Bonding Notes
What is a metallic bond ?
• Occurs when both atoms have low ionization energies and low electronegativities and will lose electrons easily (i.e. 2 metals)
• In metallic bonding, positive metal ions are arranged with valence electrons delocalized around them.
• Since the electrons are delocalized, they are mobile and able to move throughout the metal structure.
Properties of Metallic Compounds
1. Malleable and ductile because atoms are not restricted to one position by a fixed bond and the ions can roll past each other.
2. Good conductors of heat and electricity because the electrons are mobile and can transmit energy rapidly.
3. Shiny because when light strikes a metal, the valence electrons absorb energy, oscillate at the same frequency as the incident light (incoming light) and then emit light as a reflection of the original light.
4. Solids at room temperature (except mercury) due to the strong bonds (intermolecular and intramolecular forces are the same).
5. They exhibit the photoelectric effect which is electron emission caused by heat or light. This occurs when the frequency and therefore the energy of light striking a metal is sufficient to overcome the attractive forces and an electron escapes the metal decreasing the energy of the photon.
IONIC BONDS
l Occur between an element with low ionization energy (a metal) and an element with high ionization energy (a non-metal).
l An actual transfer of electrons from the metal (becoming a cation) to the non-metal (becoming an anion) occurs.
l This transfer results in the formation of 2 oppositely charged ions.
l The electrostatic interaction between the 2 ions holds the compound together.
Properties of Ionic Compounds
1. Solids at room temperature because the attraction between the ions is very strong.
2. The intramolecular forces are the same as the intermolecular forces (i.e. the electrostatic attraction of oppositely charged ions)
3. Atoms are arranged in a highly ordered crystal lattice structure which
maximizes the attractive forces between oppositely charged ions and
minimizes the repulsion between like charged ions (the crystal
structures are determined experimentally using X-ray
crystallography).
4. Soluble in water (the ions will dissociate in water).
5. Will conduct electricity as a liquid or aqueous solution because the ions
are free to move to oppositely charged electrodes.
COVALENT BONDS
l Occur between 2 atoms with high ionization energies (i.e. 2 non-metal atoms).
l Result from a sharing of electrons to obtain a full outer energy level (octet rule).
l If the electronegativity difference between the two atoms is less than 0.4, the bond is a true covalent bond and the electrons are shared equally.
l If the electronegativity difference between the two atoms is between 0.4 and 1.7, the bond is polar and there is an unequal sharing of electrons.
l If a molecule contains polar covalent bonds and is asymmetrical, the molecule will be polar.
l If both electrons being shared come from the same atom, the bond is a coordinate covalent bond.
Properties of Polar Covalent Molecules
1. Since these compounds exhibit polarity or dipoles, their intermolecular forces will be greater than non-polar covalent compounds (the presence of dipole-dipole forces and possibly H-bonding).
2. These compounds will have higher melting and boiling points and are more likely to be liquids or solids at room temperature (may even exhibit a crystal lattice like sugar).
3. Will dissolve in polar solvents if H-bonding is present (sugar in water).
4. Will not conduct electricity to any appreciable degree (only ionize to a
very small degree)
Properties of Non-Polar Covalent Compounds
1. Have low melting and boiling points and are usually gases at room temperature. This is due to the low intermolecular forces that exist in these molecules (London Dispersion Forces only).
2. If solid at room temperature, the solid is usually soft and waxy.
3. Soluble in non-polar solvents such as ethers.
4. Will not conduct electricity in any form due to the fact that there are no ions present.
LEWIS STRUCTURES
Follow Step by Step Method
1. Total all valence electrons. [Consider Charge]
2. Write symbols for the atoms and guess skeleton structure [ define a central atom ].
3. Place a pair of electrons in each bond.
4. Complete octets of surrounding atoms. [ H = 2 only ]
5. Place leftover electrons in pairs on the central atom.
6. If there are not enough electrons to give the central atom an octet, look for multiple bonds by transferring electrons until each atom has eight electrons around it.
Non-Octet Compounds
• Some compounds will contain central atoms that do not follow the octet rule.
• The four possibilities for non-octet compounds are:
– Where more than 4 atoms are bonded to the central atom such as PCl5.
– A noble gas is participating in bonding such as XeF4.
– Where the central atom has less than 8 valence electrons such as BH3.
– Where molecules contain an odd number of nonbonding electrons such as NO.
Resonance Structures
• For some molecules, there are multiple ways of placing the electrons between the atoms.
• Structures that differ only in the arrangement of the electrons are called RESONANCE STRUCTURES.
• Resonance structures are indicated using a double headed arrow.
Coordinate Covalent Bonds
• A coordinate covalent bond results when both electrons in the bond are donated from the same atom.
• To determine if a bond is coordinate covalent, compare the number of non-bonding electrons around an atom to the number of valence electrons the atom has.
Shapes of molecules
A molecule consists of 2 or more atoms joined by covalent bonds.
The shape of a molecule is a description of the way the atoms in the molecule occupy space.
A diatomic molecule, a molecule composed of only 2 atoms, must always be linear in shape as the centers of the 2 atoms will always be in a straight line.
'Electron Cloud' Repulsion Theory (Valence Shell Electron Pair Repulsion, VSEPR) is used to predict shapes and bond angles of simple molecules
a. an 'electron cloud' may be a single, double or triple bond, or a lone pair of electrons
b. a lone pair of electrons is a non-bonding pair of electrons
c. 'electron clouds' are negatively charged since the electrons are negatively charged, so electron clouds repel one another and try to get as far away from each other as possible
d. lone pairs of electrons exert a greater repelling effect than bonding pairs do
e. lone pair-bonding pair repulsion is greater than bonding pair-bonding pair repulsion
f. lone pair-lone pair repulsion > lone pair-bonding pair repulsion > bonding pair-bonding pair repulsion
LEWIS STRUCTURES
1. Draw the Lewis structure or diagram for the following substances:
1) ZnI2
2) CS2
3) SnCl2
4) CCl4
5) NH3
6) SeCl2
7) PCl5
8) SF4
9) ClF3
10) SF6
11) BrF5
12) I3-
13) ICl4-
14) ICl2-
15) BF3
16) SO42-
2. Determine the shape of each molecule above according to the VSEPR Theory.