MCAT Content / Atomic Nucleus / Atomic Number Atomic Weight

Atomic number, atomic weight

Topic: Atomic Nucleus

The atomic number is equal to the number of protons in an atom’s nucleus. The atomic mass is the sum of the mass of protons and neutrons.

Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams, which scientists define as one atomic mass unit (AMU). Together, the number of protons and the number of neutrons determine an element’s mass number.

The number of protons determines an element’s atomic number (Z) and distinguishes one element from another. For example, carbon’s atomic number (Z) is six because it has 6 protons. 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. The average atomic mass of an atom takes into account all its naturally occurring isotopes.


Key Points

• An atom is composed of two regions: the nucleus, which is in the centre of the atom and contains protons and neutrons, and the outer region of the atom, which holds its electrons in orbit around the nucleus.

• Charges of proton (+1), neutrons (0) and electron (-1). Neutral atoms of each element contain an equal number of protons and electrons.

• Proton and Neutron have approximately the same mass, about 1.67 × 10-24 grams, or one atomic mass unit (AMU) or one Dalton.

• Atomic mass is the sum of the mass of protons and neutrons of that atom.


Key Terms

mass number: the sum of the number of protons and the number of neutrons in an atom.

isotope: any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei.

atomic mass: the sum of the mass of protons and neutrons of that atom.

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

atomic number: the number of protons in the nucleus of an atom

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



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