The document discusses the structure of an atom. It explains that an atom contains protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit the nucleus. John Dalton, J.J. Thomson, and Ernest Rutherford contributed theories to our understanding of atomic structure through experiments. Rutherford determined that the nucleus is very small and contains a positive charge, with electrons orbiting the outside. The number of protons defines the atomic number of an element.

1. Atomic Structure

2. The structure of an atom
The smallest particle of matter is called as an
atom.

3. Electrons, Protons and neutrons are the
fundamental particles of the atom.
+- N
Electron Proton Neutron
The structure of an atom

4. 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.
The structure of an atom

5. The structure of an atom
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

6. The structure of an atom
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)

7. The structure of an atom
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.

8. The structure of an atom
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.

9. The structure of an atom
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.

10. 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 .
The structure of an atom

11. 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.
The structure of an atom

12. 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
The structure of an atom

13. 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.
The structure of an atom

14. Element Atomic No. Figure
Hydrogen 1
Helium 2
Carbon 6
Sodium 11
Chlorine 17
The structure of an atom

15. 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
The structure of an atom

16. 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.
The structure of an atom

17. 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.
The structure of an atom

18. 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 structure of an atom

19. 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.
The structure of an atom

20. Every element has a definite capacity for
combining with other elements. This capacity is
called Valency of that element.
Element Valency
Sodium 1
Chlorine 1
Oxygen 2
Aluminium 3
Carbon 4
The structure of an atom

21. When atoms of different elements combine
with each other then molecules of
compounds are formed.
The structure of an atom