What does the mass number represent?

What does the mass number represent?

• A. Sum of Protons and Neutrons
• B. Number of Protons
• C. Number of Electrons
• D. Atomic Weight

Answer: (A)

The mass number tells you how many particles are in the nucleus. It’s the total of protons and neutrons, so you add those two types of particles together. Electrons don’t count toward the mass number because they’re outside the nucleus and contribute almost no mass. For a given isotope, the mass number is an integer equal to protons plus neutrons, and it helps identify the specific isotope. Atomic weight, by contrast, is a weighted average of masses of all the element’s isotopes and is usually a decimal value, not the mass number of a single atom. The number of protons (the atomic number) identifies the element itself and is different from the mass number. For example, a nucleus with 6 protons and 6 neutrons has a mass number of 12.

The mass number tells you how many particles are in the nucleus. It’s the total of protons and neutrons, so you add those two types of particles together. Electrons don’t count toward the mass number because they’re outside the nucleus and contribute almost no mass. For a given isotope, the mass number is an integer equal to protons plus neutrons, and it helps identify the specific isotope. Atomic weight, by contrast, is a weighted average of masses of all the element’s isotopes and is usually a decimal value, not the mass number of a single atom. The number of protons (the atomic number) identifies the element itself and is different from the mass number. For example, a nucleus with 6 protons and 6 neutrons has a mass number of 12.