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
Ionization energy
Electron affinity
Lattice energy
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
Ionization energy
Electron affinity
Lattice energy
+ 496
– 349
– 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)
A metal + non-metal gives what kind of bond?
What is necessary for any stable compound to form from its elements?
Define cation. Define anion.
List the 3 energies involved in forming an ionic bond
What term describes a release of energy?
Is breaking a bond endothermic or exothermic?
Define lattice energy.
Explain why metals form cations and non-metals form anions.
Explain why calcium exists as Ca2+ but not as Ca3+ in ionic compounds.
Explain why most transition metals form a 2+ ion.
7.1 Answers
Ionic
There must be a net lowering of energy
Cation: a positively charged ion, Anion: a negatively charged ion
Ionization energy, electron affinity, lattice energy
Exothermic
Endothermic
The energy released by the imaginary process in which isolated ions come together to form a crystal of an ionic compound
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.
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.
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)
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
For more lessons, visit www.chalkbored.com