Edexcel GCSE Chemistry (1CH0) — Key Concepts in Chemistry
Every element is defined by the number of protons in its nucleus. However, atoms of the same element can have different numbers of neutrons. These variations are called isotopes.
Isotopes are defined as different atoms of the same element that contain an identical number of protons but a different number of neutrons within their nuclei (the plural of nucleus). Because they belong to the same element, all isotopes of a specific element share the same atomic number (). However, because their neutron counts vary, they have a different mass number ().
Almost all of an atom's mass is concentrated in the tiny, dense, positively charged nucleus at its centre. The nucleus contains nucleons, which is the collective name for protons and neutrons. Electrons orbit the nucleus in shells and have a negligible mass.
The properties of these subatomic particles are:
You can determine the exact number of neutrons in a specific isotope using the following calculation:
Isotopes are often written in "dash" notation (e.g., Carbon-12) or nuclear symbol notation ().
Naturally occurring chlorine exists as two main isotopes: Chlorine-35 and Chlorine-37.
In 'Describe isotopes' questions, Edexcel examiners look for two specific points: (1) same number of protons (or same atomic number) AND (2) different number of neutrons (or different mass number). Failing to mention 'same element' or 'same number of protons' often results in zero marks.
Terminology Tip: Use the plural term 'nuclei' when describing where the neutrons differ across various isotopes of an element.
Do not confuse 'mass number' (the whole number for a single isotope) with 'relative atomic mass' (the weighted average for the element found on the Periodic Table).
Isotopes
Different atoms of the same element that contain the same number of protons but different numbers of neutrons in their nuclei.
Protons
Positively charged subatomic particles found in the nucleus of an atom, with a relative mass of 1 and a relative charge of +1.
Neutrons
Neutral subatomic particles found in the nucleus of an atom, with a relative mass of 1 and a relative charge of 0.
Nuclei
The plural of nucleus; the tiny, dense, positively charged regions at the centre of atoms containing protons and neutrons.
Nucleus
The central part of an atom where almost all of its mass is concentrated, containing nucleons (protons and neutrons).
Atomic number (Z)
The number of protons in the nucleus of an atom, which is unique to each element.
Mass number (A)
The total number of protons and neutrons in the nucleus of an atom.
Nucleons
The collective name given to protons and neutrons because they are located in the nucleus.
Electrons
Negatively charged subatomic particles that orbit the nucleus, with a relative charge of -1 and a negligible mass of roughly 1/1836.
Put your knowledge into practice — try past paper questions for Chemistry
Isotopes
Different atoms of the same element that contain the same number of protons but different numbers of neutrons in their nuclei.
Protons
Positively charged subatomic particles found in the nucleus of an atom, with a relative mass of 1 and a relative charge of +1.
Neutrons
Neutral subatomic particles found in the nucleus of an atom, with a relative mass of 1 and a relative charge of 0.
Nuclei
The plural of nucleus; the tiny, dense, positively charged regions at the centre of atoms containing protons and neutrons.
Nucleus
The central part of an atom where almost all of its mass is concentrated, containing nucleons (protons and neutrons).
Atomic number (Z)
The number of protons in the nucleus of an atom, which is unique to each element.
Mass number (A)
The total number of protons and neutrons in the nucleus of an atom.
Nucleons
The collective name given to protons and neutrons because they are located in the nucleus.
Electrons
Negatively charged subatomic particles that orbit the nucleus, with a relative charge of -1 and a negligible mass of roughly 1/1836.
