First of all, "atomic mass number" is an oxymoron. There is one value that is called atomic mass, and there is a completely different value that is called mass number. Granted, they will be approximately the same number.
Atomic number is the quantity of protons in the element's atoms' nuclei. This defines the element itself. For instance, all carbon atoms have exactly 6 proton them. Change the quantity of protons, and you change the identity of the element.
While qty of protons is specifies the element, the quantity of neutrons isn't necessarily the same. It really can be any number. There is a general trend based upon stable combinations. To specify the quantity of neutrons in a nucleus for any given VERSION (isotope) of the element, the information is embedded in the value called mass number for the isotope.
The mass number indicates the total quantity of neutrons and protons for the element. In otherwords, how many particles are there of significant mass within the element's atoms? To get the number of neutrons, you need to subtract out the atomic number from the mass number.
It is important to note that the mass number is just a nameplate of an isotope. It is always an integer, as it is a counting of particles. It isn't necessarily equal to the mass of anything (although it will be close).
The mass of a proton and the mass of a neutron aren't necessarily always the same value. Other nuclear factors come in to play when determining the mass of any given isotope. Only Carbon-12 atoms have a mass of exactly 12 amu by definition. Every other element's isotopes will have an isotope mass that is slightly different, due to these other nuclear factors.
Since we don't always want to think about isotopes, it helps to have an averaging strategy when we are interested in mass of a bulk sample. For this reason, we use the atomic mass as a "weighted average" based upon isotope mass and isotope abundance. It adds up each isotope's mass (not mass number, but isotope mass) multiplied with the isotope abundance percentage. This gives a working value for mass per average atom that can be put to use to understand mass of a bulk sample in terms of qty of atoms.
Atomic number is the quantity of protons in the element's atoms' nuclei. This defines the element itself. For instance, all carbon atoms have exactly 6 proton them. Change the quantity of protons, and you change the identity of the element.
While qty of protons is specifies the element, the quantity of neutrons isn't necessarily the same. It really can be any number. There is a general trend based upon stable combinations. To specify the quantity of neutrons in a nucleus for any given VERSION (isotope) of the element, the information is embedded in the value called mass number for the isotope.
The mass number indicates the total quantity of neutrons and protons for the element. In otherwords, how many particles are there of significant mass within the element's atoms? To get the number of neutrons, you need to subtract out the atomic number from the mass number.
It is important to note that the mass number is just a nameplate of an isotope. It is always an integer, as it is a counting of particles. It isn't necessarily equal to the mass of anything (although it will be close).
The mass of a proton and the mass of a neutron aren't necessarily always the same value. Other nuclear factors come in to play when determining the mass of any given isotope. Only Carbon-12 atoms have a mass of exactly 12 amu by definition. Every other element's isotopes will have an isotope mass that is slightly different, due to these other nuclear factors.
Since we don't always want to think about isotopes, it helps to have an averaging strategy when we are interested in mass of a bulk sample. For this reason, we use the atomic mass as a "weighted average" based upon isotope mass and isotope abundance. It adds up each isotope's mass (not mass number, but isotope mass) multiplied with the isotope abundance percentage. This gives a working value for mass per average atom that can be put to use to understand mass of a bulk sample in terms of qty of atoms.
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atomic number = number of protons in atom
atomic mass = number of protons + neutrons in atom
atomic mass = number of protons + neutrons in atom