MCAT Content / Atomic Nucleus / Neutrons Protons Isotopes

Neutrons, protons, isotopes

Topic: Atomic Nucleus

The number of protons determines an element’s atomic number. The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom.

An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. For example, water is composed of hydrogen and oxygen atoms that have combined to form water molecules.

Atoms are made up of sub-atomic particles called protons, neutrons, and electrons, which are responsible for the mass and charge of atoms. An atom is composed of two regions: the nucleus, which is in the center of the atom and contains protons and neutrons. And the outer region of the atom, which holds its electrons in orbit around the nucleus. The number of protons is equal to the number of electrons in neutral atoms. This is the atomic number. Proton is positively charged (+1), a neutron is neutral in charge, and the electron is negatively charged (-1)

The number of neutrons can vary in the nucleus to produce isotopes, which are atoms of the same element that have different numbers of neutrons. Despite having different numbers of neutrons, isotopes of the same element have very similar physical and chemical properties. Some isotopes are unstable and will undergo radioactive decay to become other elements.

– Carbon-12 (or 12C) contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 amu (six protons and six neutrons).
– Carbon-14 (or 14C) contains six protons, eight neutrons, and six electrons; its atomic mass is 14 AMU (six protons and eight neutrons).

The predictable half-life of different decaying isotopes allows scientists to date material based on its isotopic composition, such as with Carbon-14 dating.


Practice Questions

Khan Academy

MCAT Official Prep (AAMC)

Official Guide C/P Section Passage 4 Question 18

Key Points

• The number of protons determines an element’s atomic number and is used to distinguish one element from another.

• The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess.

• The atomic mass is calculated by obtaining the mean of the mass numbers for its isotopes.

• Some isotopes are unstable and will undergo radioactive decay to become other elements.


Key Terms

radioactive decay: the process by which an unstable atomic nucleus loses energy by radiation

atom: The smallest possible amount of matter which still retains its identity as a chemical element, consisting of a nucleus surrounded by electrons.

proton: Positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element. It weighs 1 amu.

neutron: A subatomic particle forming part of the nucleus of an atom. It has no charge. It is equal in mass to a proton, or it weighs 1 amu.

electron: Each electron has a negative charge (-1) with weight so small it’s normally negligible as compared to proton or neutron.

element: atoms with the same number of protons

molecules: where atoms of the same element bond together

subatomic: particles that are smaller than atoms

nucleus: the centre of the cell containing protons and neutrons

AMU: an atomic mass unit is defined as precisely 1/12 the mass of an atom of carbon-12

half-life: the time it takes for half of the original concentration of an isotope to decay back to its more stable form.



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