31Alpha-decay occurs in very heavy elements, like Uranium. If their nucleus has too many protons (i.e. it is proton-rich), it is unstable. Such a nucleus can become more stable by emitting an alpha particle.
Alpha decay leaves a daughter nucleus that is of a different element to the parent one. When a nucleus emits an alpha-particle:
atomic mass decreases by 4.
atomic number decreases by 2
e.g. Americium-241 (an α-source used in smoke detectors), which has an atomic number of 95 and an atomic mass of 241, will decay to Neptunium-237 (which has an atomic number of 93 and an atomic mass of 237).
The equation would look like this:
241 Am
→
237 Np + α
95 93
Now an alpha particle is the same as the nucleus of a Helium atom (2 protons and 2 neutrons). So it helps to balance equations if we write He instead of α as follows:
241 Am
→
237 Np + 4 He
95 93 2
Alpha decay leaves a daughter nucleus that is of a different element to the parent one. When a nucleus emits an alpha-particle:
atomic mass decreases by 4.
atomic number decreases by 2
e.g. Americium-241 (an α-source used in smoke detectors), which has an atomic number of 95 and an atomic mass of 241, will decay to Neptunium-237 (which has an atomic number of 93 and an atomic mass of 237).
The equation would look like this:
241 Am
→
237 Np + α
95 93
Now an alpha particle is the same as the nucleus of a Helium atom (2 protons and 2 neutrons). So it helps to balance equations if we write He instead of α as follows:
241 Am
→
237 Np + 4 He
95 93 2
Beta decay occurs in nuclei, such as Strontium-90, that are unstable because they have too many neutrons in their nuclei (i.e. they are very neutron-rich). They become more stable by emitting a beta particle.
Beta decay also leaves a daughter nucleus that is of a different element to the parent one.
A beta particle is just an electron. What is an electron doing coming out of a nucleus? Under certain conditions, a neutron can decay, turning into a proton plus an electron. The proton remains in the nucleus, whilst the electron (the beta particle) flies off at high speed.
This means that when a nucleus emits a beta-particle:
atomic mass is unchanged
atomic number increases by 1
e.g. Strontium-90 undergoes beta decay and forms Yttrium-90
90 Sr
→
90 Y + β-
38 39
Because a beta particle is just an electron, we could write e instead of β in the equation as follows:
90 Sr
→
90 Y + 0 e
38 39 -1