Chemistry
(14 - 16)
The Atom
(Basic Structure - 1)
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The Atom
All matter throughout the Universe is made up of atoms.
Atoms are minute particles of matter. A speck of dust contains
billions of atoms.
As each individual atom is so tiny, it is impossible to see an
individual atom in sufficient detail to describe its structure.
The Atom
The mass of an atom is very small and is not usually expressed in
grams. One gram of hydrogen atoms will contain more than
600,000 million, million, million atoms!!!
Development of the Atomic Theory
Over the last two hundred years, scientific experiments have led us
to a model of the atom which is widely accepted by modern
scientists – this is known as the ‘Atomic Theory’.
John Dalton
In 1806, the elements were
defined by Dalton as being
substances composed of only
one type of atom.
He also introduced the idea
that atoms had a ‘solid ball’
structure.
John Dalton 1766 - 1844
We now define an element as a
substance composed of atoms,
each of which has the same
number of protons.
J. J. Thompson
In 1897, J. J. Thompson showed that an
atom contained small negatively charged
particles.
He called these particles electrons.
Thompson knew that the atom was
neutral (had no electrical charge).
J. J. Thompson 1856 - 1940
He proposed that the atom must
be made of a positively charged
sphere which contained the
negatively charged electrons.
This became known as the
‘plum pudding’ model.
Positively charged
sphere
Negatively charged
electron
Earnest Rutherford
In 1910, Earnest Rutherford and his research
team carried out experiments in which
particles were fired at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass
straight through...
Earnest Rutherford
In 1910, Earnest Rutherford and his research
team carried out experiments in which
particles were fired at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass
straight through...
Earnest Rutherford
Press ‘Start’ to take a closer look at Rutherford’s experiment...
From these results, Rutherford concluded that the atom consisted
of a central nucleus, which contained most of the atom’s mass,
surrounded by the electrons. He also predicted the existence of
neutrons in the nucleus.
The Structure of the Atom
Today scientists accept that an atom contains a central nucleus
around which electrons orbit in different levels.
The nucleus is composed of neutrons and protons.
The Structure of the Atom
Protons, neutrons, and electrons are types of sub-atomic particle.
Each element has a unique combination of the three types of
sub atomic particles...
= Neutrons
=
Protons
=
Electrons
N.B. This diagram represents a lithium atom. In reality, the
electrons are very much smaller than the neutrons or protons.
The Atomic Nucleus
The nucleus of an atom is tiny compared to the volume occupied
by the electrons of the atom.
If an atom could be enlarged to the size of a football stadium, the
nucleus of the atom would be about the size of a golf ball.
An electron has an incredibly small mass, which is 1/1840 the
mass of a neutron or a proton. Therefore, electrons are not
considered to contribute anything to the mass of an atom. Hence,
the vast majority of the mass of an atom is found in the nucleus.
The Atomic Nucleus
So far, the nucleus has
been shown as a static
body, whose sub-atomic
particles fill the space of
the nucleus. In fact, the
nuclear particles are in
constant motion.
Sub-Atomic Particles – Mass and Charge
Charge
 Protons
 Neutrons
 Electrons
Mass
1
0
1
1/1840
The charges on the proton and electron are opposite but equal in
magnitude, so they cancel each other out exactly.
The Atom – Sub Atomic Particles
For each particle drag and drop the correct answers into the table.
Does the Atom have an Electrical Charge?
= 4 Neutrons = 4 x 0 charge = 0
= 3 Protons = 3 x +1 charge = +3
= 3 Electrons = 3 x -1charge = -3
Total Charge = 0 + (+3) + (-3) = 0
All atoms have an overall neutral electrical charge and therefore
any atom always has an equal number of protons and electrons.
The Atom – Atomic Number and Atomic Mass
To describe an atom fully, we need to know the numbers of
each type of sub-atomic particle in the atom.
We use these numbers to generate the atomic number and the
mass number of the atom.
The atomic number is the number of protons in an atom.
The mass number is the sum of protons and neutrons in the atom.
N.B. The electrons have an insignificant mass.
The Atom – Atomic Number and Atomic Mass
An atom of Lithium (Li) can be represented as follows:
Mass Number
Atomic Number
7
Li
3
Atomic
Symbol
This shows that a Lithium (Li) atom has
three protons, and four neutrons.
The Lithium Atom
Mass Number
Atomic Number
= 4 x Neutrons
= 3 x Protons
7
Li
3
= 3 x Electrons
The Carbon Atom
Mass Number
12
Atomic Number
6
= 6 x Neutrons
= 6 x Protons
C
= 6 x Electrons
The Lithium and Carbon Atoms
All atoms of a particular element have the same number of
protons and electrons. All lithium atoms have 3 protons and
3 electrons whereas all carbon atoms have 6 protons and 6
electrons.
The Lithium Atom
The Carbon Atom
Therefore, the atoms of each different element have a unique
number of protons and electrons.
Atomic Structure and the Periodic Table
Drag the following elements, based on their atomic number, into the
correct place in the periodic table.
Electron Levels (Shells)
In an atom, the electrons are arranged in levels (sometimes called
shells or orbitals) around the nucleus.
The electrons fill the levels from the nearest level to the nucleus,
outwards.
Each electron level has a maximum number of electrons it can
hold.
Electron Levels (Shells)
The first electron level can hold up to 2 electrons.
The second electron level can hold up to 8 electrons.
The third electron level can hold up to 18 electrons.
In this atom (Na) the first two
electron levels are full but the
third shell is incomplete.
Note that only some nuclear
particles are shown.
The Structure of Atoms
Electrons do not orbit in concentric circles but have more
elaborate orbits.
The Structure of Atoms
If the electrons in an atom were particles
...and if we could freeze the atom at a moment in time!
...and if we had a microscope that could see the atom!
...is this what an atom would look like?
The Structure of Atoms
OR ... if the electrons in an atom were particles
…and in very rapid motion around the nucleus, so rapid
that they appear as a blur...
...is this what an atom would look like?
Electron Levels (Shells)
The electrons in different levels (shells) are represented in the
following way:
The first level can hold 2 electrons
2
The second level can hold 8 electrons
2,8
The third level can hold up to 18 electrons
2,8,18
The Electronic Structure for Lithium
Lithium is the first element in the first short period of the
Periodic Table. Its atomic number is 3, which tells us that an
atom of lithium contains 3 electrons.
The electron configuration for a lithium atom can be drawn
like this:
The electronic configuration for lithium is written as:
2, 1
The Electronic Structure for Chlorine
Chlorine is in the second short period of the Periodic Table. Its
atomic number is 17, which tells us that an atom of chlorine
contains 17 electrons.
The electrons in a chlorine atom fill the levels starting from the
lower level.
The electronic configuration for chlorine is written as:
2, 8, 7
Electron Levels (Shells)
Drag the electrons to form the correct electronic configuration
for neon (atomic number 10)…
Electron Levels (Shells)
Type the correct electronic configurations of these four elements…
End of Show
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