Isotopes and radioactive decay

In a nutshell

An isotope is an atom of the same element that has a different amount of neutrons. Some isotopes are unstable and will radioactively decay into other elements. The radioactive decay types are alpha, beta and gamma. The mass and atomic numbers change during alpha and beta decay.

Key properties of decay

Isotopes

Whilst the nucleus of an atom of a certain element always has a specific number of protons, atoms of the same element may differ in nuclear mass. As nuclear mass concerns neutrons and protons and the number of protons does not change, it means that the neutron number changes for an atom of different nuclear mass.

An isotope is an atom of the same element that has a different amount of neutrons. A neutron has no charge so this doesn't affect the charge of an atom.

Example

Helium, $$^4_2He$$, has two protons and two neutrons. The helium isotope $$^{3}_{2}He$$​ has two protons and one neutron. 

Isotopes occur naturally, but some are rarer than others.

Example

Only about two in every $$10,000$$​ hydrogen atoms is the deuterium isotope $$^2_1H$$. Only about one in every $$10^{18}$$​ hydrogen atoms is the tritium isotope $$^3_1H$$​. 

Isotopes are not always stable, generally only one or two isotopes of an element will be stable. An unstable isotope will decay into another element and give out radiation. They do this in an attempt to become more stable. This process is called radioactive decay.

Curiosity: Isotopes of large atoms (like uranium) tend to be unstable due to their large size and the imbalance of protons and neutrons in the nucleus. 

Ionising radiation is radiation that gives electrons enough energy to leave the atom. This results in positive ions. A radiation source's ionising power is how easily it can do this.

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Radioactive decay

There are three types of radioactive decay to learn: alpha, beta and gamma. These are named after the particle they emit in the process.

1. Alpha particle 2. Gamma particle 3. Beta particle

Penetrating power

1. Alpha particle 2. Beta particle 3. Gamma ray 4. Paper 5. A few $$cm$$​ of aluminium 6. A few $$mm$$​ of lead

Nuclear equations

Radioactive decays can be represented by nuclear equations using element symbols. Element symbols show the atomic (proton) number, the mass (sum of nucleons) number and the corresponding element. 

Example

An alpha particle is written as $$^4_2He$$. A beta particle is written as $$^0_{-1}e$$. A gamma particle is written as $$\gamma$$ (it does not have a mass or atomic number).

The total mass number and the total atomic number must be equal on both sides of a nuclear equation.

Alpha decay

The nuclear equation for alpha decay is 

​$${^A_BX}\to{^{A-4}_{B-2}Y}+{^4_2He}$$​​

Example

Uranium-$$238$$​ decays into thorium-$$234$$​ by alpha decay.

​$${^{238}_{92}U}\to{^{234}_{90}Th}+{^4_2He}$$

Tip: Notice how the quantities along the top (mass number) add up to $$238$$ on both sides of the arrow. And the quantities along the bottom (atomic number) add up to $$92$$ on both sides of the arrow. This follows the rule for writing nuclear equations.

Beta decay

The nuclear equation for beta decay is

​$${^A_BX}\to{^{A}_{B+1}Y}+{^0_{-1}e}$$

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Example

Lithium can undergo beta decay.

​$${^8_{3}Li}\to{^{8}_{4}Be}+{^0_{-1}e}$$

Gamma decay

Gamma rays only carry energy so do not change the mass or atomic number of an atom. The nuclear equation for gamma decay is

​$$^A_BX \rightarrow {^A_BX} + \gamma$$​​

Example

Barium-$$137$$​ can decay by gamma decay.

​$$^{137}_{56}Ba \rightarrow{ ^{137}_{56}Ba} + \gamma$$​​