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S.MORRIS 2006
Where did it all begin?
The word “atom”
comes from the
Greek word “atomos”
which means
indivisible.
The idea that all
matter is made up of
atoms was first
proposed by the
Greek philosopher
Democritus in the 5th
century B.C.
HISTORY OF THE ATOM
460 BC Democritus develops the idea of atoms
he pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
ATOMA
(greek for indivisible)
HISTORY OF THE ATOM
1803 John Dalton
suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them
ATOMS
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small particles
called atoms.
2. All atoms of a given element are identical.
The atoms of one element are different from those of
any other element
3. Atoms of different elements can combine in simple
whole number ratios to form compounds.
4. Chemical reactions only involve the rearrangement of
atoms. Atoms are not created or destroyed in chemical
reactions.
2.1
16 X + 8Y 8 X2Y
2.1
Law of Definite Proportions
• The ratio of mass of elements in a
compound is always the same
• Every Water molecule will contain 16g of
oxygen and 2 g of hydrogen
2
2.1
HISTORY OF THE ATOM
1898 Joseph John Thompson
found that atoms could sometimes eject a far
smaller negative particle which he called an
ELECTRON
HISTORY OF THE ATOM
1890
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 charge
like plums surrounded by pudding.
PLUM PUDDING
MODEL
HISTORY OF THE ATOM
1910 Ernest Rutherford
1910
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
(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
Rutherford’s Model of
the Atom
atomic radius ~ 100 pm = 1 x 10-10 m
nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m
2.2
HISTORY OF THE ATOM
gold foil
helium nuclei
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.
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.
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.
Chadwick’s Experiment (1932)
H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
measured mass He/mass H = 4
+ 9Be 1n + 12C + energy
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g
2.2
Subatomic Particles
Mass Charge Charge
Particle
(g) (Coulombs) (units)
Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1
Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1
Neutron (n) 1.67 x 10-24 0 0
mass p = mass n = 1840 x mass e-
2.2
Bohr’s Atom
electrons in orbits
nucleus
Atoms
• # of protons &
neutrons
• The basic unit of
Matter
• The smallest particle
of an element that
retains the properties
of that element.
HELIUM ATOM
Shell
proton
+
N
-
+
- N
electron neutron
What do these particles consist of?
ATOMIC STRUCTURE
All About Atoms
Particle Charge Mass
Proton + charge 1amu
Neutron No charge 1 amu
electron - charge 1/1836
ATOMIC STRUCTURE
2
He
Atomic number
Represents the number of
protons in an atom
Never changes
P+ equal to the number of e-
4 Mass Number
the number of protons and
neutrons in an atom
Neutrons equal
mass # - atomic #
number of electrons = number of protons
Ions
Charged particles due to the loss or gain of electrons
2) Atoms that are called cations lose e- thus
becoming positive
11 protons 11 protons
Na 11 electrons Na+ 10 electrons
1) Atoms that are called anions gain e- thus become
negative
17 protons 17 protons
Cl 17 electrons Cl- 18 electrons
2.5
Isotopes
• Atoms with the same atomic number but
different mass number
• Atoms having the same number of protons
but different numbers of neutrons
• Average Atomic Mass of an element is the
weighted average of an element’s
naturally occurring isotopes
2.3
Do You Understand Isotopes?
Isotope Maker
How many protons, neutrons, and electrons are in 14
6 C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 11
6 C?
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
# OF NEUTRONS = mass number – atomic number
Mass Number A
ZX
Element Symbol
Atomic Number
1 2 3
1H 1H (D) 1H (T)
235 238
92 U 92 U
2.3
Bohr Model of the Atom
• Electrons travel around the nucleus in one
of several orbits/shells/ nrg levels
• Principal energy level is designated by a
quantum number (n)
• Quantum number is the same as period on
the periodic table
2.4
Noble Gas
Halogen
Group
Period
Alkali Earth Metal
Alkali Metal
ATOMIC STRUCTURE
Electrons are arranged in Energy Levels or
Shells around the nucleus of an atom.
• first shell maximum of 2 electrons
• second shell maximum of 8 electrons
• third shell max of 18 electrons
• fourth shell max of 32 electrons
ATOMIC STRUCTURE
There are two ways to represent the atomic
structure of an element or compound;
1. Electronic Configuration
2. Electron dot diagrams
ELECTRONIC CONFIGURATION
With electronic configuration elements are represented
numerically by the number of electrons in their shells
and number of shells. For example;
Nitrogen configuration = 2 , 5
N
2 in 1st shell 14
5 in 2nd shell
7
2 + 5 = 7
ELECTRONIC CONFIGURATION
Write the electronic configuration for the following
elements;
20 11 8
a) Ca b) Na c) 16O
40 23
2,8,8,2 2,8,1 2,6
14
d)
17
Cl e) Si f)
5
B
35 28 11
2,8,7 2,8,4 2,3
Valence
• Valence shell is outermost occupied
energy level and is the same as the period
number in the periodic table
• Valence electrons are the electrons in the
outer energy level of an atom
DOT & CROSS DIAGRAMS
With Dot & Cross diagrams elements and compounds
are represented by Dots or Crosses to show electrons,
and circles to show the shells. For example;
X
N
Nitrogen X X N X X 7
14
XX
DOT & CROSS DIAGRAMS
Draw the Dot & Cross diagrams for the following
elements;
8 17
X
a) O b) Cl 35 X X
16 X
X X X
X
X X X X Cl X X X
X O X
X X
X X X
X X X
X
SUMMARY
1. 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.
