Atoms are the basic building blocks of all matter. John Dalton proposed that atoms are indivisible and identical for each element. Rutherford discovered that atoms have a small, dense nucleus at the center surrounded by electrons. Niels Bohr modeled atoms with electrons orbiting the nucleus in fixed energy levels or shells. The nucleus contains protons and neutrons, while electrons orbit outside. The number of protons determines the element, and protons and neutrons together equal the mass number.

2. PREPARED BY
LEKSHMI.S.G
PHYSICAL SCIENCE
REG NO: 13357003
BNVCTE,THIRUVALLAM

4. All matter is made up of atoms.
Atoms of an element are
identical.
Each element has different
atoms.
Atoms can engage in a chemical
reactions.
Atoms can neither be created
nor be destroyed.
Atoms are indivisible.
Atomic Theory
John Dalton
(1776-1884)

5. atom has a positively charged
central part (nucleus)
Electrons are distributed
around nucleus
Mass of an atom is
concentrated at nucleus.
Compared with total volume
of an atom, the volume of
nucleus is meager
Rutherford’s Atomic
Model
Ernest Rutherford
(1871-1937)

6. Solar system Rutherford‘s atom model
Planetary model of atom

7. Bohr's Model of the Atom
Niels Bohr
1913

8. Bohr's Model of the Atom
 electrons orbit the nucleus like planets
orbit the sun

9. Bohr's Model of the Atom
 electrons fills the orbits closest to the
nucleus
e.g. fluorine:
#P = 9
#e-
= 9
#N = 10
9P
10N

10.  each orbit can hold a specific maximum
number of electrons
Shell maximum no: of
electrons
1 - K
2 - L
3 - M
4 - N
Bohr's Model of the Atom
2
8
18
32

11. Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels

12. What is the structure of an atom?
• Nucleus - center of the atom
Home of Protons and Neutrons
Has a positive charge
• Proton
Has a relative mass of 1
Has a positive (+) charge
Determines the atomic number
Found inside the nucleus

13. What is the structure of an atom?
• Neutron
Has no (0) charge
Has a relative mass of 1
Found inside the nucleus

14. What is the structure of an atom?
• Electron
Has a negative (-) charge
Found outside the nucleus
• Rutherford atom model - electrons are
around the nucleus
• Bohr model – electrons are in specific
energy levels called shells

15. How are p, n, e related?
• No: protons = No: electrons
• No: protons = atomic number
• No: protons + No:neutrons = mass
number
• No: neutrons=mass no: - atomic no:

16. ATOMIC STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and
neutrons in an atom
HeHe
22
44Atomic mass
Atomic number
number of electrons = number of protons