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3. Atoms and Molecules

Ions: The charged atoms are known as ions. 

Ions have two types of charge.

(1) Anion : A negatively charged ion is called an ‘anion’.

   Eg: Cl^-, O^-2, S^2-, N^3-, Br^-, H^- etc. 

These all atoms have negative charged so they are ions with anoin. 

Note: All non-metals form anoin. 

(2) Cation : The positively charged ion is called a ‘cation’.

   Eg. Na⁺, K⁺, Ag⁺, Cu⁺, Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺, Al³⁺ etc. 

These all atoms have positive charged so they are ions with cation.  

Note: All metals form cation.

For example, sodium chloride (NaCl). Its constituent particles are positively
charged sodium ions (Na+) and negatively charged chloride ions (Cl–). Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

Polyatomic ion: A group of atoms carrying a charge is known as a polyatomic ion.

Valency: The combining power (or capacity) of an element is known as its valency.

Writing Chemical Formulae

The chemical formula of a compound is a symbolic representation of its composition.

First many questions might have risen in the mind.

(i) How are atoms charged? 

(ii) How different aions have different charges like -, 2-, 3- or +, 2+, and 3+ etc ?

(iii) What is combining power or capacity of an element ? 

(iv) In waht manner compounds form? 

For understanding this ion and its type anion and cation as well as valancy we need to learn more about the structure of atom before learning writing chemical formula.

Structure of atom:

Always rememer some points these are as follow:

• An atom has three non-atomic particles i.e electrons, protons and neutrons.
• All electrons moves around the nucleus of an atom in different shells. 
• Protons and neutrons are present in nucleus of an atom. 
• All electrons have same negative charge and no mass. 
• All protons have same positive charge and mass as 1 unit. 
• Neutrons have no charge but have mass as 1 unit.
• The numbers of electrons and protons are equal in an atom.
• The numbers of neutrons may be greater, equal or less than protons. 
• The number of protons present in nucleus of an atom is also calculated as atomic number of the same atom. 
• The sum of protons and neutrons present in nucleus of an atom is calculated as atomic mass of the same atom. 
• Hydrogen has no any neutron in its nucleus. There is only 1 proton.
• Each orbit is also known as shell and they are called energy levels.
• These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

Distribution of electrons in different shells or orbits.

The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index, 1,2,3,….

For example:

It means the first shell can have only maximum 2 electrons, second can have maximum 8 electrons, third shell can have maximum 18 electrons and forth shell can have maximum 32 electrons. 

See the table:

3. Atoms and Molecules

The rules that you have to follow while writing a chemical formula are as follows:

(i) The valencies or charges on the ion must balance.

(ii) When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc.

(iii) Where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

(iv) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

Binary Compounds: The simplest compounds, which are made up of two different elements are called binary compounds.

Writing Formula of Simple Compounds: 

While writing the chemical formulae for compounds, we write the constituent
elements and their valencies as shown below. Then we must crossover the valencies of the combining atoms.

There are two non-metals which participate in molecule

Example 1: 

Formula of Hydrogen Compound: 

Therefore formula: HCl

Example 2: 

Formula of Hydrogen Sulphide: 

Therefore formula:H₂O

Example 3: 

Formula of Carbon Tetrachloride: 

Therefore formula: CCl₄

Formula of Ionic Compound: When the First element is Metal which forms cation (+) and Second element is non-metal which form anion (-).  

Formula of Magnesium Chloride:  

For finding the the formula of Magnesium Chloride, we write symbol of cation with element like this (Mg⁺), After this we write the symbol of anion of (Cl^-). Then we crossover the charges we obtain formula.

Example 4:

Therefore formula: MgCl₂

Example 5:

Formula of Alumunium Oxide: 

Therefore formula:Al₂O₃

The Compound of Polyatomic Ions: 

Example 6:

Formula of Sodium Nitrate: 

Therefore formula: NaNO₃

Example 6:

Therefore formula: (NH₄)₂SO₄

Molecular Mass: 

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. E.g- Molecular Mass of water(H₂O).

Atomic mass of Hydrogen = 1 u 

Atomic mass of  Oxygen = 16 u 

There are two atoms of hydrogen and one atom of oxygen in a water molecule.  

∴ Molecular mass of water = 2 x 1 + 16 = 18 u 

It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Formula Unit Mass: 

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. 

Example - 

NaCl, CaCl₂, Al₂O₃, MgCl₂ etc.  

Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

• The amount of substances can be calculated by its mass or by the number of atoms.

Mole: 

Mole is the amount of substance in which the number of particles of any species (atom, molecules, ions or particles) are equal to the atoms present in carbon-12 in 12 gram. This amount of species is known as mole.  

Mole is a heap of many atoms. In which numbers of atoms, molecules or ions of any element is equal to 6.022 x 10²³​. 

Avogadro Constant or Avogadro Number: The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 10²³​. This is an experimentally obtained value. This number is called the Avogadro Constant or Avogadro Number (represented by N0).

Example:  

See the table and try to understand the following things: 

- There is 1 mole in 1 g of hydrogen atoms and 6.022 x 10²³ atoms in that of 1 mole.   

-- There is 1 mole in 12 g of Carbon atoms and 6.022 x 10²³ atoms in that of 1 mole. 

-- There is 1 mole in 16 g of Oxygen atoms and 6.022 x 10²³​  atoms in that of 1 mole.  

Similarily you can find the mole of every element and you can also find the number of atoms in this mole. 

Now understand this: 

-- There is 1 atom in 1 u of hydrogen. 

-- There is 1 atom in 12 u of Carbon. 

-- There is 1 atom in 14 u of Nitrogen.

-- There is 1 atom in 16 u of Oxygen.  

Similarly, you can find atom of all elements.

Molar mass : The Mass of one mole of atoms of that element is known as  molar mass.  Molar mass of atoms is also known as gram atomic mass.

For example:

Atomic mass of hydrogen=1u.

So, gram atomic mass of hydrogen = 1 g. and 1 g of hydrogen has 1 mole. 1 mole of hydrogen has  6.022 x 10²³​ atoms.  

Similarly; The molar mass of carbon is 12 g.

Molar mass of 1 mole of oxygen is 16 g. 

Molar mass of 1 mole of sodium is 23 g. 

Similarily: 

In 1 u of Hydrogen has only 1 Hydrogen atom.  

In 4 u of Helium has only 1 Helium atom. 

In 12 u of Carbon has only 1 Carbon atom. 

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