1.6 Oxidation numbers

Oxidation numbers

1. Give the oxidation numbers for each element in the following chemical compounds:
   1. NO ${}_2$
   2. BaCl ${}_2$
   3. H ${}_2$ SO ${}_4$
2. Give the oxidation numbers for the reactants and products in each of the following reactions:
   1. $\mathrm{C}+{\mathrm{O}}_2\to {\mathrm{CO}}_2$
   2. ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3$
   3. Magnesium metal burns in oxygen

Summary

• A chemical bond is the physical process that causes atoms and molecules to be attracted together and to be bound in new compounds.
• Atoms are more reactive , and therefore more likely to bond, when their outer electron orbitals are not full. Atoms are less reactive when these outer orbitals contain the maximum number of electrons. This explains why the noble gases do not combine to form molecules.
• There are a number of forces that act between atoms: attractive forces between the positive nucleus of one atom and the negative electrons of another; repulsive forces between like-charged electrons, and repulsion between like-charged nuclei.
• Chemical bonding occurs when the energy of the system is at its lowest.
• Bond length is the distance between the nuclei of the atoms when they bond.
• Bond energy is the energy that must be added to the system for the bonds to break.
• When atoms bond, electrons are either shared or exchanged.
• Covalent bonding occurs between the atoms of non-metals and involves a sharing of electrons so that the orbitals of the outermost energy levels in the atoms are filled.
• The valency of an atom is the number of electrons in the outer shell of that atom and valence electrons are able to form bonds with other atoms.
• A double or triple bond occurs if there are two or three electron pairs that are shared between the same two atoms.
• A dative covalent bond is a bond between two atoms in which both the electrons that are shared in the bond come from the same atom.
• Lewis and Couper notation are two ways of representing molecular structure. In Lewis notation, dots and crosses are used to represent the valence electrons around the central atom. In Couper notation, lines are used to represent the bonds between atoms.
• Electronegativity measures how strongly an atom draws electrons to it.
• Electronegativity can be used to explain the difference between two types of covalent bonds: polar covalent bonds (between non-identical atoms) and non-polar covalent bonds (between identical atoms).
• An ionic bond occurs between atoms where the difference in electronegativity is greater than 1.7. An exchange of electrons takes place and the atoms are held together by the electrostatic force of attraction between oppositely-charged ions.
• Ionic solids are arranged in a crystal lattice structure.
• Ionic compounds have a number of specific properties , including their high melting and boiling points, brittle nature, the lattice structure of solids and the ability of ionic solutions to conduct electricity.
• A metallic bond is the electrostatic attraction between the positively charged nuclei of metal atoms and the delocalised electrons in the metal.
• Metals also have a number of properties, including their ability to conduct heat and electricity, their metallic lustre, the fact that they are both malleable and ductile, and their high melting point and density.
• The valency of atoms, and the way they bond, can be used to determine the chemical formulae of compounds.
• The shape of molecules can be predicted using the VSEPR theory, which uses the arrangement of electrons around the central atom to determine the most likely shape of the molecule.
• Oxidation numbers are used to determine whether an atom has gained or lost electrons during a chemical reaction.

Summary exercise

1. Give one word/term for each of the following descriptions.
   1. The distance between two atoms in a molecule
   2. A type of chemical bond that involves the transfer of electrons from one atom to another.
   3. A measure of an atom's ability to attract electrons to it.
2. Which ONE of the following best describes the bond formed between an H ${}^{+}$ ion and the NH ${}_3$ molecule?
   1. Covalent bond
   2. Dative covalent (coordinate covalent) bond
   3. Ionic Bond
   4. Hydrogen Bond
3. Explain the meaning of each of the following terms:
   1. valency
   2. bond energy
   3. covalent bond
4. Which of the following reactions will not take place? Explain your answer.
   1. $\mathrm{H}+\mathrm{H}\to {\mathrm{H}}_2$
   2. $\mathrm{Ne}+\mathrm{Ne}\to {\mathrm{Ne}}_2$
   3. $\mathrm{Cl}+\mathrm{Cl}\to {\mathrm{Cl}}_2$
5. Draw the Lewis structure for each of the following:
   1. calcium
   2. iodine (Hint: Which group is it in? It will be similar to others in that group)
   3. hydrogen bromide (HBr)
   4. nitrogen dioxide (NO ${}_2$ )
6. Given the following Lewis structure, where X and Y each represent a different element...

   

   1. What is the valency of X?
   2. What is the valency of Y?
   3. Which elements could X and Y represent?
7. A molecule of ethane has the formula C ${}_2$ H ${}_6$ . Which of the following diagrams (Couper notation) accurately represents this molecule?

   

8. Potassium dichromate is dissolved in water.
   1. Give the name and chemical formula for each of the ions in solution.
   2. What is the chemical formula for potassium dichromate?
   3. Give the oxidation number for each element in potassium dichromate.