Every atom's mass is determined by the particles packed into its nucleus — the Atomic Mass Calculator takes your number of protons and number of neutrons and returns the atom's atomic mass in unified atomic mass units (u). Secondary outputs include the atomic mass in kilograms and the mass number (A), which is the total proton + neutron count.
Atomic Mass
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Atomic Mass in Kilograms
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Mass Number (A)
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Atomic mass is the mass of an atom, including its protons, neutrons, and electrons. It's usually expressed in atomic mass units (u) or daltons (Da), where 1 u equals 1/12 the mass of a carbon-12 atom.
To calculate atomic mass, add the mass of protons and neutrons. Each proton and neutron has approximately 1 atomic mass unit, so atomic mass ≈ number of protons + number of neutrons.
Mass number is the total count of protons and neutrons (always a whole number), while atomic mass is the actual mass measured in atomic mass units (can have decimal places due to binding energy effects).
Neutrons help stabilize the atomic nucleus by providing the strong nuclear force without adding electrical charge. They prevent protons from repelling each other due to electromagnetic force.
An atomic mass unit (u) is a unit of mass equal to 1/12 the mass of a carbon-12 atom. One u equals approximately 1.66 × 10⁻²⁷ kg.
Carbon-12 has an atomic mass of exactly 12.000000 u by definition, as it serves as the reference standard for atomic mass units. It has 6 protons and 6 neutrons.