In 1803, Dalton first proposed to take the mass of hydrogen atom as the standard of atomic weight - absolute mass.Measure the relative mass of atoms of other elements by comparison (hydrogen is the smallest known element, so the atomic weight of other elements is greater than 1), so the atomic weight of oxygen is 15.88.Later, in view of the fact that there are far less hydrogen compounds than oxygen compounds, for the convenience of determining atomic weight (in the past, the atomic weight was mostly calculated by measuring the element equivalent), the atomic weight of other elements was measured by using the standard of 16 for one atom of oxygen (actually the average mass of several oxygen isotopes).In this way, the atomic weight of hydrogen is 1.008.Since no isotope of oxygen element was found at that time, the comparison standard of 16 atomic weight of oxygen has been used for more than 60 years.
From 1927 to 1929, it was found that oxygen in nature contains three isotopes, namely 16O, 17O and 18O.According to the relatively accurate mass spectrometry in 1940, the abundance of the three isotopes in nature accounted for 99.759% of 16O, 0.037% of 17O, and 0.204% of 18O.Therefore, it is not perfect to use natural oxygen as the atomic weight standard.At that time, the physics community immediately changed to use 16O equal to 16 as the standard, but the chemical community still used natural oxygen equal to 16 as the standardstandard。When using physical standards, the atomic weights of various oxygen isotopes are:
16O=16.0000
17O=17.0045
18O=18.0049
Therefore, the average atomic weight of oxygen in nature should be:
In this way, the atomic weight used in physics and the atomic weight used in chemistry have the following ratios due to different selection standards:
That is to say, the difference between the two standards is about 3/10000.
After 1940s
Since the 1940s, the International Atomic Weight Commission has adopted 1.000275 as the conversion factor of two standards, namely:
Physical atomic weight=1.000275 × chemical atomic weight
Physics and chemistry are closely related, and different atomic standards can easily lead to confusion.1959International Union of Pure and Applied Chemistry(IUPAC) proposed to use carbon isotope 12C=12 as the atomic weight standard (that is, 1/12 of the mass of 12C as the standard), and obtained the consent of the International Union of Pure and Applied Physics (IUPAP). In August 1961, it formally decided to use carbon isotope 12C=12 as the new atomic weight standard.In the same year, the new International Atomic Scale was released.
The reason why 12C is adopted as the atomic weight standard is roughly as follows: (1) carbon forms many high-quality "molecular ions" and hydrides, which is conducive to the determination of mass spectrometry;(2) 12C can be easily measured in mass spectrometer, and mass spectrometer is the most accurate method to measure atomic weight in modern times;(3) After 12C, the atomic weights of all elements have little change, only 0.0043% less than the past;(4) The abundance of such carbon atoms in nature is relatively stable;(5) Carbon is widely distributed in nature, and its compounds, especially organic compounds, are numerous;(6) The atomic weight of hydrogen with the smallest density is still not less than 1.
The absolute mass of an atom is very small. It is inconvenient to express it in kilograms.Therefore, 1/12 of the mass of an atom at 12C is used as the standard. The mass of other atoms compared with it is the relative atomic mass of this atom.
12C Absolute mass of an atom ×
=1.993×10-26 kg×
=1.66×10-27 kg
Therefore, the relationship between the absolute mass of an atom and the relative atomic mass of an element is:
The relative atomic mass is a ratio, its SI unit is one, and its symbol is 1.
Research achievements in China
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Famous chemistAcademician of CASProfessor Zhang Qinglian of Peking University is a Chinese participantInternational Union of Pure and Applied ChemistryMember of the Committee on Atomic Weight and Isotope Abundance.The atomic weights of indium, antimony, cerium, europium, iridium, erbium, germanium, zinc and dysprosium measured by Zhang Qinglian and other scientists were adopted as the new standard for atomic weight data by the Atomic Weight and Isotope Abundance Committee of the International Union of Pure and Applied Chemistry
Static mass
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The so-called static mass refers to the mass measured when an object can be decelerated by physical means so that it is relatively stationary with a certain reference system that is considered stationary. For general objects, we can decelerate it to be relatively stationary with the ground surface. The mass measured in this way is called "static mass".The rest mass of the same object is a Lorentz invariant.
The static mass does not refer to the mass of the object measured under the absolute static state, because the absolute static state does not exist at all, it is just the mass of the material in the low-speed static state that is artificially recognized.
The mass of a stationary object is called a stationary mass, and the mass of a moving object increases with its speed.Speed squared divided bylight speedThis is the factor that relates speed to mass.(See the formula in the upper right corner) So if the object moves at a low speed, the increase of its mass will be very small;However, if an object moves at a speed close to the speed of light, its mass will increase greatly.It can be seen from the formula that the mass of any object whose speed increases close to the speed of light tends to infinity, so it is impossible for a solid object to reach or exceed the speed of light.
The relationship between the mass of the object and the speed of motion is shown in figure.If the rest mass of an object is m0, then when its speed of motion is v, its mass m is equal to 1 - (v/c) ^ 2 under the root of m0/
