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Chemical Ideas 2.1
A simple model of the atom
What is inside atoms?
In a simple model of atomic structure the atoms are thought to be made from
three types of sub-atomic particle: protons, neutrons and electrons. Protons
and neutrons form the dense nucleus of atoms. Electrons are much more diffuse
and move around the nucleus (on orbits/shells).
The nucleus is tiny compared with the volume occupied by the electrons.
Protons and
neutrons in
nucleus
electrons
Sub-atomic particles
Particle
proton
neutron
electron
Mass on relative
atomic mass scale
1
1
0.00055
Charge (relative
to proton)
+1
0
-1
Location in atom
in nucleus
in nucleus
around nucleus
Protons and electrons have equal but opposite electrical charges. Protons and
neutrons have almost equal masses – the nucleus accounts for almost all the mass
of atom and hardly any of its volume.
Most of the atom is empty space.
Nuclear symbols
The nucleus can be described by:
 atomic number (symbol Z) represents the number of protons in the nucleus
 mass number (symbol A) is the number of protons and neutrons in the nucleus
A = Z + N
Number of neutrons
Nuclear symbols identify the mass number and the atomic number as well as the
symbol for the element.
What are isotopes?
Isotopes are atoms of the same element which have different mass numbers.
The difference in mass arises from different numbers of neutrons.
Because most elements exist naturally as a mixture of isotopes, the relative
atomic mass (Ar) is an average of the masses of the isotopes (relative isotopic
masses) taking into account their abundances. (see table 2)
What is a mass spectrometer?
This is an instrument that compares the masses of different particles (i.e. 37Cl
and 35Cl) and measures their relative abundances.
The essential parts of the instrument are:
 a sample inlet – small quantities of gases and liquids are injected
 an ionisation chamber – heated filament gives electrons that pass into the ionisation
chamber. Here the sample of the element is bombarded by a stream of high-energy
electrons. The bombardment knocks off electrons from the outsidew of the atoms, producing
positive ions.
X-atom of the element as a gas
X (g) + e
X (g) + 2e

an electric field to accelerate a beam of positively charged ions – electrically
charged plates attract the positive ions from the mixture in the ionisation chamber and
accelerate them. By arranging the plates and choosing their voltage a fine beam of ions with
only a narrow range of kinetic energies is passed into the magnetic field.
 a magnetic field for deflecting the ion beams – an electromagnet produces a
magnetic field. The field deflects the beam of ions into curved path. Lighter ions are pushed
further off course than heavy ions. With a low magnetic field these light ions can be
targeted at the detector, but the field will have to be increased before a beam of heavy ions
is deflected enough to take the same course.
 an ion detector – produces an electric current when hit by ions. The detector signal is
recorded as the magnetic field is gradually changed. This produces a mass spectrum.
Calculating Ar
Iron’s Ar = 56 as a rounded number. The mass spectrum for naturally occurring
iron is: 54, 56, 57 & 58. The percentage abundance of 56Fe is the largest, so its
Ar actually equals 55.91 (see table 3).
Chemical Ideas 2.2
Nuclear reactions
Emissions from radioactive substances
Some isotopes of some elements are unstable, so their nuclei break down
spontaneously and emit rays and particles called emissions. They are radioactive.
This breakdown or RADIOACTIVE DECAY occurs without any need to be
triggered off by something.
There are 3 different kinds of emissions identified: andemissions. All
three types of emissions are capable of knocking electrons of the atoms they
collide with, which ionises the atoms. These emissions are sometimes referred to
as ionising radiation. (see table 4)
Nuclear equations
Nuclear equations summarise the processes which produce 
They include the :
 mass number
 nuclear charge
 chemical symbol for each particle involved.
Both, mass and charge must balance in a nuclear equation.
and  radiation.
-decay involves the emission of -particles, which are helium nuclei,
-decay reduces the mass of heavy nuclei:
238
92
U
234
90Th
+
4
4
2He.
2He
the isotope produced will have a mass number 4 units lower and a nuclear charge
2 units lower than the original atom.
-decay involves the emission of electrons, written as
among lighter elements:
14
6
C
14
7N
+
1p
+
0
0
-1
e. -decay is common
-1e
during -decay the mass number (A) remains constant but the nuclear charge
(Z) increases by 1 unit. This means that neutron is converted into a proton plus
an electron which is ejected from the nucleus:
1
0
n
1
0
-1e
Both - and -decay result in the production of a different element.
-decay is the emission of energy from a nucleus which is changing from high
energy level to a lower one. It often accompanies other two emissions. (see
Storyline EL2)
Half-lives
DEF1. Half-life is the time taken for the activity of the sample to fall to half its
original volume.
DEF2.The time for half of the nuclei to decay is called the half life. Decay is
independent of the chemical state of the isotope (it happens the same way
whether the isotope was in a compound or in the free state as an element). For
any given isotope the half-life is fixed (table 5).
Calculating and using half-lives
One half-life of the radioisotope is equal to 2 hours. If you know the half-life of
a radioisotope, you can calculate:
 the time needed for the activity or the mass of the isotope to fall to a certain
value
 the mass of the isotope or its activity after it has decayed for a certain time.
(see examples)
 radioactive decay curve
mass of sample
(g)
4
2
a shape of the curve is reciprocal y=1/x
t1
t2
t1/2=t2 –t1
activity never
reaches zero
time (years)
A line which a graph approaches without touching is called ASYMPTOTE.
Nuclear fusion
In this reaction two light atomic nuclei, when they are very close to each other,
fuse together to form a single heavier nucleus of a new element. The process is
exothermic (release of energy). The nuclear fusions occur at only very high
temperatures.
When 2 hydrogen nuclei fuse together by nuclear fusion,
hydrogen turns into helium.
Isotopes of hydrogen: 11H – hydrogen, 21H – deuterium, 31H - tritium
Nuclear reactions that take place in the Sun:
1
1H
+
2
1H
3
2H e
+γ
2
1H
+
3
1H
4
2He
+
1
0n
NB atomic numbers and mass numbers must balance in nuclear equations
See relevant Storyline EL4
The gases in the gas clouds of stars exist in an ionised form called plasma. In
plasma the positive atomic nuclei exist in a ‘sea’ of delocalised electrons.