Atomic Theory
Democritus
• 440 B.C.E• All matter is made up of atoms,
which are indivisible• he pounded up materials in his pestle
and mortar until he had reduced them to smaller and smaller particles which he called
ATOMOSATOMOS (Greek for indivisible)
Dalton•In the early 1800’s, the
English chemist John Dalton did a number of experiments that eventually led to the acceptance of the idea of atoms.
Dalton’s Atomic Theory•All elements are composed
of atoms. Atoms are solid small spheres and indivisible particles.
•BILLIARD BALL MODEL
continued...
•Atoms of the same element are exactly alike.
H H
continued...
•Atoms of different elements are different.
O H
continued...
•Compounds are formed by the joining of atoms of two or more elements.
H H O
Was he right?•Dalton’s atomic theory of
matter became one of the foundations of chemistry.
•BUT it didn’t explain attraction of repulsion of objects.
J.J.Thomson•In 1897, the work of an English
scientist J. J. Thompson provided the first hint that atoms were made up of even smaller particles
•Atoms are divisible…Dalton was wrong!
Thompson’s Model•He stated that atoms are
made up of positively (+) and negativelynegatively (-) charged particles
And...•He thought the positive (+)
and negative (-)negative (-) particles were s p r e a d o u t in atoms likeplums in pudding or raisins in a bun.
Questions...•What kind of particles did Thompson find in atoms?
•What did his idea of an atom look like?
Was Thomson Right?•Not really…there are positively (+) and negativelynegatively (-) charged particles inside atoms.
•BUT...
Rutherford’s Model•In 1908, Rutherford discovered
the nucleus.•He stated that atoms have a
small, dense, positively (+) charged center called a nucleus.
.
Alpha Particle ScatteringThe Nuclear Atom 26/04/23
NextBack
An atom
Alpha Source
+
Rutherford’s ModelThe Nuclear Atom 26/04/23
NextBack
He suggested that all of the atom’s positive charge, together with most of its mass, is concentrated in the centre.
Alpha particles which travel close to the nucleus are strongly deflected. The degree of deflection depends on how close it approaches.
Rutherford’s ModelThe Nuclear Atom 26/04/23
NextBack
The nucleus must be very small in comparison to the atom.
This will account for the vast majority making it through unaffected.
So...•Particles are not spread out like
plums in pudding…•Positive particles (protons) are
in the center (nucleus).•Where are the negativenegative particles (electrons)?
Bohr’s Model•In 1913, Niels Bohr
proposed an improvement to Rutherford’s Model:
•Electron’s move in definite orbits around the nucleus, like planets around the sun.
+
NextBack
Are we there yet?•Bohr’s model was used for a
long time…
•BUT...
Modern Atomic Model
•An atom has a small, positively charged nucleus surrounded by a large region in which there are enough electrons (-) to make the atom neutral (0).
P
E
E
E
E
E
E
E
E
E
E
E
PPPPPPPP P
PP
E
Bohr’s Atom
electrons in orbits
nucleus
HELIUM ATOM
+N
N
+-
-
proton
electron
neutron
Shell
What do these particles consist of?
ATOMIC STRUCTUREATOMIC STRUCTURE
Particle
proton
neutron
electron
Charge
+ ve charge
-ve charge
No charge
1
1
nil
Mass
ATOMIC STRUCTUREATOMIC STRUCTURE
the number of protons in an atom
the number of protons and neutrons in an atom
HHee
22
44 Atomic mass
Atomic number
number of electrons = number of protons
HeliumChemical Name
Chemical Symbol
ATOMIC STRUCTUREATOMIC STRUCTURE
Electrons are arranged in Energy Levels
or Shells around the nucleus of an atom.
• first shell a maximum of 2 electrons
• second shell a maximum of 8
electrons
• third shell a maximum of 8
electrons
• forth shell a maximum of 8
electrons
ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION
With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example;
NNitrogen
7
14
2 in 1st shell 5 in 2nd shell
configuration = 2 , 5
2 + 5 = 7
ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION
Write the electronic configuration for the following elements;
Ca O
Cl Si
Na20
40
11
23
8
17
16
35
14
28B 11
5
a) b) c)
d) e) f)
2,8,8,2 2,8,1
2,8,7 2,8,4 2,3
2,6
Bohr Rutherford DiagramsBohr Rutherford Diagrams
With Bohr Rutherford diagrams, elements and compounds are represented by Dots to show electrons, and circles to show the shells. For example;
Nitrogen N OO O
O
OO
O N 7
14
Bohr Rutherford DiagramsBohr Rutherford Diagrams
Draw the Bohr Rutherford Diagrams for the following elements;
O Cl8 17
16 35a) b)
O
OO
O
O
O
O
O
O
Cl
O
O
O
O O
OO
O
O
O
O
O
O
OO
O
O
O
SUMMARYSUMMARY1. The Atomic Number of an atom = number of protons in the nucleus.
2. The Atomic Mass of an atom = number of Protons + Neutrons in the nucleus.
3. The number of Protons = Number of Electrons.
4. Electrons orbit the nucleus in shells.
5. Each shell can only carry a set number of electrons.
Top Related