The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. Also called nucleon number . The number of protons plus the number of neutrons is called the mass number. Atomic mass value sometimes change over time in publications as scientists revise the natural isotope abundance of elements. In effect all atoms are isotopes of one element or another. Sodium has 11 protons and 12 neutrons. Number of neutrons = mass number − atomic number. The value of A is calculated by summing the number of protons and neutron in the nucleus of an atom. It is a whole number. Related Terms & Definitions. mass number synonyms, mass number pronunciation, mass number translation, English dictionary definition of mass number. Here are some related terms and definitions we found for this term. It is defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The dalton or unified atomic mass unit (symbols: Da or u) is a unit of mass widely used in physics and chemistry. For example, the most common isotope of carbon has 6 protons and 6 neutrons. Notation and Formula. What is the Mass Number? n. The sum of the number of neutrons and protons in an atomic nucleus. Mass number is the number of nucleons (the sum of protons and neutrons) present in an atom. The mass number is the sum of the number of protons and neutrons in an atom. However, since everything on Earth is subject to about the same gravity, we usually drop the "g" part of the equation and just report weight in the same units as mass. The mass number of an atom is never smaller than the atomic number. These variants have the same atomic number but a different mass number. The mass number is 11 + 12 = 23. Mass number is also called as atomic number or nucleon number. To determine this, you would subtract as shown: 52 − 24 = 28 neutrons in a chromium atom. The atomic mass constant, denoted m u is defined identically, giving m u = m(12 C)/12 = 1 Da.. It cannot be a fraction. » Neutron » Nuclide Symbol » Beta Particle Decay » Electron » Mass Spectrometer » Coordination Number » Atomic Mass Unit (AMU) » Molar Mass » Mass It can be the same, but is usually bigger. Where is the mass number and atomic number on a full chemical symbol? The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus. Define mass number. However, it exists in more variants as He-2, He-3, He-5, He-6, He-7, He-8, He-9, He-10. Isotope definition. The mass number is the total number of particles in the nucleus. Mass number is denoted as A. Definition. For example, the mass number of argon atoms and calcium atoms can both be 40. 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