Bonding
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 bonding
Ionic 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
Cl
Cl–
Ionization energy
+ e–
+ Na+
e– + Na+
Electron affinity
Lattice energy
Cl–
Cl– Na+
Ionic 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 bonding
Ionic 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
Cl
Cl–
Ionization energy
+ e–
+ Na+
e– + Na+
Electron affinity
Lattice energy
Cl–
+ 496
– 349
Cl– Na+ – 766
Ionic 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 the
energy needed
• Note that although we represent this as a
three step process it actual occurs all at once
7.1 questions (pg. 225 – 8)
1. A metal + non-metal gives what kind of bond?
2. What is necessary for any stable compound to form
from 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 Answers
1. Ionic
2. There must be a net lowering of energy
3. Cation: a positively charged ion, Anion: a
negatively charged ion
4. Ionization energy, electron affinity, lattice
energy
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 (and IE).
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 is
not energetically favorable because of the
large third IE for Ca (the energy required to
remove the third electron can not be made
up 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 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
–
+
–
• 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 to
free 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 energy
• The energy associated with moving atoms
closer and farther is shown in 7.3 (pg. 231)
Energy of
separated atoms
• It requires lots of energy to push nuclei close
together (top). The atoms bond where
energy is most favorable (lowest) similar to
position marble would find on a track
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