

3. Define cation. Define anion.

4. List the 3 energies involved in forming an ionic bond

5. What term describes a release of energy?

6. Is breaking a bond endothermic or exothermic?

7. Define lattice energy.

8. Explain why metals form cations and non-metals

form anions.

9. Explain why calcium exists as Ca2+ but not as Ca3+

in ionic compounds.

10.Explain why most transition metals form a 2+ ion.



7.1 Answers7.1 Answers1. Ionic

2. There must be a net lowering of energy3. Cation: a positively charged ion, Anion: a

negatively charged ion

4. Ionization energy, electron affinity, latticeenergy

5. Exothermic

6. Endothermic

7. The energy released by the imaginary

process in which isolated ions come

together to form a crystal of an ionic

compound



8. Metals form cations because they have small

IEs (and EA), non-metals form anions

because they have large EAs (andIE).These trends are energetically favorable.

9. The first two electrons from the 4s subshell

are easily lost (they can be made up for by

the lattice energy). Losing a third electron isnot energetically favorable because of the

large third IE for Ca (the energy required to

remove the third electron can not be madeup by the lattice energy.

10.The loss of two electrons from the s subshell

accounts for the typical 2+ charge of the

transition elements.



Covalent bondingCovalent bonding Just as with ionic bonds, covalent bonds

must involve a net lowering of energy

We can explain this net lowering of energy

in two ways:

1) visualizing the combination of attraction

as two atoms approach each other

2) drawing and combining orbital diagrams



As atoms approach As atoms approach Recall that EA for all atoms, except

the noble gasses is negative

In other words we have no trouble

adding electrons to atoms

The attraction for electrons is not limited tofree electrons, but also involves electrons

that are part of other atoms.

+ ±

±

+ ± ±

Thus, atoms are pulled toward each other

How far they are pulled together will depend

on a balance of attraction (nucleus to

electrons) and repulsion (nucleus to nucleus

and electrons to electrons)



Reducing energyReducing energy The energy associated with moving atoms

closer and farther is shown in 7.3 (pg. 231)

It requires lots of energy to push nuclei closetogether (top). The atoms bond where

energy is most favorable (lowest) similar to

position marble would find on a track

Energy of separated atoms

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Bonding Ionic Covalent