Relative atomic mass is a way to calculate atomic mass becauseatomThe actual mass of, for examplehydrogen atomThe actual mass of is 1.674 × 10 ⁻ ² ⁷ kg, oneOxygen atomIts mass is 2.657 × 10 ⁻ ² ⁶ kg.The mass of a carbon-12 atom is 1.993 × 10 ⁻ ² ⁶ kg.The relative atomic mass of an element isisotopeRelative atomic massweightingAverage.periodic table of ele mentsThe bottom number in is the relative atomic mass.[1]
）It refers to the ratio of the real mass of any atom to 1/12 of the mass of a carbon-12 atom, which is called the relative atomic mass of the atom.[2]The atomic weight is the unit of mass, with the symbol U, which is defined as 1/12 of the atomic mass of carbon 12.
development
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Atomic weight was first determined by British scientistsDaltonPut forward.He said that "atoms of the same element have the same weight, and atoms of different elements have different weights." Therefore, atomic weight is translated into "atomic weight" in Chinese.However, since weight and mass are the same concepts at that time, they are still called atomic weight although the relative mass of atoms is obtained in practice.
In 1803, Dalton usedhydrogenThe atomic weight of is 1 as the reference of relative atomic weight.
In 1826,Jons Jakob Berzelius Change toOxygen atom1/100 of the quantity as the benchmark;In 1860,J. - S. STAIt is suggested to use 1/16 of the atomic weight of oxygen as the reference, which has been used for a long time.
In 1929, W.F. Giorgio and H50. Jansden found that there issixteenO、seventeenO、eighteenThe distribution of O isotopes in nature is not completely uniform, so it is inappropriate to use natural oxygen as the atomic weight standard.Later, the physics community used 1/16 of O as the atomic weight benchmark, and the chemical community also used the original benchmark. Since then, there have been two scales of atomic weight. In 1940, the International Atomic Weight Commission decided to use 1.000275 as the conversion factor of the two scales: physical atomic weight=1.000275 × chemical atomic weight.The existence of two scales is bound to often cause confusion.
In 1959MunichAt the International Union of Pure and Applied Physics (IUPAP for short), JH. Martauch's suggestionC=12.0000As the atomic weight benchmark, it was submitted to the International Union of Pure and Applied Chemistry for consideration, which accepted this proposal in 1960.In 1961, this new benchmark was officially adopted at the International Union of Pure and Applied Chemistry held in Montreal.In 1979, byInternational Committee on Relative Atomic MassThe definition of atomic weight is proposed.
characteristic
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When we calculate the molecular weight of a water molecule, we will find that it is extremely inconvenient to calculate.If other more complexmolecular mass Then it's even more troublesome.Therefore, the relative atomic mass andrelative molecular massTo representatom、moleculeQuality relationship.The relative molecular mass is numerically equal toMolar mass, but the unit is different.The unit of relative molecular weight is "1", while the unit of molar mass is g/mol.
International regulations
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Divide the mass of a carbon-12 atom into 12 equal parts（carbon atomThere are several kinds, one of which is carbon atomNucleusIncluding 6protonAnd 6neutronIt adds up to 12, so we call it carbon-12.Of course, there are others such asCarbon-14It contains 8 neutrons and 6 protons, which add up to 14.The reason why carbon-12 is chosen instead of carbon-14 internationally is that when carbon-12 is chosen as the standard, the relative atomic masses of other atoms areNear Integer, easy to remember and use).The mass of each portion is: the mass of an atom/(1/12 × the mass of a carbon-12 atom)=the mass of an atom/1.661e ⁻ ² ⁷ kg.
computing method
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（1.993e⁻²⁶）/12=1.661e⁻²⁷Kg。Then compare the actual mass of some other atom with this number, and the value of this result is called the relative atomic mass of this atom.asOxygen atomThe calculation method of the relative atomic mass of the oxygen atom is: (2.657e ⁻ ² ⁶)/(1.661e ⁻ ² ⁷)=16 (approximately), that is, the relative atomic mass of the oxygen atom is about 16. We use 16 in our calculation.This is much easier.
The relative atomic mass of other atoms is calculated in the same way.
The international basic unit of relative atomic mass is 1.
The concept of relative atomic mass is based oncarbon atom(There are 6 in the nucleusprotonAnd 6neutron1/12 (about 1.661e ⁻⁻⁷⁷⁷ kg) of the mass of a carbon atom of C-12) as the standard, and the ratio of the mass of other atoms to it is the relative atomic mass of this atom.
The actual mass of an atom of the atom (kg)=the relative atomic mass of the atom x 1/12 (kg) ne of the actual mass of a carbon-12 atom.
The mass of 1mol material is calledMolar mass, the unit is generally g/mol.
The molar mass of an atom (molecule, ion) is numerically equal to its relative atomic mass（Formula quantity）, but please note that only when the atom, moleculeIonsOfMolar massWhen the unit of is g/mol, this rule is met.
formula
From "the atomic mass is mainly concentrated on the nucleus"
The derivation formula for calculating the relative atomic mass can be obtained:
Relative atomic mass
=The mass of an atom/(1/12) of carbon atomic mass
=Nuclear mass+extranuclear electron mass/[（1/12]mC
≈ Nuclear mass/（1/12）mC
=Mass of proton+mass of neutron/（1/12）mC
=[Proton number*Mass of a proton+number of neutrons * mass of a neutron]/（1/12）mC
=[Proton number * (1/12) mc+neutron number * (1/12) mC]/（1/12）mC
*These are elements without stable isotopesOne or more known isotopes are listed in "Table 3" in "Reference Materials", and appropriate relative atomic mass and half-life are given.However, the three elements (thorium, protactinium and uranium) have certain characteristic surface isotope compositions, so the atomic weights of these elements are also listed in the table above.
Common atomic weight
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Usually, both junior high school and senior high school use integers or at most one decimal point. The following table is attached:
Nuclear charge number
Element Name
Element symbol
Relative atomic mass
one
hydrogen
H
one
two
helium
He
four
six
carbon
C
twelve
seven
nitrogen
N
fourteen
eight
oxygen
O
sixteen
nine
fluorine
F
nineteen
ten
neon
Ne
twenty
eleven
sodium
Na
twenty-three
twelve
magnesium
Mg
twenty-four
thirteen
aluminum
Al
twenty-seven
fourteen
silicon
Si
twenty-eight
fifteen
phosphorus
P
thirty-one
sixteen
sulfur
S
thirty-two
seventeen
chlorine
Cl
35.5（It can only be written as 35.5）
eighteen
argon
Ar
forty
nineteen
potassium
K
thirty-nine
twenty
calcium
Ca
forty
twenty-five
manganese
Mn
fifty-five
twenty-six
iron
Fe
fifty-six
twenty-nine
copper
Cu
63.5（Can write 64 or 63.5）
thirty
zinc
Zn
sixty-five
forty-seven
silver
Ag
one hundred and eight
fifty-three
iodine
I
one hundred and twenty-seven
fifty-six
barium
Ba
one hundred and thirty-seven
seventy-eight
platinum
Pt
one hundred and ninety-five
seventy-nine
gold
Au
one hundred and ninety-seven
eighty
mercury
Hg
two hundred and one
Defining differences
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Difference between relative atomic mass and atomic mass
The mass of an atom is the actual mass or real mass of the substance that actually exists in the atom. The relative atomic mass is the relative mass of the substance represented by 1/12 of the atomic mass of C as the standard. The unit of atomic mass is kg, and the unit of relative atomic mass is 1. The atomic mass is mainly concentrated on the atomic nucleus
Among the three particles constituting the atom, the mass of one proton and one neutron is about 1 compared with the standard of relative atomic mass (that is, 1/12 of the mass of a carbon 12 atom). The mass of electrons is very small and can be ignored. Therefore, the mass of atoms is mainly concentrated on protons and neutrons (that is, atomic nuclei). Therefore, the relative atomic mass is ≈ the number of protons+the number of neutrons[3]
