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Chapter 5 The Cosmic Engine
Unit 5.2 The Big Bang
The Structure of the Atom
In order to understand how the universe formed and evolved we need to know some basic things
about the structure of simple atoms. An atom is made up of electrons, orbiting a compact
nucleus composed of protons and neutrons. The protons and neutrons are responsible for 99%
of the mass of the atom.
The atom is thus composed of three particles: the proton, neutron and electron. These
particles are responsible for the atom’s chemical characteristics and mass.
Chemical characteristics
Each element has a different number of protons in the nucleus of its atoms. Hydrogen has one
proton, helium two and lithium three protons. The number of protons in the atomic nucleus is
the atomic number of the element on the chemists’ periodic table. This was first shown by
Moseley in 1913.
Small, light, negatively charged electrons orbit the nucleus at radii of about 5 × 10–11 m. The
electrons determine the chemical behaviour of the atom.
Mass
Most of the mass of an atom is due to the protons and neutrons. These particles have similar
masses, 1833 times the electron mass and are grouped together in a nucleus. The nucleus is very
small in volume compared to the volume of the atom and has a radius of about 10–14 m. The
protons differ from the neutrons in that they carry a charge, which is equal in size to that of the
electron but positive. Atoms are electrically neutral and contain the same number of protons as
electrons. The neutrons are neutral and carry no charge. The relative masses and charges of the
atomic particles are shown in Table 1.
Table 1
The mass and charge of the atomic particles
Particle
Mass
Charge
Proton
Neutron
Electron
1.6726 × 10–27 kg
1.6750 × 10–27 kg
9.1095 × 10–34 kg
+1.6 × 10–19 C
0
–1.6 × 10–19 C
A representation of a hydrogen and a helium atom is shown in Figure 1. The size of the
nucleus is greatly exaggerated, for clarity. The hydrogen atom has a single electron orbiting a
nucleus composed of a single proton. The helium atom contains two protons and two neutrons
in its nucleus, and two orbiting electrons.
Figure 1
Representation of
a hydrogen and
b helium atoms.
a
b
electron
electrons
nucleus
containing
1 proton
nucleus
containing
2 protons and
2 neutrons
The atomic number (Z) of an atom or nucleus is the number of protons contained within the
nucleus. Since atoms are electrically neutral and contain the same number of positive and
negative charges, this is also the number of electrons in the atom. The chemical symbol given to
the atom depends on its atomic number. For example, all nuclei with an atomic number 8 are
oxygen atoms and behave in chemically similar ways.
The atomic mass number (A) is the numbers of protons and neutrons in the nucleus. This is
related to the mass of the atom.
Several atoms can have the same number of electrons orbiting their nuclei, and have the same
chemical symbol, yet differ in the number of neutrons in their nuclei. Their atomic numbers will
be identical but their atomic mass numbers will differ. The different atoms are known as isotopes
of the element concerned. The chemical properties of the isotopes of an element are identical
but the physical properties may vary due to the differences in mass.
For example, there are three isotopes of hydrogen. They are called hydrogen, deuterium and
tritium. Their particle composition is given in Table 2 and they are illustrated in Figure 2.
Table 2
The isotopes of hydrogen
Hydrogen
Deuterium
Tritium
Figure 2
Proton number
Electron number
Neutron number
1
1
1
1
1
1
0
1
2
The isotopes of hydrogen: a hydrogen, b deuterium and c tritium.
We refer collectively to the isotopes of hydrogen, but describe each individual nucleus as a
nuclide. The term nuclide is thus used to describe a particular atomic species.
The shorthand way of describing a nuclide is:
For example the isotopes of hydrogen are represented using this notation as: 11H, 21H, 31H.
Quarks
The study of radioactive isotopes led to the discovery of even more particles inside the atomic
nucleus: positrons (symbol e+), which are positively charged electrons; neutrinos and antineutrinos, which are almost massless particles with high kinetic energies. The increasingly high
energies used in atom smashing accelerators revealed a wealth of particles and antiparticles, and
provided clues to the fundamental building blocks of nucleii. It now appears that protons and
neutrons are made up of smaller particles called quarks. As well as possessing mass and charge,
Figure 3
Protons and
neutrons are
composed of
quarks.
u
d
u
d
u
a proton
d
a neutron
the quarks have other fundamental properties, for example colour, charm, and spin, which are
difficult to explain. The six basic quarks can each come in one of three colours.
Six leptons, including the electron, complete the suite of fundamental particles.
A proton, for example, is made up of three quarks: two up quarks and one down quark, one of
which is red, one blue and one green. These quarks are bound together by particles called gluons.
Table 3
Characteristics of quarks
Quark
Colour
Charge (electron)
Spin
Up
Down
Charm
Strange
Top
Bottom
red, green, blue
red, green, blue
red, green, blue
red, green, blue
red, green, blue
red, green, blue
+2/3
–1/3
+2/3
–1/3
+2/3
–1/3
1/2
1/2
1/2
1/2
1/2
1/2
A Dictionary of Particles and Antiparticles
The following table includes some sub-atomic particles.
Particle
Symbol
Antiparticle
Rest mass (MeV c–2)
Leptons Interact via weak force; spin 1/2
Photon
γ
Electron
e
Neutrino (e)
νe
Muon
µ–
Neutrino (µ)
ν–µ
Tau
τ–
self
e+
νe
µ+
νµ
τ+
0.511
0?
105.7
0?
1784
Mesons Interact via strong force; spin 0
Pion
π+
π0
+
Kaon
K
Eta
η0
π–
self
–
K
self
139.6
135.0
493.7
548.8
Baryons Interact via strong force; spin 1
Proton
p
Neutron
n
Lambda
Λ0
Sigma
Σ+
Xi
Ξ0
Omega
Ω–
p
n
Λ0
Σ–
Ξ0
Ω+
938.3
939.6
1115.6
1189.4
1315
1672
Quarks
Up
Down
Charm
S
Top
Bottom
u
d
c
s
t
b
u
d
c
s
t
b
360
360
1500
540
100 000
5000
Exchange particles
W
Z
W
Zo
W
Zo
86 000
93 000
E x e r ci s e s
1
Draw a diagram of a helium atom; clearly label the protons, neutrons and
electrons.
2
A proton is made up of two ‘up’ and one ‘down’ quarks. What is the total
charge on the proton?
3
A neutron is made up of one ‘up’ and two ‘down’ quarks. What is its charge?