          # Atomic mass

• this article needs to be updated. in particular: it needs to reflect the 2019 redefinition of the si base units, which came into effect on 20 may 2019. please update this article to reflect recent events or newly available information. (january 2020) stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~​14300th of the mass of the nucleus). it has a mass of 7.016 da. rare lithium-6 (mass of 6.015 da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941.

the atomic mass (ma or m) is the mass of an atom. although the si unit of mass is kilogram (symbol: kg), the atomic mass is often expressed in the non-si unit dalton (symbol: da, or u) where 1 dalton is defined as ​112 of the mass of a single carbon-12 atom, at rest. the protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. thus, the atomic mass measured in da has nearly the same value as the mass number. conversion between mass in kg and mass in da can be done using the atomic mass constant .

the formula used for conversion is: where n a {\displaystyle n_{\rm {a}}} is the avogadro constant and is the experimentally determined molar mass of carbon-12.

the relative isotopic mass (see section below) can be obtained by dividing the atomic mass ma of an isotope by the atomic mass constant mu yielding a dimensionless value. thus, the atomic mass of a carbon-12 atom is 12 da (or 12 u), but the relative isotopic mass of a carbon-12 atom is simply 12. the sum of relative isotopic masses of all atoms in a molecule is the relative molecular mass.

the atomic mass of an isotope and the relative isotopic mass refers to a certain specific isotope of an element. because usually substances are not isotopially pure, it is convenient to use the elemental atomic mass which is the average (mean) atomic mass of an element, weighted by the abundance of the isotopes. the dimensionless (standard) atomic weight is the weighted mean relative isotopic mass of a (typical naturally-occurring) mixture of isotopes.

the atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to binding energy mass loss (as per e = mc2).

• relative isotopic mass
• similar terms for different quantities
• mass defects in atomic masses
• measurement of atomic masses
• relationship between atomic and molecular masses
• history

## Not to be confused with Standard atomic weight, Mass number, or Relative atomic mass. For the band, see Atomic Mass (band). This article needs to be updated. In particular: it needs to reflect the 2019 redefinition of the SI base units, which came into effect on 20 May 2019. Please update this article to reflect recent events or newly available information. (January 2020) Stylized lithium-7 atom: 3 protons, 4 neutrons, and 3 electrons (total electrons are ~​1⁄4300th of the mass of the nucleus). It has a mass of 7.016 Da. Rare lithium-6 (mass of 6.015 Da) has only 3 neutrons, reducing the atomic weight (average) of lithium to 6.941. The atomic mass (ma or m) is the mass of an atom. Although the SI unit of mass is kilogram (symbol: kg), the atomic mass is often expressed in the non-SI unit dalton (symbol: Da, or u) where 1 dalton is defined as ​1⁄12 of the mass of a single carbon-12 atom, at rest. The protons and neutrons of the nucleus account for nearly all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, the atomic mass measured in Da has nearly the same value as the mass number. Conversion between mass in kg and mass in Da can be done using the atomic mass constant $m_{\rm {u}}$ . The formula used for conversion is: $1\ {\rm {Da}}=m_{\rm {u}}={M_{\rm {u}} \over {N_{\rm {A}}}}={M(^{12}C) \over {12\ N_{\rm {A}}}}=1.66053906660(50)\cdot 10^{-27}\ {\rm {kg}}$ where $M_{\rm {u}}$ N A {\displaystyle N_{\rm {A}}} is the Avogadro constant and $M(^{12}C)$ is the experimentally determined molar mass of carbon-12. The relative isotopic mass (see section below) can be obtained by dividing the atomic mass ma of an isotope by the atomic mass constant mu yielding a dimensionless value. Thus, the atomic mass of a carbon-12 atom is 12 Da (or 12 u), but the relative isotopic mass of a carbon-12 atom is simply 12. The sum of relative isotopic masses of all atoms in a molecule is the relative molecular mass. The atomic mass of an isotope and the relative isotopic mass refers to a certain specific isotope of an element. Because usually substances are not isotopially pure, it is convenient to use the elemental atomic mass which is the average (mean) atomic mass of an element, weighted by the abundance of the isotopes. The dimensionless (standard) atomic weight is the weighted mean relative isotopic mass of a (typical naturally-occurring) mixture of isotopes. The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to binding energy mass loss (as per E = mc2). Contents 1 Relative isotopic mass 2 Similar terms for different quantities 3 Mass defects in atomic masses 4 Measurement of atomic masses 5 Relationship between atomic and molecular masses 6 History 7 See also 8 References 9 External links

Other Languages
Afrikaans: Atoommassa
العربية: كتلة ذرية
aragonés: Peso atomico
asturianu: Masa atómica
azərbaycanca: Atom kütləsi
Bân-lâm-gú: Goân-chú chit-liōng
български: Атомна маса
bosanski: Atomska masa
brezhoneg: Mas atomek
dansk: Atommasse
Deutsch: Atommasse
eesti: Aatommass
Ελληνικά: Ατομικό βάρος
español: Masa atómica
Esperanto: Atomomaso
euskara: Masa atomiko
فارسی: جرم اتمی
Fiji Hindi: Atomic mass
français: Masse atomique
Frysk: Atoommassa
한국어: 원자 질량
Bahasa Indonesia: Massa atom
íslenska: Atómmassi
italiano: Massa atomica
ქართული: ატომური მასა
Kiswahili: Uzani atomia
Kreyòl ayisyen: Mas atomik
Кыргызча: Атомдук масса
latviešu: Atommasa
Lëtzebuergesch: Atommass
lietuvių: Atominė masė
Limburgs: Atoomgewich
la .lojban.: na'orteryratni
lumbaart: Massa atomica
magyar: Atomtömeg
македонски: Атомска маса
मराठी: अणुभार
Bahasa Melayu: Jisim atom
монгол: Атом масс
Nederlands: Atoommassa

norsk: Atommasse
norsk nynorsk: Atommasse
oʻzbekcha/ўзбекча: Atom massasi
پنجابی: ایٹمی بھار
Piemontèis: Massa atòmica
Plattdüütsch: Atommass
polski: Masa atomowa
português: Massa atômica
română: Masă atomică
Runa Simi: Iñuku wisnu
Seeltersk: Atommasse
Simple English: Atomic mass
slovenčina: Atómová hmotnosť
slovenščina: Atomska teža
Soomaaliga: Cuf atam
српски / srpski: Atomska masa
srpskohrvatski / српскохрватски: Relativna atomska masa
Sunda: Massa atom
suomi: Atomimassa
svenska: Atommassa
தமிழ்: அணு நிறை
Taqbaylit: Takura tabelkamt
Türkçe: Atom kütlesi
українська: Атомна маса
Tiếng Việt: Nguyên tử khối
Winaray: Masa atomika  