Atoms and Molecuels

Atoms or molecules having any charge over them are called ions.

Compounds of composed of metals and non metals contains charged species. These charged species are known as ions.

Example: Sodium Chloride (NaCl)

This (sodium chloride) is a compound formed by the combination of one metal, i.e. sodium and other non metal, i.e. chlorine atoms.

In the compound of sodium chloride, sodium and chlorine are charged species and are called ions.

Sodium ion have positive charge over it and chloride ion has negative charge over it.

Potassium chloride (KCl)

This (Potassium chloride) is a compound. This compound has two atoms: Potassium and chloride. Potassium is a metal and chlorine is a non metal. And potassium and chloride contains charges, and thus are called ions.

Types of Ions

Ions can be divided into two types on the basis of chare over them : Positive ions and Negative ions.

Positive Ions

Particles having positive charge are called POSITIVE IONS. Positive ions are called "Cation" also.

Example: Sodium ion (Na⁺). Since sodium ion has one positive charge over it, thus sodium ion is called positive ion or Cation.

Potassium ion (K⁺). Since Potassium ion has one positive charge over it, thus Potassium ion is called positive ion or Cation.

Hydrogen ion (H⁺). Since Hydrogen ion has one positive charge over it, thus Hydrogen ion is called positive ion or Cation.

Magnesium ion (Mg^{+ +} ). Since Magnesium ion has one positive charge over it, thus Magnesium ion is called positive ion or Cation.

Negative Ion

Particles having negative charge over them are called Negative Ions. Negative ions are known as "Anion" also.

Example: Chloride ion, Fluoride ion, Iodide ion, Sulphate ion, etc.

Chloride ion (Cl^–). Since Chloride ion has one negative charge over it, thus Chloride ion is called negative ion or Anion.

Fluoride ion (F^–). Since Fluoride ion has one negative charge over it, thus Fluoride ion is called negative ion or Anion.

Bromide ion (Br^–). Since Bromide ion has one negative charge over it, thus Fluoride ion is called negative ion or Anion.

Monoatomic Ion and Polyatomic Ion

Ions may consist of single charged atom or a group of atoms that have a net charge on them.

Ions having single charged atom are called Monoatomic Ions.

Example: Sodium ion (Na⁺), Calcium ion (Ca²⁺), Potassium ion (K⁺), etc. are examples of monoatomic ions.

In sodium ion, there is only one atom. And thus this sodium ion is called Monoatomic ion

Potassium ion (K⁺), Calcium ion (Ca⁺⁺), Hydrogen ion (H⁺), Lithium ion (Li⁺), etc. are some examples of monoatomic ions.

A group of atoms carrying a charge is known as Polyatomic Ion.

Example: Sulphate ion (SO^2–). There are two atoms in sulphate ion, i.e. sulphur and oxygen. And hence sulphate ion is called polyatomic ion.

Polyatomic = Poly + Atomic. Here "Poly" means "many (more than one)" and "atomic" means "of atom". This means substance having more than one atom. Clearly polyatomic ion means ions having more than one atoms.

Carbonate ion (CO^–), Nitrate ion (NO_–3), Phosphate ion (PO_–2), etc. are some examples of polyatomic ion.

Valency

Definition of Valency

The combining capacity of an atom is known its VALENCY. In other words, VALENCY is the Combining Capacity of an atom.

Valency can be negative or positive.

Example: Valency of Sodium (Na) = +1

Valency of potassium (K) = +1

Valency of Chlorine (Cl)= – 1, etc.

How to know the valency of an atom?

There is no need to remember valency of atoms. Valency can be calculated easily. Only we have to remember the valency of Hydrogen atom.

The valency of Hydrogen atom = +1

A compound is electrically neutral.

This means that total charge over a compound is equal to zero.

Points to be remembered while calculating the valency.

(a) Valency of atom of Hydrogen is equal to 1.

(b) Valency of atom of element which is written at the left hand side in a compound is considered as positive.

(c) Valency of atom of an element which is written at the right hand side in a compound is considered as negative.

(d) All compounds are electrically neutral and hence the sum of their valency is equal to zero.

Calculate the valency of Oxygen atom in water molecule (H₂O)

The chemical formula of water: H₂O

Here, we know that, valency of Hydrogen atom = +1

Now, Let valency of Oxygen (O) atom `=x`

Since, the total charge over a compound = 0

This means that, total charge over H₂O (water) = 0

Thus,

`H_2O`

`1xx2 + x = 0`

[∵ valency of hydrogen atom = +1 and we assume the valency of oxygen `=x`]

`=>2+x=0`

`=>x = -2`

Thus, valency of Oxygen atom `=-2`

Calculate valency of Chlorine in Hydrochloric Acid (HCl)

Here, we know that, valency of hydrogen = +1

Now, Let, valency of `Cl = x`

And we know that, total valency, i.e. charge over HCl = 0

Thus, `1+x=0`

`=>x = -1`

Thus, valency of chlorine (Cl) in HCl `=-1`

Calculate the valency of Potassium (K) in KCl (Potassium Chloride)

Here, chemical formula of Potassium chloride = KCl

We know that, valency of Cl (chlorine) in HCl `=-1` [As calculated in the case of HCl]

Let, valency of Potassium (K) `=x`

Now, since, total charge over KCl = 0

`:. x + (-1)=0`

`=>x-1=0`

`=> x = 1`

Thus, valency of Potassium (K) = 1

Calculate the valency of Carbon (C) in carbon dioxide (CO₂)

We know that valency of Oxygen `=-2` as calculated in the case of H₂O

Let, valency of carbon (C) `=x`

∵ valency of `CO_2 = 0`

`:. x + (-2xx2)=0`

`=>x +(-4)=0`

`=>x-4=0`

`=>x = +4`

Thus, valency of carbon in Carbon dioxide (CO₂) = +4

Calculate the valency of Mg (Magnesium) in MgCl₂

Solution:

You know the valency of Cl is –1 as calculated in HCl.

Let the valency of Mg in MgCl₂ is = m

Now, we know that total charge or valency of a compound = 0

Therefore,

Valency of Mg + Valency of Cl × 2 = 0

⇒ m + (–1 × 2) = 0

⇒ m – 2 = 0

⇒ m = 2

Hence the valency of Mg is equal to –2

Calculate the valency of sulphate (SO₄) in Sodium sulphate (Na₂SO₄)

Solution:

We know that the valency of Na is +1 as calculated in the case of NaCl.

Let the valency of Sulphate (SO₄) in Na₂SO₄ `= m`

We know that valency or charge over a compound = 0

Therefore,

Valency of Na × 2 + Valency of Sulphate = 0

⇒ (1×2)+m = 0

⇒ 2+m = 0

⇒ m = –2

Hence the valency of Sulphate (SO₄) in Na₂SO₄ = –2

Find the valency of calcium (Ca) in Calcium carbonate (CaCl₂)

Solution:

We know that the valency of chlorine (Cl) is -1 as calculated in the case of NaCl.

Let the valency of Calcium (Ca) in CaCl₂ `= m`

We know that valency or charge over a compound = 0

Therefore,

Valency of Cacium + Valency of chloride = 0

⇒ m+ (–1 × 2)= 0

⇒ m – 2 = 0

⇒ m = 2

Hence the valency of Calcium (Ca) in CaCl₂ = 2

Find the valency of carbonate in calcium carbonate (CaCO₃)

Soltuion:

We know that valency of calcium = 2 [as calculated in the case of calcium chloride (CaCl₂) ]

Let valency of carbonate (CO₃) `=x`

We know that a compound is electrically neutral, thus

Valency of calcium + valency of carbonate = 0

`=>2+x=0`

`=>x=-2`

Thus, valency of carbonate (CO₃) = –2

Valency of ions

Charge over an ion shows its valency. This means valency of an ion is equal to charge over it.

Thus, valency of sodium ion (Na⁺) = +1 [Since it has +1 charge over it]

Valency of potassium ion (K⁺) = +1 [Since it has +1 charge over it]

Valency of Calcium ion (Ca⁺⁺) = +2 [Since it has +2 charge over it]

Valency of Magnesium ion (Mg⁺⁺) = +2 [Since it has +2 charge over it]

Valency of Copper ion (Cu⁺⁺) = +2 [Since it has +2 charge over it]

Valency of Aluminium ion (Al⁺⁺⁺) = +3 [Since it has +3 charge over it]

Valency of Chloride ion (Cl^–) = –1 [Since it has –1 charge over it]

Valency of Bromide ion (Br^–) = –1 [Since it has –1 charge over it]

Valency of Carbonate ion [CO₃^–2] = –2 [Since it has –2 charge over it]

Valency of Oxide ion [O^2––2] = –2 [Since it has –2 charge over it]

Names, Symbols and Valency of some of the ions