Transcript Atomic Structure PPT

ATOMIC
STRUCTURE
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
ATOMA
(greek for indivisible)
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
Democritus’ Theory
1. All matter is composed of atoms, which are bits
of matter too small to be seen. These cannot
be further split into smaller portions.
2. There is a void, which is empty space between
atoms.
3. Atoms are completely solid.
4. Atoms are homogenous, with no internal
structure.
5. Atoms are different in their: sizes, shapes and
weight.
HISTORY OF THE ATOM
1808
John Dalton
ATOMS
suggested that all matter was made up of tiny
spheres that were able to bounce around with
perfect elasticity and called them
16 X
+
8Y
8 X2Y
Law of Conservation of Mass
2.1
HISTORY OF THE ATOM
1898
Joseph John Thompson
ELECTRON
found that atoms could sometimes eject a far
smaller negative particle which he called an
1904
HISTORY OF THE ATOM
Thompson develops the idea that an atom was made up of electrons
scattered unevenly within an elastic sphere surrounded by a soup of
positive charge to balance the electron's negative charge like plums
surrounded by
pudding.
PLUM PUDDING
MODEL
2.2
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
2.2
Measured mass of e(1923 Nobel Prize in
Physics)
e- charge = -1.60 x 10-19 C
Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
2.2
HISTORY OF THE ATOM
1910
Ernest Rutherford
oversaw Geiger and Marsden carrying out his
famous experiment.
they fired Helium nuclei at a piece of gold foil
which was only a few atoms thick.
they found that although most of them passed
through. About 1 in 10,000 hit the detector
plate.
HISTORY OF THE ATOM
helium nuclei
gold foil
helium nuclei
They found that while most of the helium nuclei passed
through the foil, a small number were deflected and, to their
surprise, some helium nuclei bounced straight back.
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
2.2
HISTORY OF THE ATOM
Rutherford’s new evidence allowed him to propose a more
detailed model with a central nucleus.
He suggested that the positive charge was all in a central
nucleus. With this holding the electrons in place by electrical
attraction
However, this was not the end of the story.
Because there was a problem….
It was known that H contains only 1
proton, He contains 2. So, the ratio of
mass must be H:He=1:2. However, the
ratio is 1:4!!
Rutherford said there must be another
subatomic particle…. But, unfortunately
he couldn’t find what it was…
HISTORY OF THE ATOM
1913
Niels Bohr
studied under Rutherford at the Victoria
University in Manchester.
Bohr refined Rutherford's idea by adding
that the electrons were in orbits. Rather
like planets orbiting the sun. With each
orbit only able to contain a set number of
electrons.
Bohr’s Atom
nucleus
electrons in orbits
ATOMIC 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 electron
•
third shell
a maximum of 18 electrons
•
fourth shell
a maximum of 32 electrons
ATOMIC STRUCTURE
There are two ways to represent the atomic
structure of an element or compound;
1.
Electron Configuration
2.
Electron Dot Structure
ELECTRON CONFIGURATION
With electronic configuration elements are represented
numerically by the number of electrons in their shells and
number of shells. For example;
Nitrogen
2 in 1st shell
5 in 2nd shell
configuration = 2 , 5
2
+
5 = 7
N
7
14
ELECTRON CONFIGURATION
Write the electronic configuration for the following
elements;
a)
Ca
20
b)
Na
40
2,8,8,2
d)
Cl
17
35
2,8,7
11
23
c)
2,8,1
e)
Si
14
28
2,8,4
O
8
16
2,6
f)
B
5
11
2,3
Electron Dot Structure
With Dot Structure diagrams elements and compounds
are represented by Dots or Crosses to show electrons
in the valence, outermost, shell. For example:
X
Nitrogen
X
N
XX
X
X
X
N
7
14
Electron Dot Structure
Draw the Electron Dot Structure diagrams for the
following elements:
XX
8
17
a)
O
b)
16
Cl
35
XX
X X
X X
X
O
X
X
X
X
X
X
Cl
X
XX
X
X X
X
X
X
X
HISTORY OF THE ATOM
1932
James Chadwick
He found the presence of neutron by
bombarding a thin sheet of a metal with
radiation. A high-energy radiation was emitted
by the metal.
However, the rays are not distracted by
electrical charge. That’s why it’s named
neutron because it is neutral. It’s mass is
slightly different from the proton.
ATOMIC STRUCTURE
Particle
proton
Charge
Mass (g)
+ ve charge 1.6 x 10-24
neutron
No charge
1.6 x 10-24
electron
-ve charge
9.1 x 10-28
HELIUM ATOM
Shell
proton
+
-
N
N
+
electron
What do these particles consist of?
-
neutron
ATOMIC STRUCTURE
He
2
4
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
number of electrons = number of protons
ATOMIC
STRUCTURE
SUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
•
The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
•
The number of Protons = Number of Electrons.
•
Electrons orbit the nucleus in shells.
•
Each shell can only carry a set number of electrons.