Chemical formulae of compounds
A substance formed by the chemical union of two or more A substance made of one type of atom only. is called a A substance formed by the chemical union of two or more elements.. The chemical A combination of symbols that indicates the chemical composition of a substance. for a compound shows:
- the symbols for each element in the compound
- the number of The smallest part of an element that can exist. of each element in a unit of the compound
Covalent compounds
A bond formed between two atoms, where Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. are shared, is called a A bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons..Compounds, commonly formed between non-metal elements, by sharing electrons between atoms to form covalent bonds. All molecular compounds are covalent. usually form between Element that is a poor conductor of electricity and heat, and which forms acidic oxides. elements and are held together by covalent bonds. All compounds that exist as A collection of two or more atoms held together by chemical bonds. are covalent. Here are some examples:
| Compound | Formula |
| Ammonia | NH3 |
| Carbon dioxide | CO2 |
| Methane | CH4 |
| Sulfur dioxide | SO2 |
| Water | H2O |
| Compound | Ammonia |
|---|---|
| Formula | NH3 |
| Compound | Carbon dioxide |
|---|---|
| Formula | CO2 |
| Compound | Methane |
|---|---|
| Formula | CH4 |
| Compound | Sulfur dioxide |
|---|---|
| Formula | SO2 |
| Compound | Water |
|---|---|
| Formula | H2O |
The subscript numbers in formulae show how many atoms of that element appear in the molecule.
So, the formula NH3 shows that ammonia contains one nitrogen and three hydrogen atoms.
Ionic compounds
A bond formed between two atoms where an electron is transferred from one atom to the other is called an Ionic bonding forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair.. An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). are made up of atoms joined together by ionic bonds. They usually (but not always) contain at least one Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. element and one non-metal element.
Ions
An Electrically charged particle, formed when an atom or molecule gains or loses electrons. is a charged A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. formed when an atom, or a group of atoms, loses or gains electrons. The number and sign of its electrical Electrical charge can be positive or negative. Like charges (+ and + or - and -) repel but unlike charges (+ and -) attract each other. are shown in superscript text.
Names and formulae of some common ions:
| Name of ion | Formula of ion | Electrical charge(s) |
| Sodium | Na+ | One positive |
| Magnesium | Mg2+ | Two positive |
| Chloride | Cl- | One negative |
| Oxide | O2- | Two negative |
| Name of ion | Sodium |
|---|---|
| Formula of ion | Na+ |
| Electrical charge(s) | One positive |
| Name of ion | Magnesium |
|---|---|
| Formula of ion | Mg2+ |
| Electrical charge(s) | Two positive |
| Name of ion | Chloride |
|---|---|
| Formula of ion | Cl- |
| Electrical charge(s) | One negative |
| Name of ion | Oxide |
|---|---|
| Formula of ion | O2- |
| Electrical charge(s) | Two negative |
Simple formulae
The formula of an ionic compound can be predicted by using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.
| Name of compound | Formula | Electrical charges |
| Sodium cloride | NaCl | One positive, one negative |
| Sodium oxide | Na2O | Two positive, two negative |
| Magnesium oxide | MgO | Two positive, two negative |
| Magnesium chloride | MgCl2 | Two positive, two negative |
| Name of compound | Sodium cloride |
|---|---|
| Formula | NaCl |
| Electrical charges | One positive, one negative |
| Name of compound | Sodium oxide |
|---|---|
| Formula | Na2O |
| Electrical charges | Two positive, two negative |
| Name of compound | Magnesium oxide |
|---|---|
| Formula | MgO |
| Electrical charges | Two positive, two negative |
| Name of compound | Magnesium chloride |
|---|---|
| Formula | MgCl2 |
| Electrical charges | Two positive, two negative |
More guides on this topic
- Hazards and risks - Edexcel
- Atomic structure - Edexcel
- The periodic table - Edexcel
- Ionic compounds - Edexcel
- Simple molecular substances - Edexcel
- Giant covalent substances - Edexcel
- Metals and non-metals - Edexcel
- Calculations for all students - Edexcel
- Higher tier only calculations - Edexcel
- Sample exam questions - key concepts in chemistry - Edexcel