Section 1 of 5What Isotopes Are
Isotopes are different versions of the same element that differ in mass but not in their chemistry.
Definition
- Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.
Same electrons
- Isotopes have the same number of electrons, which determines their chemical properties.
Different mass
- Isotopes have different masses because they contain different numbers of neutrons.
Same element only
- Only atoms of the same element can be isotopes, so carbon-13 and hydrogen-2 are not isotopes of each other.
Section 2 of 5Hydrogen Isotopes
Hydrogen has three isotopes, which are all written with the same atomic number of 1.
Hydrogen Isotopes
Protons
- Equal to the atomic number, so every isotope of hydrogen has 1 proton.
Electrons
- Equal to the protons in a neutral atom, so every neutral hydrogen atom has 1 electron.
Neutrons
- Calculated as mass number minus atomic number, so each hydrogen isotope has a different number.
Mass number
- Equal to protons plus neutrons, so it changes between the isotopes of the same element.
| Isotope | Notation | Protons | Neutrons | Electrons |
|---|---|---|---|---|
| Hydrogen-1 | ¹₁H | 1 | 0 | 1 |
| Hydrogen-2 | ²₁H | 1 | 1 | 1 |
| Hydrogen-3 | ³₁H | 1 | 2 | 1 |
Section 3 of 5Counting Particles in Isotopes and Ions
The same steps work for isotopes and for charged ions.
1
Read the atomic number from the notation to find the number of protons.
2
Subtract the atomic number from the mass number to find the number of neutrons.
3
For a neutral atom, make the number of electrons equal to the number of protons.
4
For a positive ion, subtract the charge from the protons; for a negative ion, add the charge to the protons.
Section 4 of 5Worked Example
Question Determine the number of protons, neutrons and electrons in a chloride ion, ³⁷₁₇Cl⁻.
- Protons = atomic number = 17.
- Neutrons = mass number − atomic number = 37 − 17 = 20.
- Chloride has gained one electron, so electrons = 17 + 1 = 18.
Answer The chloride ion has 17 protons, 20 neutrons and 18 electrons.
Section 5 of 5Exam Checklist & Tips
1.2.4 – Key Points for the Exam
- Isotope Atoms of the same element with the same number of protons but different numbers of neutrons.
- Mass number The total number of protons and neutrons in an atom.
- Atomic number The number of protons in an atom of an element.
- Ion A charged atom formed by gaining or losing electrons.
- Notation The way of writing an element with its mass number and atomic number, such as ²³₁₁Na.
Common Mistakes
- Saying that isotopes have different numbers of protons;
- they have the same number of protons and different numbers of neutrons.
- Calculating neutrons as atomic number minus mass number, which gives a negative value.
- Forgetting that a negative ion has more electrons than protons, so the electron count goes up.
- Claiming that isotopes have different chemical properties;
- they behave the same because they have the same electron arrangement.