C2aL3 Ionic Bonding and Compounds

Key Words

Atoms - the smallest particle into which an element can be broken down

Charge - an electrical property that can be either positive or negative.

Dissolved - ionic compounds seem to disappear when added to water.

Electrons - tiny particles of negative charge.

Forces of attraction - forces that pull atoms together.

Giant structure - lots of atoms bonding to each other a long range of regularly repeating ions.

Ion - a charged atom.

Ionic bond - two atoms held together by opposite charges.

Metal - elements that are good conductors of heat and electricity.

Molten - melted.

Negative - charge carried by electrons.

Noble gas - unreactive gassy elements that have full outer shells.

Non-metal - any element that is not a metal (really?).

Positive - charge carried by protons.

Protons - positively charged particles in the nucleus.

Grade E

All atoms in elements have equal numbers of protons and electrons.  They have no overall charge, which we call neutral.

If an atom loses an electron, it gains a positive charge to become a positive ion.

If the atom gains an electron, it gets a negative charge to become a negative ion.

Protons never move.

Look at this sodium atom (we won't show the protons and the neutrons in the nucleus):

The sodium has 1 outer shell electron.  If it loses this electron, it will gain a structure like a noble gas.  Atoms want to do this.

Now look at the chlorine atom:

It has 7 outer shell electrons.  If the chlorine wants to have a noble gas structure, it can either shed 7 electrons or gain 1 electron.  Gaining 1 electron is much easier.

Ionic compounds always have a metal ion (positive) and a non-metal ion (negative).  When sodium reacts with chlorine, it transfers its outer shell electron to the chlorine:

The sodium has lost the electron to be positively charged, while the chlorine has gained an electron to become negatively charged.  Since opposite charges attract, the two atoms in this compound are held together with an ionic bond, which is strong.  Both atoms have full outer shells of electrons.

When there are lots of ions, we get a giant structure with positive ions and negative ions placed alternately.  These giant structure are seen as crystals.  Electrostatic attraction from the oppositely charged ions cause  very strong forces of attraction.  Therefore:

• Ionic compounds have high melting points and boiling points.

• Ionic solids don't conduct electricity.

Ionic compounds do conduct electricity when:

• molten;

• dissolved.

Grade C

Magnesium has 2 outer shell electrons.  To gain noble gas structure, it transfers them to other elements.  Oxygen wants to gain 2 outer shell electrons to become a noble gas structure.  So 1 atom of magnesium will give two electrons to 1 atom of oxygen.  We have MgO.

If magnesium reacts with chlorine, it can only give one electron to the chlorine atom.  It cannot give both.  Instead it gives one to a first chlorine atom, then the other goes to a second chlorine atom to give MgCl₂.

Giant structures from ionic compounds look like this:

This structure is often called the crystal lattice.  Crystals have a shape that is determined by the lattice.

Melting of ionic structures require high temperatures (600 ^oC or more).  When molten, the ions are free to move and conduct electricity.  Ionic solutions free the ions to move about as well.  They conduct electricity.

Grade A

Ionic compounds generally dissolve in water.  Pure water on its own is a very bad conductor of electricity.  When an ionic compound like sodium chloride is dissolved, the positive sodium ions and the negative chloride ions carry the electrical charge through the water.  Therefore salt water is a good conductor of electricity.

We can use the movement of ions in electroplating.

When a current flows, two electrons are removed from the copper atoms.  The copper ions migrate from the copper positive electrode (anode).  They are attracted to the negative electrode (cathode) and are deposited there, having gained back two electrons for each atom.