Compounds

There are 2 different types of compounds.

* Ionic
* Covalent

Ionic compounds are made of a metal and a non-metal while covalent compounds are only made of non-metals.

## Ionic Compounds

Ionic compounds are formed when metallic atoms give electrons to non-metallic atoms and stick together.

* Metallic atoms lose their electrons and form positively charged ions
* Non-metallic atoms gain electrons and form negatively charged ions
* A strong electrostatic force holds these ions together due to their opposite charges

Oppositely charged ions attract while like ions repel.

### Ions

Electrons are tiny negatively charged particles with very little mass. They zoom around the nucleus quickly. Atoms have the same number of electrons as protons but ions do not.

* Ions are charged atoms
* They have lost or gained electrons

Elements in different groups of the periodic table (columns) form different ions. Group 1, 2, and 3 elements lose electrons to get a full outer shell and form 1+, 2+, and 3+ ions respectively. Group 5, 6, and 7 elements gain electrons to get a full outer shelld and form 3-, 2-, and 1- ions respectively. Group 4 elements are less reactive than the rest and tend to form covalent bonds.

* A full outer shell is 8 electrons for elements between lithum (3) and calcium (20)
* Metallic atoms have 0 electrons in the outer shell because:
	+ It is easier to lose electrons
	+ An empty shell is ignored
* Noble gases have full outer shells
	+ Helium = 2
	+ Neon = 2 + 8
	+ Argon = 2 + 8 + 8

Electron configurations are written using commas instead of plus signs. Ions have noble gas electron configurations, but their proton number does not change.

> Write `2, 8, 8, 2` for the electron configuration of Ca and `2, 8, 8` for Ca2+
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### Rules of Ionic Bonding

Metallic elements usually have a group number less than 4. However, there are some trends in the periodic table which change the metallic quality of an element. Hydrogen, for example, is not metallic and has a group number of 1. It is also why hydrogen forms a covalent compound with chlorine. There are some rules for ionic compounds.

* The compound must consist of a metal and a non-metal
* The overall or net charge of the compound must be 0
* Noble gases almost never form compounds because they already have a full outer shell

Atoms form ions because they want to get a full outer shell of electrons. Having a full outer shell makes an atom more stable. Algebra skills will help with writing formulas for ionic compounds.

The positively charged ions need to balance with the negatively charged ions. A commonly asked ionic compound is aluminium oxide.

* Al3+ has a charge of +3 while O2- has a charge of -2
* Find the lowest common multiple and multiply both ions or cross them over and simplify later
	+ The lowest common multiple of 2 and 3 is 6
	+ There must be 2 aluminium atoms and 3 oxygen atoms to produce a +6 and -6 charge
* Balance the equation
	+ Oxygen is only stable in its diatomic form
	+ Al + O2 → Al2O3
	+ There must be an equal number of aluminium and oxygen atoms on both sides
	+ 4Al + 3O2 → 2Al2O3

## Covalent Compounds

Covalent compounds are formed when non-metals share electrons. Atoms will always try to get a full outer shell.

* Covalent bonds involve pairs of electrons instead of exchanges
	+ One electron is contributed to the covalent bond by each atom
	+ Hydrogen can only form 1 covalent bond because it only needs 1 more electron to get a full outer shell of 2 electrons
	+ Carbon can form up to 4 covalent bonds because it is in group 4 of the periodic table
* The overall charge does not have to equal 0 unlike ionic compounds
	+ H2O and HCl are covalent compounds that have a neutral charge
	+ OH- is a covalent compound and has a negative charge

Covalent compounds can form ionic bonds with metallic elements. Exams require students to know common covalent compounds.

> H+ is a positive ion that does not form ionic bonds, but H- is an unusual hydride ion that can form ionic compounds like MgH2
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### Common Covalent Compounds

Some covalent compounds like water **H2O**, hydrogen chloride (or hydrochloric acid in aqueous solution) **HCl**, nitrate **SO42-**, and carbonate **CO32-** are commonly seen in exams. There are even more covalent compounds in the table below.

Common Covalent Compounds
* 2 (C : O) Bonds
* 4 + 3(-2) = -2
 Sulphate
* 2 (S : O) Bonds
* 6 + 4(-2) = -2
 Nitrate
* 2 (N : O)
* 5 + 3(-2) = -1