Download An atom - Ms. Buicke maths and science

The Atom
By Miss Buicke
What we must know from the syllabus:
OC39
Describe the structure of the atom, state the location, relative charge, and
atomic mass of the sub-atomic particles, and define atomic number and
isotope.
OC40
Draw the Bohr structure of the first 20 elements.
An atom:
The smallest part of an element which still has the
properties of that element.
The word atom comes from a Greek word meaning
something which cannot be split.
Inside the atom:
The atom consists of three sub-atomic particles
 Protons
 Neutrons
 Electrons
Properties of each of the sub-atomic particles:
Particle in
an atom
Mass
Charge
Location
proton
1 unit
Positive (+) nucleus
neutron
1 unit
neutral
electron
negligible
Negative (-) Orbiting
the nucleus
nucleus
All the particles are extremely small. Electrons are
so small it would take almost 2000 of them to have
the same mass as a single proton or neutron. As a
result the mass of an electron is rounded off to zero.
Protons and neutrons are of similar mass.
Atomic and Mass numbers
The atomic number of an element:
The number of protons in an atom of that element.
The mass number:
The total number of protons and neutrons in an atom
of that element.
Atoms of different elements differ from each other by the
number of protons, neutrons and electrons they have.
Each element in the periodic table has a number written
above and below its symbol. The smaller number is called
the atomic number. This tells us how many protons every
atom of that element has.
The number of protons in an atom is always equal to the
number of electrons.
Because of this atoms are neutral. The charge of the proton
cancels out the charge on the electron.
Examples:
Atomic number: 6 protons
Mass number:
Number of protons and neutrons
12-6 =6 neutrons
6 protons + 6 neutrons= 12
Atomic number:
Number of protons = 3
Mass number:
Number of protons and
neutrons.
Number of neutrons: 7-3 =4
Number of protons 3 + number
of neutrons 4.
Mass number= 3 + 4 = 7
How electrons are arranged:
Electrons whizz around the nucleus in energy levels
called shells or orbitals.
The first shell can hold two electrons.
The second and third shell can hold 8 electrons each
Electrons fill up the shells one by one, starting with
the first shell. When a shell is full, they start a new
one.
Two electrons can only go in the first shell
The atomic structure (The Bohr Model)
We must be able to draw the atomic structures of the first
twenty elements in the periodic table.
Niels Bohr was the first person to put forward the idea of
electron shells containing electrons orbiting the nucleus.
The way these electrons are arranged in each shell is called
the electronic configuration.
Examples using the Bohr model
ATOM
NO.OF ELECTRONS
ELECTRONIC
CONFIGURATION
HELIUM
2
2
LITHIUM
3
(2,1)
CARBON
6
(2,4)
12 electrons
ELECTRONIC
CONFIGURATION: (2,8,2)
18 electrons
ELECTRONIC
CONFIGURATION:
(2,8,8)
Isotopes:
Atoms of the same element which have different numbers
of neutrons
A popular pair of isotopes are carbon-12 and
carbon-13 which are used in carbon dating.
Both of these atoms have an atomic number of 6
which means they are both carbon.
However they have different mass numbers and as a
result they have different numbers of neutrons in
their nucleus.