Atoms and radioactivity
Electrons, protons, and neutrons
Atomic particle Relative mass Relative charge
Electron 1 -1
Proton 2000 +1
Neutron 2000 0
The atom
Atomic number Chemical behaviour depends on how atoms combine with other atoms = no.
of electrons
Protons in atom are always the same- atomic number shows no. of protons
Atomic mass Total no. of protons + neutrons in the nucleus
16
Atomic notation 8O has 16 nucleons and 8 protons. 16-8 = 8 neutrons. 8 protons = 8
electrons (neutral)
Isotopes
- No. of protons identifies element
- Chemical behaviour depends on electrons
Atoms of an element with a different number of neutrons are called isotopes.
- No. of neutrons will affect mass of atom, but atomic number will be the same
Stability of isotopes
Isotopes = different physical properties, (mass and stability of nucleus)
- Protons are held in nucleus by nuclear force- strong, over small distance, counteracts
electric force (like charge of protons)
- Neutrons affect this balance- too many/few neutrons will make nucleus unstable = decay
(release α, β, γ)
Ionising radiation
It is unstable nuclei decay- causes atoms to gain/lose charge randomly
Properties of ionising radiation
Ionising radiation uncharged atoms lose electrons. Three types of radioactive emission can form
ions. When ionising radiation passes through mater, its energy is absorbed.
ALPHA particles BETA particles GAMMA waves
Helium nucleus 42α (2+) 0
β electron (1-)
-1
0
γ EM wave (neutral)- no
0
1
Relatively large mass n 11p (stays in nucleus) + 0-1e (i.e.
0 mass or charge
neutron splits into proton + electron) Short wavelength
Very ionising Medium ionising Poorly ionising unless
Rapidly give off energy Smaller, less charged, and have intense
greater range than alpha
, Loses energy after hitting each Blocked by few mm Al/plastic Very penetrating:
atom so short range 2-10m air reduced by lead
Blocked by paper or few cm air Interact with matter less frequently Long range
Short range than alpha
222
Rn 42α + 21884Po
86
14
C 147N + 0-1β
6
60
27 Co 00γ + 6027Co
Rn is parent nucleus, α and Po C is parent nucleus, N and β are
are daughter nuclei daughter nuclei
Neutron radiation
Same mass as proton, no charge. If emitted, mass number decreases by one, atomic number
unchanged. (vice versa if absorbed)
1
n
0
- Do not cause ionisation (neutral particles)
- They are absorbed by nuclei of other atoms = radioactive (unstable nuclei)
Practical: penetrating powers of different kinds of radiation
Radioactive sources must be stores in lead-lined boxes + warning label. Must be handled with tongs,
away from body.
1) Before source is removed from box, measure background radiation using Geiger-Muller
(GM) tube. Write no. of counts after 5 mins, repeat three times to find average count
2) Take alpha source, set it up at measured distance from GM tube (2-4cm)
3) Measure counts in 5 min period, repeat count with thick paper in front of the source (count
should have dropped).
4) Replace alpha with beta, repeat steps but use thin sheets of aluminium between source and
detector. When thickness is 1-2mm, count should have dropped to background level
5) Repeat with gamma source- should be blocked only by few cm of lead.
Alpha decay
- Numbers on top (mass number) are equal on both sides
- Numbers of bottom (atomic number) are equal on both sides
General equation:
Beta decay
General equation:
Electrons, protons, and neutrons
Atomic particle Relative mass Relative charge
Electron 1 -1
Proton 2000 +1
Neutron 2000 0
The atom
Atomic number Chemical behaviour depends on how atoms combine with other atoms = no.
of electrons
Protons in atom are always the same- atomic number shows no. of protons
Atomic mass Total no. of protons + neutrons in the nucleus
16
Atomic notation 8O has 16 nucleons and 8 protons. 16-8 = 8 neutrons. 8 protons = 8
electrons (neutral)
Isotopes
- No. of protons identifies element
- Chemical behaviour depends on electrons
Atoms of an element with a different number of neutrons are called isotopes.
- No. of neutrons will affect mass of atom, but atomic number will be the same
Stability of isotopes
Isotopes = different physical properties, (mass and stability of nucleus)
- Protons are held in nucleus by nuclear force- strong, over small distance, counteracts
electric force (like charge of protons)
- Neutrons affect this balance- too many/few neutrons will make nucleus unstable = decay
(release α, β, γ)
Ionising radiation
It is unstable nuclei decay- causes atoms to gain/lose charge randomly
Properties of ionising radiation
Ionising radiation uncharged atoms lose electrons. Three types of radioactive emission can form
ions. When ionising radiation passes through mater, its energy is absorbed.
ALPHA particles BETA particles GAMMA waves
Helium nucleus 42α (2+) 0
β electron (1-)
-1
0
γ EM wave (neutral)- no
0
1
Relatively large mass n 11p (stays in nucleus) + 0-1e (i.e.
0 mass or charge
neutron splits into proton + electron) Short wavelength
Very ionising Medium ionising Poorly ionising unless
Rapidly give off energy Smaller, less charged, and have intense
greater range than alpha
, Loses energy after hitting each Blocked by few mm Al/plastic Very penetrating:
atom so short range 2-10m air reduced by lead
Blocked by paper or few cm air Interact with matter less frequently Long range
Short range than alpha
222
Rn 42α + 21884Po
86
14
C 147N + 0-1β
6
60
27 Co 00γ + 6027Co
Rn is parent nucleus, α and Po C is parent nucleus, N and β are
are daughter nuclei daughter nuclei
Neutron radiation
Same mass as proton, no charge. If emitted, mass number decreases by one, atomic number
unchanged. (vice versa if absorbed)
1
n
0
- Do not cause ionisation (neutral particles)
- They are absorbed by nuclei of other atoms = radioactive (unstable nuclei)
Practical: penetrating powers of different kinds of radiation
Radioactive sources must be stores in lead-lined boxes + warning label. Must be handled with tongs,
away from body.
1) Before source is removed from box, measure background radiation using Geiger-Muller
(GM) tube. Write no. of counts after 5 mins, repeat three times to find average count
2) Take alpha source, set it up at measured distance from GM tube (2-4cm)
3) Measure counts in 5 min period, repeat count with thick paper in front of the source (count
should have dropped).
4) Replace alpha with beta, repeat steps but use thin sheets of aluminium between source and
detector. When thickness is 1-2mm, count should have dropped to background level
5) Repeat with gamma source- should be blocked only by few cm of lead.
Alpha decay
- Numbers on top (mass number) are equal on both sides
- Numbers of bottom (atomic number) are equal on both sides
General equation:
Beta decay
General equation:
