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The Atom
Scientists discovered that materials around
us are made up of millions of tiny particles.
They called these particles atoms.
Atom
Word atom comes from the Greek word for
“cannot be split up”
e.g. the smallest piece of copper is called an
atom of copper.
Atom Definition
An atom is the smallest particle of an
element which still retains the properties of
that element.
Each element is made up of atoms of that
element e.g. the element gold is made up
of atoms of gold
Size of an Atom
Atoms are very small
16
A full stop . Contains about 10 atoms of
carbon
A special microscope called an electron
microscope can be used to see atoms
Dalton’s Theory of Atoms
In 1808 John Dalton proposed
a theory on atoms after doing
many experiments.
He imagined atoms to be like
marbles tiny, round and hard
only smaller. He though
atoms could not be divided
into anything simpler.
Particles inside Atoms
In the 1890’s other Scientists began to study
atoms. Found that Dalton was incorrect
when he said atoms could not be split up
into anything simpler.
They found ting particles in side atoms
called sub-atomic particles. Sub atomic
means “inside atoms”
Chapter 29 – The Atom
The atom
An atom is made up of three subatomic particles:
• Protons
• Neutrons
• Electrons
The Atom
The electrons which is negatively charged
was discovered in 1897
The proton was then discovered it was
positively charged
The neutron which has no charge (neutral)
was discovered in 1932
Chapter 21 – The Atom
Subatomic particles
• The nucleus of an atom is made up of protons and
neutrons.
• The electrons whizz around the nucleus in paths called
orbits or shells.
Chapter 21 – The Atom
Differences in sub-atomic particles
Mass
The mass of these particles is very small.
A new unit of mass called the Atomic Mass
Unit (a.m.u) is used to measure their
mass.
1 proton = 1 a.m.u.
1 neutron = 1 a.m.u.
1 electron = 1/1840 a.m.u.
Charge
An electron has a negative charge (-1)
A proton has a positive charge (+1)
A neutron has no charge (0 charge)
Structure of an Atom
The Planetary Model
Niels Bohr proposed that
electrons move around the
nucleus in fixed paths called
orbits.
Planetary Model
Review
Complete the table:
Name of
Where
particle
situated in
atom
Proton
Nucleus
Outside the
nucleus
Relative
Mass
Relative
Charge
0
- 1 unit
Answer
Name of
particle
Where
situated in
atom
Relative
Mass
Relative
Charge
Proton
Nucleus
1 a.m.u.
+1
Neutron
Nucleus
1 a.m.u.
0
Electron
Outside the 1/1840units - 1 unit
nucleus
Quiz
1. ________ was a famous Scientist who
put forward an atomic theory in 1808?
2. List two words to describe Daltons theory
of an atom? _______ __________
3. The centre of an atom is called the
_____
Answers
1. John Dalton
2. Small and hard
3. Nucleus
Electron Cloud
The movement of the electrons around the
nucleus gives rise to an electron cloud
Size of the nucleus
The nucleus of an atom could be imagined
as a marble at the centre of Croke Park
pitch. The electrons would be like specks
of dust at various parts of the stands.
Everything in between would be just empty
space and the electrons are free to move
in this space.
Chapter 21 – The Atom
Atomic number and mass number
Each element in the periodic table has its own atomic
number and mass number.
Chapter 21 – The Atom
Atomic number
Atomic number of an atoms is the number
of protons in the nucleus of that atom.
Atomic Number
The atomic number of an atom is equal to
the number of protons and the number of
electrons in an atom.
e.g. atomic number of copper is 29
Number of protons in copper is 29
Number of electrons in copper is 29
Complete the following table
Element
Hydrogen
Lithium
Oxygen
Sodium
Magnesium
Atomic
Number
Number of
protons
Number of
electrons
Periodical Table
Chapter 21 – The Atom
Mass number
Mass number is the number of protons and neutrons
that an atom at that element possesses.
Mass number = number of protons + number of neutrons
Mass number
Find the Mass number of:
Chapter 21 – The Atom
For example
So an atom of magnesium has
Atomic number = 12
• 12 protons
• 12 electrons
• Mass number = 24
24 – 12 = 12
Number of neutrons = 12
Example
Helium:
Atomic number =
Number of protons =
Number of electrons =
Mass number =
Number of neutrons =
Helium
Atomic number = 2
Number of protons = 2
Number of electrons = 2
Mass number = 4
Number of neutrons = 4 - 2 = 2
Example
Chlorine
Atomic number =
Number of protons =
Number of electrons =
Mass number =
Number of neutrons =
Chlorine
Atomic number = 17
Number of protons = 17
Number of electrons = 17
Mass number = 35
Number of neutrons = 35-17 = 18
Complete the table for the first 20
elements p.g. 207 t/b
Element
Hydrogen
Helium
Lithium
Berillium
Boron
Carbon
Nitrogen
Oxygen
flourine
Atomic
Number
Number
Protons
Number
Electrons
Mass No.
Number of
Neutrons
Elements
Element
Hydrogen
Helium
Lithium
Berillium
Boron
Carbon
Nitrogen
Oxygen
flourine
Atomic
Number
Number
Protons
Number
Electrons
Mass No.
Number of
Neutrons
Chapter 21 – The Atom
Electronic configuration
Electronic configuration refers to the way electrons are
arranged in each shell or orbit.
How electrons are arranged
• Electrons whizz around the nucleus in
shells or orbits.
• The first shell can hold up 2 electrons and
the other shells can hold up to 8 electrons.
• Electrons must fill a shell before starting a
new shell.
Chapter 21 – The Atom
Magnesium electronic configuration
As magnesium has 12 electrons, they will be arranged:
 2 in the first shell
 8 in the second shell
 2 in the third shell
or, more simply:
(2, 8, 2)
Chapter 21 – The Atom
Reactivity
Elements react with each other to obtain a full outer shell
of electrons.
Chapter 21 – The Atom
Isotopes
Isotopes are atoms at the same element that have the
same atomic number but different mass numbers.
In other words, the same numbers of protons but different
numbers of neutrons.
Chapter 21 – The Atom