LAXMIBAI BHAURAO PATIL HIGH SCHOOL AHMEDNAGAR The structure

LAXMIBAI BHAURAO PATIL HIGH SCHOOL, AHMEDNAGAR The structure of an atom • MR. SINARE JALINDAR

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The structure of an atom 1. Dalton’s atomic theory: Dalton proposed that matter is made of very small particles called atoms. An atom is a hard and solid ball and indivisible. Hard and solid sphere

Drawbacks of Dalton’s atomic theory Dalton’s theory does not mention anything about the positive and negative charges on an atom. Hence , many of the properties of a substance can not be explained.

Thomson’s atomic model Thomson proposed that the atom is a homogeneous sphere of positive charge in which electrons are embedded. Electrons are negative charged particles. As the positive and negative charges are equal, the atom as a whole does not have any charge.

Rutherford’s atomic model • Postulates of Rutherford’s atomic theory: 1 The centre of the atom is called the nucleus and it has positive charge. 2 Most of the mass of the atom is concentrated in the nucleus. 3 Electrons having negetive charge , they revolve around the nucleus in the specific orbit.

Fundamental particles in an atom • 1 Proton: - Positively charged particle and denoted by symbol p+ • It is located in the nucleus of an atom. • 2 Neutron: - Neutron is electrically neutral and denoted by symbol n. It is located in the nucleus of an atom. • 3 Electron: - Negatively charged particles and denoted by symbol e- It revolve around the nucleus of an atom in specific orbits.

Atomic number • Definiation: - The number of protons or electrons present in an atom. • It is denoted by Z. • Ex. Atomic number of sodium 11

Atomic mass number • Definiation: - The sum of the number of protons and neutrons in the nucleus of an atom. • It is denoted by symbol A

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