WHAT IS ATOMIC MASS UNIT- (amu)
As we know there should be some standard(invariant) physical constant for measurement mass. an atom made up with proton, neutron, and electron.
the nucleus consists of a proton as well as neutron, this two-particle also known as nucleons which contribute (almost) the total mass of an atom.
these nucleons are basic unit of mass of an atom or simply we say atomic mass unit (amu).
When the mass is expressed in amu, it roughly reflects the sum of the number of protons and neutrons in the atomic nucleus.
It is defined precisely as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state.
Its symbol is represented by "u" (unified atomic mass unit) or Da (Dalton), although amu may still be used.
amu is the unit that was based on oxygen-16 until 1961 when it was redefined based on carbon-12.
why is carbon-12 taken as a standard reference for measuring atomic masses? why not oxygen?
As we know there should be some standard(invariant) physical constant for measurement mass. an atom made up with proton, neutron, and electron.
the nucleus consists of a proton as well as neutron, this two-particle also known as nucleons which contribute (almost) the total mass of an atom.
these nucleons are basic unit of mass of an atom or simply we say atomic mass unit (amu).
When the mass is expressed in amu, it roughly reflects the sum of the number of protons and neutrons in the atomic nucleus.
It is defined precisely as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state.
Its symbol is represented by "u" (unified atomic mass unit) or Da (Dalton), although amu may still be used.
amu is the unit that was based on oxygen-16 until 1961 when it was redefined based on carbon-12.
why is carbon-12 taken as a standard reference for measuring atomic masses? why not oxygen?
The vast majority of all carbon found on Earth is 12C. Almost 99% of all carbon on Earth is of this form. While only approximately 1% of all carbon on Earth is of the 13C isotopic form, 14C is still much rarer. Only one out of every trillion carbon atoms is 14C.
Carbon-12 is the standard while measuring the atomic masses. Because no other nuclides other than carbon-12 have exactly whole-number masses in this scale. This is due to two factors:
A) the different mass of neutrons and protons acting to change the total mass in nuclides with proton/neutron ratios other than the 1:1 ratio of carbon-12; and
B) an exact whole-number will not be located if there exists a loss/gain of mass to difference in mean binding energy relative to the mean binding energy for carbon-12.
Oxygen was used as the standard for quite some time, but the results seem to be different in certain experiments. This was because oxygen-17 and oxygen-18 are also present in natural oxygen this led to 2 different tables of atomic mass. The unified scale based on carbon-12, 12C, met the physicists' need to base the scale on a pure isotope while being numerically close to the chemists' scale.
How can we prove that 1 amu is equal to $1.66\times 10^{-27} kg$?
consider a sample of C-12 and take exactly 1mole of it i.e 12g.
now we know that 1 mole of carbon sample contains $6.022\times 10^{23}$ atoms of carbon-12
$\therefore$ mass of single carbon -12 atom is $ \dfrac {12g}{6.022\times 10^{23}}$
mass of single carbon-12=1.992693457$\times 10^{-23}$g
This is the mass of a single neutral carbon-12 atom.
as $\dfrac {1}{12}$ of the mass of an unbound neutral atom of carbon-12 is 1 amu
therefore, $1amu = \dfrac {1}{12}$ mass of single carbon -12
$1amu =\dfrac {1.99269345}{12}\times 10^{-23}$ g
mass of single carbon-12=1.992693457$\times 10^{-23}$g
This is the mass of a single neutral carbon-12 atom.
as $\dfrac {1}{12}$ of the mass of an unbound neutral atom of carbon-12 is 1 amu
therefore, $1amu = \dfrac {1}{12}$ mass of single carbon -12
$1amu =\dfrac {1.99269345}{12}\times 10^{-23}$ g
1amu = $0.1665077881\times 10^{-23}$g
or
1amu = $0.1665077881\times 10^{-27}$Kg
or
1amu = $0.1665077881\times 10^{-27}$Kg
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