1.2 Isotopes
- Syllabus
- 9701–2028–2029
- Topic
- 1.2
- Level
- AS
Learning objectives
1.2.1Terms: isotope• Define: isotope in terms of numbers of protons and neutrons1.2.2The notation x• Understand the notation x y A for isotopes, where x is the mass/nucleon number and y is the atomic/proton number1.2.3Isotopes of the same element have the same• State that and explain why isotopes of the same element have the same chemical properties1.2.4Isotopes of the same element have different• State that and explain why isotopes of the same element have different physical properties, limited to mass and density
Isotopes differ only in neutron number
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The shared proton number keeps the element identity the same; the different neutron number gives a different mass number.
Carbon-12 and carbon-14 are both carbon because each atom has 6 protons. Carbon-12 has 6 neutrons, while carbon-14 has 8, so their mass numbers are 12 and 14.
Changing neutron number makes a different isotope. Changing electron number makes an ion, and changing proton number makes a different element.
Read isotope notation without swapping the numbers
An isotope is written with its mass number at the upper left of the element symbol and its atomic number at the lower left.
^{A}_{Z}X
A is the mass (nucleon) number, Z is the atomic (proton) number and X is the element symbol. Therefore the isotope contains Z protons and A−Z neutrons.
In 1737Cl, the lower number 17 identifies chlorine and gives 17 protons. The upper number 37 counts all nucleons, so the neutron number is 37−17=20. Isotopes of chlorine keep Z=17 but have different values of A.
Why isotopes have the same chemical properties
Isotopes of the same element have the same chemical properties because their atoms have the same electron configuration. Chemical reactions involve electrons, especially those in the outer shell, rather than neutrons in the nucleus.
The isotopes have the same proton number. As atoms, they therefore have the same number of electrons, arranged in the same shells and outer-shell pattern. They form the same types of bonds and undergo the same chemical reactions.
Different neutron numbers change nuclear mass, not the electron arrangement that controls chemistry. Do not use the isotope’s different mass number as a reason for different chemical behaviour.
Why isotope mass and density can differ
Isotopes of one element have different physical properties because they contain different numbers of neutrons. The syllabus limit here is mass and density.
| Property | Effect of more neutrons | Reason |
|---|---|---|
| mass | increases | each added neutron contributes nuclear mass |
| density | can increase | under the same physical conditions, greater mass is present in approximately the same volume |
A 22Ne atom has two more neutrons and greater mass than a 20Ne atom. A sample enriched in neon-22 therefore has a higher density than a comparable sample enriched in neon-20 when state, temperature and pressure are the same.
The neutron difference explains mass-related physical changes. It does not change the element’s proton number or the electron arrangement responsible for its chemical properties.