Ions, Formula Unit Mass & Ionic Compounds
Formula Unit Mass
Na₂O = (2 × 23 u) + (1 × 16 u) = 62 u��Ca(NO₃)₂ = (1 × 40 u) + (2 × 14 u) + (6 × 16 u) = 164 u
Ion
An ion is a charged particle and can be negatively or positively charged.��Cations → positive ions�Anions → negative ions
Important facts about ions
• Ions may consist of a single charged atom or a group of atoms having a net charge.�• Ionic compounds contain charged species called ions as their smallest unit.�• A group of atoms carrying a fixed charge is called a polyatomic ion or radical.�• The chemical formula of a compound is a symbolic representation of its composition.
Names and Symbols of Some Ions — Cations
Valency | Name of ion | Symbol |
1 | Sodium | Na⁺ |
1 | Potassium | K⁺ |
1 | Silver | Ag⁺ |
1 | Copper (I) | Cu⁺ |
2 | Magnesium | Mg²⁺ |
2 | Calcium | Ca²⁺ |
2 | Zinc | Zn²⁺ |
2 | Iron (II) | Fe²⁺ |
2 | Copper (II) | Cu²⁺ |
3 | Aluminium | Al³⁺ |
3 | Iron (III) | Fe³⁺ |
* Some elements show variable valency, represented by Roman numerals in brackets.
Names and Symbols of Some Ions — Anions
Valency | Non-metallic element | Symbol | Polyatomic ions | Symbol |
1 | Hydrogen | H⁺ | Ammonium | NH₄⁺ |
1 | Hydride | H⁻ | Hydroxide | OH⁻ |
1 | Chloride | Cl⁻ | Nitrate | NO₃⁻ |
1 | Bromide | Br⁻ | Hydrogen carbonate | HCO₃⁻ |
1 | Iodide | I⁻ | | |
2 | Oxide | O²⁻ | Carbonate | CO₃²⁻ |
2 | Sulphide | S²⁻ | Sulphite | SO₃²⁻ |
2 | | | Sulphate | SO₄²⁻ |
3 | Nitride | N³⁻ | Phosphate | PO₄³⁻ |
Note: Polyatomic ions are groups of atoms carrying a fixed charge.
Rules for Writing the Formula of a Compound
1
The formula of a compound is written by writing side by side the symbols of the constituent elements.
2
The symbol of the more metallic element is written first in the formula.
3
The number of atoms of each constituent element is indicated by a subscript.
4
When either or both ions are polyatomic and their valency is more than one, enclose the polyatomic ion in brackets. No brackets are necessary if the valency is 1.
5
If the valencies have a Highest Common Factor (H.C.F.), divide the valencies by the H.C.F. to get the simplest ratio.
A compound is electrically neutral: total positive and negative charges must balance to zero.
Valency, Criss-Cross Method & Key Points
Criss-Cross Method
• The charges/valencies on ions must be balanced.��• For a binary compound, cross the valencies of the elements to obtain subscripts.��Example: Mg²⁺ and Cl⁻ → MgCl₂��Example: Al³⁺ and O²⁻ → Al₂O₃
Important Points
• A chemical compound is always electrically neutral.�• Positive and negative charges add up to zero.�• Use the simplest whole-number ratio.�• Relative molecular mass is expressed in atomic mass unit (u).�• Atomic masses are compared using the C-12 standard.
Atoms combine with one another to form chemical bonds.
The main types of chemical bonds at this level are ionic (electrovalent) bonds and covalent bonds.
Key Words
Atom
Smallest particle of an element that shows all the properties of an element.
Radical
An atom or group of atoms carrying a positive or negative charge that behaves as a single unit in a chemical reaction.
Binary compound
A simple compound made up of two different elements. Examples: HCl, H₂O.
Gram atomic mass
Atomic mass of an element expressed in terms of grams.
Atoms and Molecules — Difference
Basis | Atom | Molecule |
Definition | Smallest particle of an element that may or may not exist independently. | Smallest particle of a substance that can exist independently. |
Existence | May or may not exist freely (except noble gases). | Exists independently. |
Composition | Single particle. | Two or more atoms chemically combined. |
Example | Helium (He), sodium (Na). | Water (H₂O), oxygen (O₂). |
Atoms are the basic building blocks; molecules are groups of atoms chemically bonded together.