Automic structure

Atomic Structure

The structure of an atom

The smallest particle of matter is called as an atom.

Electrons, Protons and neutrons are the fundamental particles of the atom.

+- N

Electron Proton Neutron


Protons and neutrons ,the fundamental particles of the atom are present in the nucleus of an atom while the electrons revolve around the nucleus in the outer orbit. They revolve in different orbits.



In every atom number of protons and electrons in is equal. That is, positive and negative charges are equal, and hence atom as a whole is electrically neutral.

Proton

Electron


After studying the structure of an atom John Dalton, J.J. Thomson and Earnest Rutherford put forth their theories.

- Matter is made of small particles. - These particles are called atoms. - Atom is like a hard solid ball - Atom is indivisible

- - Does not propose anything about the p positive and negative charges on an a atom.

John Dalton

(1808)


J. J. Thomson

(1897)

-Compared an atom to a watermelon.

- Atom has a positively charged part like the red part of the watermelon and in it are embedded like the seeds, negatively charged particles called electrons. -Proved the existence of negatively charged particles ,electrons with an experiment. Awarded the noble prize in the year 1906 for the same.


Earnest Rutherford wanted to know about the incomplete theory of Thomson . So he bombarded positive charged alpha rays on a thin gold sheet. This experiment is called as Rutherford's scattering experiment.


In this experiment Rutherford made following observations :

- Most of the alpha particles passed straight through the sheet without any obstacles.

- Some of the alpha particles turned back from the gold sheet.

Based on this experiment Rutherford made following inferences:

- The fact that most of the alpha particles pass through gold sheet means that the atom consists mainly of empty space.

- The part from which the positively charged particles are turned back is positively charged but very small in size as compared to the empty space .


From these inferences, Rutherford proposed his

theory of structure of atoms:- The nucleus at the centre of the atom has the positive charge. Most of the mass of the atom is concentrated in the nucleus.

- The negatively charged electrons revolve around the nucleus in specific orbits.

- In comparison with the size of the atom, the nucleus is

very very small.


From all these studies it is observed that structure of the

atom is same as the structure of the solar system. Centralley located protons and neutrons representing Sun whereas electrons revolving around the centre resembles planets.

+N

N

+-

-

proton

electron

neutron

Shell


Atomic number- The number of electrons or protons in an atom is called it’s Atomic number.

- Every element has a different atomic number.- All the atoms of certain element has same number of electrons/protons.


Element Atomic No. Figure

Hydrogen 1

Helium 2

Carbon 6

Sodium 11

Chlorine 17


The mass of an atom is concentrated in its nucleus. Atomic mass number (A) is equal to the sum of the number of protons (p) and neutrons (n) in the nucleus.

Atomic mass number = No. of Protons + No. of Neutrons


Isotopes : In case of certain elements, some atoms have same atomic number but a different atomic mass number.Such atoms of an elements are called isotopes of that element.


Applications of an isotopes

- Isotopes of uranium are used as a fuel in atomic reactors. - Isotopes of cobalt are used in the treatment of cancer. - Isotopes of iodine are used in the treatment of goitre.


In a chemical reaction different chemical substances are formed. During this reaction some element's atoms give their electrons to another element. That means some elements tend to give electrons and some elements tend to receive electrons.


The atom which donates electron loses it's negative electric charge. This atom has more positive charge.The atom which accepts electron has more negative charge .These positively and negatively charged atoms are called as ions.


Every element has a definite capacity for combining with other elements. This capacity is called Valency of that element.

Element Valency

Sodium 1Chlorine 1 Oxygen 2

Aluminium 3Carbon 4


When atoms of different elements combine with each other then molecules of compounds are formed.
