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# Mass number

The mass number (symbol A, from the German word Atomgewichte (atomic weight), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It determines the atomic mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus:. The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number:. This is technically redundant, as each element is defined by its atomic number, so it is often omitted. [1]

## Alpha decay

Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into an atom with a mass number that is reduced by four and an atomic number that is reduced by two.

## Alpha particle

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.

## Atom

An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.

## Atomic mass

The atomic mass (ma) is the mass of an atom.

## Atomic nucleus

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment.

## Atomic number

The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of an atom.

## Baryon

A baryon is a composite subatomic particle made up of three quarks (a triquark, as distinct from mesons, which are composed of one quark and one antiquark).

## Baryon number

In particle physics, the baryon number is a strictly conserved additive quantum number of a system.

## Beta decay

In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta ray (fast energetic electron or positron) and a neutrino are emitted from an atomic nucleus.

## Binding energy

Binding energy (also called separation energy) is the minimum energy required to disassemble a system of particles into separate parts.

## Bromine

Bromine is a chemical element with symbol Br and atomic number 35.

## Carbon

Carbon (from carbo "coal") is a chemical element with symbol C and atomic number 6.

## Carbon-12

Carbon-12 is the more abundant of the two stable isotopes of carbon (Carbon-13 being the other), amounting to 98.93% of the element carbon; its abundance is due to the triple-alpha process by which it is created in stars.

## Carbon-14

Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.

## Chemical element

A chemical element is a species of atoms having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z).

## Electron

The electron is a subatomic particle, symbol or, whose electric charge is negative one elementary charge.

## Gamma ray

A gamma ray or gamma radiation (symbol γ or \gamma), is penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei.

## Ion

An ion is an atom or molecule that has a non-zero net electrical charge (its total number of electrons is not equal to its total number of protons).

## Isotope

Isotopes are variants of a particular chemical element which differ in neutron number.

## Isotopes of bromine

Bromine (35Br) has two stable isotopes, 79Br and 81Br, and 30 known radioisotopes, the most stable of which is 77Br, with a half-life of 57.036 hours.

## Isotopes of chlorine

Chlorine (17Cl) has 24 isotopes with mass numbers ranging from 28Cl to 51Cl and 2 isomers (34mCl and 38mCl).

## Isotopes of nitrogen

Natural nitrogen (7N) consists of two stable isotopes, nitrogen-14, which makes up the vast majority of naturally occurring nitrogen, and nitrogen-15, which is less common.

## Isotopes of thorium

Although thorium (90Th) has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, 232Th, is relatively stable, with a half-life of 1.405&times;1010 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe.

## Mass excess

The mass excess of a nuclide is the difference between its actual mass and its mass number in atomic mass units.

## Mass–energy equivalence

In physics, mass–energy equivalence states that anything having mass has an equivalent amount of energy and vice versa, with these fundamental quantities directly relating to one another by Albert Einstein's famous formula: E.

## Metastability

In physics, metastability is a stable state of a dynamical system other than the system's state of least energy.

## Mole (unit)

The mole, symbol mol, is the SI unit of amount of substance.

## Neutrino

A neutrino (denoted by the Greek letter ν) is a fermion (an elementary particle with half-integer spin) that interacts only via the weak subatomic force and gravity.

## Neutron

| magnetic_moment.

## Neutron number

The neutron number, symbol N, is the number of neutrons in a nuclide.

## Nuclear binding energy

Nuclear binding energy is the minimum energy that would be required to disassemble the nucleus of an atom into its component parts.

## Nuclear isomer

A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons).

## Nucleon

In chemistry and physics, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus.

## Proton

| magnetic_moment.

Radioactive decay (also known as nuclear decay or radioactivity) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, gamma ray, or electron in the case of internal conversion.

## Radioactive displacement law of Fajans and Soddy

The law of radioactive displacements, also known as Fajans and Soddy law, in radiochemistry and nuclear physics, is a rule governing the transmutation of elements during radioactive decay.

## Relative atomic mass

Relative atomic mass (symbol: A) or atomic weight is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to one unified atomic mass unit.

## Standard atomic weight

The standard atomic weight (Ar, standard, a relative atomic mass) is the atomic weight (Ar) of a chemical element, as appearing and met in the earthly environment.

## Subscript and superscript

A subscript or superscript is a character (number, letter or symbol) that is (respectively) set slightly below or above the normal line of type.

## Unified atomic mass unit

The unified atomic mass unit or dalton (symbol: u, or Da) is a standard unit of mass that quantifies mass on an atomic or molecular scale (atomic mass).

## Uranium-238

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%.

## References

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