Calculadora de Massa Atômica
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
m = Z·mp + (A−Z)·mn ; mp = 1.00728 u ; mn = 1.00867 u
About this calculator
The Atomic Mass Calculator estimates the approximate mass of an atom based on the number of protons (Z) and neutrons (A−Z). It uses the formula m = Z·mp + (A−Z)·mn, where mp = 1.00728 u and mn = 1.00867 u. The result is given in atomic mass units (u), making it easy to understand nuclear composition.
This tool is useful for chemistry and physics students who want to quickly check the mass of specific isotopes. For example, for carbon-12 (Z=6, A=12), the calculator gives a value close to 12 u, helping to visualize the contribution of protons and neutrons. Keep in mind that the actual mass of an atom differs slightly due to nuclear binding energy, but this approximation is sufficient for many introductory calculations.
To use the calculator, enter the atomic number (Z) and mass number (A) of the desired isotope. The result is instantaneous and can be used to compare different isotopes or to check general chemistry exercises. Caution: the formula does not account for electrons (negligible mass) or binding energy, so it should not replace tabulated values for precise applications.
Frequently asked questions
Why does the calculated mass differ from the tabulated atomic mass?
The formula ignores nuclear binding energy, which reduces the actual mass. For precise values, consult atomic mass tables.
Can I use this calculator for ions?
Yes, because the mass of electrons is negligible (about 0.0005 u). The formula only considers protons and neutrons.
What does 'u' in the answer mean?
u is the atomic mass unit, where 1 u = 1.660539 × 10⁻²⁷ kg. It is used to express atomic and molecular masses.
What is the difference between mass number (A) and atomic mass?
Mass number is the sum of protons and neutrons (integer). Atomic mass is the actual mass of the atom, usually non-integer due to binding energy.
Does this calculator work for all elements?
Yes, as long as you know Z and A of the isotope. For elements without stable isotopes, use the most common isotope.