elements When identical atoms combine the resultant substance is called an element. 

• They cannot be separated by physical or chemical methods. 

Compound:- 

• When two or more different elements combine or react with one another the resultant substance is called a compound. 

• They cannot be separated by physical methods but they can be separated by chemical methods. 

Mixtures:- 

• When two or more atoms or compounds or elements are mixed it is called a mixture. 

• They can be separated by physical and chemical methods. 

Structure of an atom:- 

• The basic unit or smallest particle of an element is an atom. 

• Atoms cannot survive independently. 

• ​The center of an atom is called as the nucleus  

• The nucleus is surrounded by an Orbit. 

Nucleus:-  

• The nucleus is divided into two parts. They are: 

1. Positively charged particles – protons. The protons can be represented with P or + 

2. Neutrally charged or particles with no charge- Neutrons. The neutrons are represented by N. 

Classification of elements:- 

They are classified into 4 types. They are: 

1.  Metals  

2. Non- metals 

3. Metalloids and  

4. Nobel gases. ​

Metalloids:-  

• They are the elements that have the properties of both metals and non-metals. ​

​Nobel gases:- 

• They are also called inert gases or non-reactive gases or trace elements. 

• They are present in very little amount or very little. 

• Helium, Krypton, Argon, Radon, Neon. 

Elements- symbol:- 

• It indicates the atom by which an element is made up of. 

• It is an abbreviated form  

• The symbol is very specific to the element ​

2. Symbols:- 

• It was proposed by Berzelius. 

• He first proposed that we will use the first letter of the element as the symbol. 

1. Hydrogen – H 

2. Helium- H 

3. Carbon – C 

4. Copper- C 

This method was not approved as the symbols were repeating. ​

• He then proposed that we can use the first two as symbols. 

1. Helium – He 

2. Copper- Co 

3. Cobalt – Co 

This method was approved to a certain extent. 

3. Symbols derivate from Latin names:- 

1. Copper- Cuprum- Cu 

2. Sodium- Natrium- Na 

3. Lead- Plumbum-Pb ​