# Relationship of atomic mass and number

### Atomic mass - Wikipedia There is a difference between the meanings of atomic mass and mass number. Here is an explanation of the terms and how you can keep them. Define atomic and mass numbers. Determine the number of protons, neutrons, and electrons in an atom. Identify the charge and relative mass. Mass number is just a count of the number of nucleons in an atomic nucleus. Atomic Mass is the actual mass of a given nucleus.

The relative isotopic mass, then, is the mass of a given isotope specifically, any single nuclidewhen this value is scaled by the mass of carbonwhen the latter is set equal to For example, the relative isotopic mass of a carbon atom is exactly For comparison, the atomic mass of a carbon atom is exactly 12 daltons or 12 unified atomic mass units. Alternately, the atomic mass of a carbon atom may be expressed in any other mass units: As in the case of atomic mass, no nuclides other than carbon have exactly whole-number values of relative isotopic mass.

As is the case for the related atomic mass when expressed in unified atomic mass units or daltonsthe relative isotopic mass numbers of nuclides other than carbon are not whole numbers, but are always close to whole numbers.

### Structure of the Atom

This is discussed more fully below. Similar terms for different quantities[ edit ] The atomic mass and relative isotopic mass are sometimes confused, or incorrectly used, as synonyms of standard atomic weight also known as atomic weight and the standard atomic weight a particular variety of atomic weight, in the sense that is a standardized atomic weight. However, as noted in the introduction, atomic weight and standard atomic weight represent terms for abundance-weighted averages of atomic masses in elemental samples, not for single nuclides. As such, atomic weight and standard atomic weight often differ numerically from relative isotopic mass and atomic mass, and they can also have different units than atomic mass when this quantity is not expressed in unified atomic mass units see the linked article for atomic weight. The atomic mass or relative isotopic mass of each isotope and nuclide of a chemical element is therefore a number that can in principle be measured to a very great precision, since every specimen of such a nuclide is expected to be exactly identical to every other specimen, as all atoms of a given type in the same energy state, and every specimen of a particular nuclide, are expected to be exactly identical in mass to every other specimen of that nuclide.

For example, every atom of oxygen is expected to have exactly the same atomic mass relative isotopic mass as every other atom of oxygen However, such an error can exist and even be important when considering individual atoms for elements that are not mononuclidic. For non-mononuclidic elements that have more than one common isotope, the numerical difference in relative atomic mass atomic weight from even the most common relative isotopic mass, can be half a mass unit or more e.

### Mass number - Wikipedia

The atomic mass relative isotopic mass of an uncommon isotope can differ from the relative atomic mass, atomic weight, or standard atomic weight, by several mass units. Atomic masses expressed in unified atomic mass units i. The ratio of atomic mass to mass number number of nucleons varies from about 0. Since neutrons do not affect the charge, the number of neutrons is not dependent on the number of protons and will vary even among atoms of the same element.

An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 its nucleus contains 8 protons is an oxygen atom, and any atom with a different number of protons would be a different element.

The periodic table see figure below displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus.

## Mass number

Lithium atoms have three protons, beryllium atoms have four, and so on. Since atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Helium, with two protons, will have two electrons.