Notes 3. Atoms and Molecules - Ions And Atoms | Class 9 Science - Toppers Study

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: