Atoms and isotopes
All substances contain atomThe smallest part of an element that can exist.. There is a nucleusThe central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. at the centre of each atom. Atomic nucleiNuclei is the plural of nucleus. The nucleus is the central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. consist of two types of subatomic particles:
- protonSubatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1., which each carry a single positive chargeProperty of matter that causes a force when near another charge. Charge comes in two forms, positive and negative. For example, a negative charge causes a repulsive force on a neighbouring negative charge.
- neutronUncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0., which are electrically neutral
This means that all atomic nuclei are positively charged.
Atomic nuclei are surrounded by negatively charged electronSubatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. Electrons are important for understanding chemical reactions.
Elements
There are over 100 chemical elementA substance made of one type of atom only.. Each element has a unique number of protons in the nuclei of its atoms. This means that the nucleus of each element has a characteristic positive charge. For example:
- hydrogen nuclei have one proton, so each carry a +1 charge
- lithium nuclei have three protons, so each carry a +3 charge
The atomic numberThe number of protons in the nucleus of an atom. Also called the proton number. of an atom is the number of protons in its nucleus. For example:
- the atomic number of hydrogen is 1 because its nuclei each contain one proton
- the atomic number of lithium is 3 because its nuclei each contain three protons
Isotopes
Atoms of the same element can have different numbers of neutrons in the nuclei of their atoms. This means that atoms of the same element can differ in the mass of their nucleus.
The mass numberThe number of protons and neutrons found in the nucleus of an atom. of an atom is the total number of protons and neutrons in its nucleus. Isotopes of an element will have the same atomic number but a different mass number.
The mass number of an atom is never smaller than its atomic number. It can be the same as the atomic number but is usually greater.
There is a conventional way to represent the nuclei of different isotopes. The chemical symbol of the element is given with:
- mass number at the top left
- atomic number at the bottom left
For example: \(_{17}^{35}Cl\) is the symbol for a chlorine-35 nucleus.
Calculating protons and neutrons
The number of neutrons in a nucleus can be calculated:
- neutrons = (mass number) - (atomic number)
The number of neutrons in a chlorine-35 nucleus is (35 - 17) = 18.
Example
Lithium isotopes
Question
Lithium atoms exist naturally as two different isotopes, \(_{3}^{6}Li\) and \(_{3}^{7}Li\). Complete the table to show the numbers of subatomic particles in each isotope and the nuclear charges.
| Isotope | Name of isotope | Number of protons | Number of neutrons | Nuclear charge |
| Isotope | |
|---|---|
| Name of isotope | |
| Number of protons | |
| Number of neutrons | |
| Nuclear charge |
| Isotope | |
|---|---|
| Name of isotope | |
| Number of protons | |
| Number of neutrons | |
| Nuclear charge |
| Isotope | Name of isotope | Number of protons | Number of neutrons | Nuclear charge |
| \(_{3}^{6}Li\) | Lithium-6 | 3 | (6 - 3) = 3 | +3 |
| \(_{3}^{7}Li\) | Lithium-7 | 3 | (7 - 3) = 4 | +3 |
| Isotope | \(_{3}^{6}Li\) |
|---|---|
| Name of isotope | Lithium-6 |
| Number of protons | 3 |
| Number of neutrons | (6 - 3) = 3 |
| Nuclear charge | +3 |
| Isotope | \(_{3}^{7}Li\) |
|---|---|
| Name of isotope | Lithium-7 |
| Number of protons | 3 |
| Number of neutrons | (7 - 3) = 4 |
| Nuclear charge | +3 |
Question
Certain uranium isotopes are useful as a nuclear fuel. The atomic number of uranium is 92. Calculate the numbers of neutrons in the isotopes uranium-235 and uranium-238.
Uranium-235: (235 - 92) = 143 neutrons
Uranium-238: (238 - 92) = 146 neutrons