How to determine the relative atomic mass of an element. Relative atomic mass of an element. The historical course of counting matter
Currently, the atomic mass unit is taken to be equal to 1/12 the mass of a neutral atom of the most common isotope of carbon 12 C, so the atomic mass of this isotope by definition is exactly 12. The difference between the atomic mass of an isotope and its mass number is called excess mass (usually expressed in MeV ). It can be either positive or negative; the reason for its occurrence is the nonlinear dependence of the binding energy of nuclei on the number of protons and neutrons, as well as the difference in the masses of the proton and neutron.
The dependence of the atomic mass of an isotope on the mass number is as follows: the excess mass is positive for hydrogen-1, with increasing mass number it decreases and becomes negative until a minimum is reached for iron-56, then it begins to grow and increases to positive values for heavy nuclides. This corresponds to the fact that the fission of nuclei heavier than iron releases energy, while the fission of light nuclei requires energy. On the contrary, the fusion of nuclei lighter than iron releases energy, while the fusion of elements heavier than iron requires additional energy.
Story
Until the 1960s, atomic mass was defined so that the nuclide oxygen-16 had an atomic mass of 16 (oxygen scale). However, the ratio of oxygen-17 to oxygen-18 in natural oxygen, which was also used in atomic mass calculations, resulted in the presence of two different tables atomic masses. Chemists used a scale based on the fact that the natural mixture of oxygen isotopes would have an atomic mass of 16, while physicists assigned the same number of 16 to the atomic mass of the most common isotope of oxygen (which has eight protons and eight neutrons).
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See what “Atomic mass” is in other dictionaries:
The mass of an atom, expressed in atomic mass units. Atomic mass is less than the sum of the masses of the particles that make up the atom (protons, neutrons, electrons) by an amount determined by the energy of their interaction (see, for example, Mass Defect) ... Big Encyclopedic Dictionary
Atomic mass is the mass of an atom of a chemical element, expressed in atomic mass units (amu). For 1 amu 1/12 of the mass of the carbon isotope with atomic mass 12 is accepted. 1 amu = 1.6605655 10 27 kg. The atomic mass consists of the masses of all protons and... Nuclear energy terms
atomic mass- is the mass of atoms of an element, expressed in atomic mass units. The mass of an element that contains the same number of atoms as 12 g of the isotope 12C. General chemistry: textbook / A. V. Zholnin ... Chemical terms
ATOMIC MASS- dimensionless quantity. A. m. mass of an atom chemical. element expressed in atomic units (see) ... Big Polytechnic Encyclopedia
- (obsolete term atomic weight), the relative value of the mass of an atom, expressed in atomic mass units (a.m.u.). A.m. is less than the sum of the masses of the constituent atoms per mass defect. A. m. was taken by D. I. Mendeleev as the basis. characteristic of the element when... ... Physical encyclopedia
atomic mass- - [Ya.N.Luginsky, M.S.Fezi Zhilinskaya, Yu.S.Kabirov. English-Russian dictionary of electrical engineering and power engineering, Moscow, 1999] Topics of electrical engineering, basic concepts EN atomic weight ... Technical Translator's Guide
The mass of an atom, expressed in atomic mass units. The atomic mass of a chemical element consisting of a mixture of isotopes is taken to be the average value of the atomic mass of isotopes, taking into account their percentage content (this value is given in periodic... ... Encyclopedic Dictionary
The concept of this quantity has undergone long-term changes in accordance with changes in the concept of atoms. According to Dalton's theory (1803), all atoms of the same chemical element are identical and its atomic mass is a number equal to... ... Collier's Encyclopedia
atomic mass- santykinė atominė masė statusas T sritis Standartizacija ir metrologija apibrėžtis Cheminio elemento vidutinės masės ir nuklido ¹²C atomo masės 1/12 dalies dalmuo. atitikmenys: engl. atomic mass; atomic weight; relative atomic mass vok. Atommasse…
atomic mass- santykinė atominė masė statusas T sritis Standartizacija ir metrologija apibrėžtis Vidutinės elemento atomų masės ir 1/12 nuklido ¹²C atomo masės dalmuo. atitikmenys: engl. atomic mass; atomic weight; relative atomic mass vok. Atommasse, f;… … Penkiakalbis aiškinamasis metrologijos terminų žodynas
In the process of the development of science, chemistry was faced with the problem of calculating the amount of substance for carrying out reactions and the substances obtained in their course.
Today, for such calculations of chemical reactions between substances and mixtures, the value of the relative atomic mass entered in the periodic table is used chemical elements D. I. Mendeleev.
Chemical processes and the influence of the proportion of an element in substances on the course of the reaction
Modern science, by the definition of “relative atomic mass of a chemical element,” means how many times the mass of an atom of a given chemical element is greater than one twelfth of a carbon atom.
With the advent of the era of chemistry, the need for precise definitions the progress of the chemical reaction and its results grew.
Therefore, chemists constantly tried to solve the problem of the exact masses of interacting elements in a substance. One of best solutions at that time there was a link to the lightest element. And the weight of its atom was taken as one.
The historical course of counting matter
Hydrogen was initially used, then oxygen. But this method of calculation turned out to be inaccurate. The reason for this was the presence of isotopes with masses of 17 and 18 in oxygen.
Therefore, having a mixture of isotopes technically produced a number other than sixteen. Today, the relative atomic mass of an element is calculated based on the weight of the carbon atom taken as a basis, in a ratio of 1/12.
Dalton laid the foundations for the relative atomic mass of an element
Only some time later, in the 19th century, Dalton proposed to carry out calculations using the lightest chemical element - hydrogen. At lectures to his students, he demonstrated on figures carved from wood how atoms are connected. For other elements, he used data previously obtained by other scientists.
According to Lavoisier's experiments, water contains fifteen percent hydrogen and eighty-five percent oxygen. With this data, Dalton calculated that the relative atomic mass of the element that makes up water, in this case oxygen, is 5.67. The error in his calculations stems from the fact that he believed incorrectly regarding the number of hydrogen atoms in a water molecule.
In his opinion, there was one hydrogen atom for every oxygen atom. Using the data of the chemist Austin that ammonia contains 20 percent hydrogen and 80 percent nitrogen, he calculated the relative atomic mass of nitrogen. With this result, he came to an interesting conclusion. It turned out that the relative atomic mass (the formula of ammonia was mistakenly taken with one molecule of hydrogen and nitrogen) was four. In his calculations, the scientist relied on Mendeleev’s periodic system. According to the analysis, he calculated that the relative atomic mass of carbon is 4.4, instead of the previously accepted twelve.
Despite his serious mistakes, it was Dalton who was the first to create a table of some elements. It underwent repeated changes during the scientist’s lifetime.
The isotopic component of a substance affects the relative atomic weight accuracy value
When considering the atomic masses of elements, you will notice that the accuracy for each element is different. For example, for lithium it is four-digit, and for fluorine it is eight-digit.
The problem is that the isotopic component of each element is different and not constant. For example, ordinary water contains three types of hydrogen isotopes. These include, in addition to ordinary hydrogen, deuterium and tritium.
The relative atomic mass of hydrogen isotopes is two and three, respectively. “Heavy” water (formed by deuterium and tritium) evaporates less easily. Therefore, there are fewer isotopes of water in the vapor state than in the liquid state.
Selectivity of living organisms to different isotopes
Living organisms have a selective property towards carbon. To build organic molecules, carbon with a relative atomic mass of twelve is used. Therefore, substances of organic origin, as well as a number of minerals such as coal and oil, contain less isotopic content than inorganic materials.
Microorganisms that process and accumulate sulfur leave behind the sulfur isotope 32. In areas where bacteria do not process, the proportion of sulfur isotope is 34, that is, much higher. It is on the basis of the ratio of sulfur in soil rocks that geologists come to a conclusion about the nature of the origin of the layer - whether it has a magmatic or sedimentary nature.
Of all the chemical elements, only one has no isotopes - fluorine. Therefore, its relative atomic mass is more accurate than other elements.
Existence of unstable substances in nature
For some elements, the relative mass is indicated in square brackets. As you can see, these are the elements located after uranium. The fact is that they do not have stable isotopes and decay with the release of radioactive radiation. Therefore, the most stable isotope is indicated in parentheses.
Over time, it turned out that some of them can get artificial conditions stable isotope. It was necessary to change the atomic masses of some transuranium elements in the periodic table.
In the process of synthesizing new isotopes and measuring their lifespan, it was sometimes possible to discover nuclides with half-lives millions of times longer.
Science does not stand still; new elements, laws, and relationships between various processes in chemistry and nature are constantly being discovered. Therefore, what form chemistry and Mendeleev’s periodic system of chemical elements will appear in in the future, a hundred years from now, is vague and uncertain. But I would like to believe that the works of chemists accumulated over the past centuries will serve new, more advanced knowledge of our descendants.
To measure the mass of an atom, relative atomic mass is used, which is expressed in atomic mass units (amu). Relative molecular weight is made up of the relative atomic masses of substances.
Concepts
To understand what relative atomic mass is in chemistry, you should understand that the absolute mass of an atom is too small to be expressed in grams, much less in kilograms. Therefore, in modern chemistry, 1/12 of the mass of carbon is taken as an atomic mass unit (amu). Relative atomic mass is equal to the ratio of the absolute mass to 1/12 of the absolute mass of carbon. In other words, relative mass reflects how many times the mass of an atom of a particular substance exceeds 1/12 the mass of a carbon atom. For example, the relative mass of nitrogen is 14, i.e. The nitrogen atom contains 14 a. e.m. or 14 times more than 1/12 of a carbon atom.
Rice. 1. Atoms and molecules.
Among all the elements, hydrogen is the lightest, its mass is 1 unit. The heaviest atoms have a mass of 300 a. e.m.
Molecular mass is a value indicating how many times the mass of a molecule exceeds 1/12 of the mass of carbon. Also expressed in a. e.m. The mass of a molecule is made up of the mass of atoms, therefore, to calculate the relative molecular mass it is necessary to add up the masses of the atoms of the substance. For example, the relative molecular weight of water is 18. This value is the sum of the relative atomic masses of two hydrogen atoms (2) and one oxygen atom (16).
Rice. 2. Carbon in the periodic table.
As you can see, these two concepts have several common characteristics:
- the relative atomic and molecular masses of a substance are dimensionless quantities;
- relative atomic mass is designated Ar, molecular mass - Mr;
- The unit of measurement is the same in both cases - a. e.m.
Molar and molecular masses are the same numerically, but differ in dimension. Molar mass is the ratio of the mass of a substance to the number of moles. It reflects the mass of one mole, which equal to the number Avogadro, i.e. 6.02 ⋅ 10 23 . For example, 1 mole of water weighs 18 g/mol, and M r (H 2 O) = 18 a. e.m. (18 times heavier than one atomic mass unit).
How to calculate
To express relative atomic mass mathematically, one should determine that 1/2 part of carbon or one atomic mass unit is equal to 1.66⋅10 −24 g. Therefore, the formula for relative atomic mass is as follows:
A r (X) = m a (X) / 1.66⋅10 −24,
where m a is the absolute atomic mass of the substance.
The relative atomic mass of chemical elements is indicated in the periodic table of Mendeleev, so it does not need to be calculated independently when solving problems. Relative atomic masses are usually rounded to whole numbers. The exception is chlorine. The mass of its atoms is 35.5.
It should be noted that when calculating the relative atomic mass of elements that have isotopes, their average value is taken into account. Atomic mass in this case is calculated as follows:
A r = ΣA r,i n i ,
where A r,i is the relative atomic mass of isotopes, n i is the content of isotopes in natural mixtures.
For example, oxygen has three isotopes - 16 O, 17 O, 18 O. Their relative mass is 15.995, 16.999, 17.999, and their content in natural mixtures is 99.759%, 0.037%, 0.204%, respectively. Dividing the percentages by 100 and substituting the values, we get:
A r = 15.995 ∙ 0.99759 + 16.999 ∙ 0.00037 + 17.999 ∙ 0.00204 = 15.999 amu
Referring to the periodic table, it is easy to find this value in the oxygen cell.
Rice. 3. Periodic table.
Relative molecular mass is the sum of the masses of the atoms of a substance:
When determining the relative molecular weight value, symbol indices are taken into account. For example, calculating the mass of H 2 CO 3 is as follows:
M r = 1 ∙ 2 + 12 + 16 ∙ 3 = 62 a. e.m.
Knowing the relative molecular weight, you can calculate the relative density of one gas from the second, i.e. determine how many times one gaseous substance is heavier than the second. To do this, use the equation D (y) x = M r (x) / M r (y).
What have we learned?
From the 8th grade lesson we learned about relative atomic and molecular mass. The unit of relative atomic mass is taken to be 1/12 of the mass of carbon, equal to 1.66⋅10 −24 g. To calculate the mass, it is necessary to divide the absolute atomic mass of the substance by the atomic mass unit (amu). The value of the relative atomic mass is indicated in the periodic table of Mendeleev in each cell of the element. The molecular mass of a substance is the sum of the relative atomic masses of the elements.
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The masses of atoms and molecules are very small, so it is convenient to choose the mass of one of the atoms as a unit of measurement and express the masses of the remaining atoms relative to it. This is exactly what the founder of atomic theory, Dalton, did, who compiled a table of atomic masses, taking the mass of the hydrogen atom as one.
Until 1961, in physics, 1/16 of the mass of an oxygen atom 16 O was taken as an atomic mass unit (amu), and in chemistry - 1/16 of the average atomic mass of natural oxygen, which is a mixture of three isotopes. The chemical unit of mass was 0.03% larger than the physical one.
Currently accepted for physics and chemistry unified system measurements. 1/12 of the mass of a 12 C carbon atom was chosen as the standard unit of atomic mass.
1 amu = 1/12 m(12 C) = 1.66057×10 -27 kg = 1.66057×10 -24 g.
DEFINITION
Relative atomic mass of an element (A r) is a dimensionless quantity equal to the ratio of the average mass of an atom of an element to 1/12 of the mass of an atom of 12 C.
When calculating relative atomic mass, the abundance of isotopes of elements in earth's crust. For example, chlorine has two isotopes 35 Cl (75.5%) and 37 Cl (24.5%). The relative atomic mass of chlorine is:
A r (Cl) = (0.755×m(35 Cl) + 0.245×m(37 Cl)) / (1/12×m(12 C) = 35.5.
From the definition of relative atomic mass it follows that the average absolute mass of an atom is equal to the relative atomic mass multiplied by amu:
m(Cl) = 35.5 × 1.66057 × 10 -24 = 5.89 × 10 -23 g.
Examples of problem solving
EXAMPLE 1
| Exercise | In which of the following substances the mass fraction of the oxygen element is greater: a) in zinc oxide (ZnO); b) in magnesium oxide (MgO)? |
| Solution |
Let's find the molecular weight of zinc oxide: Mr (ZnO) = Ar(Zn) + Ar(O); Mr (ZnO) = 65+ 16 = 81. It is known that M = Mr, which means M(ZnO) = 81 g/mol. Then the mass fraction of oxygen in zinc oxide will be equal to: ω (O) = Ar (O) / M (ZnO) × 100%; ω(O) = 16 / 81 × 100% = 19.75%. Let's find the molecular weight of magnesium oxide: Mr (MgO) = Ar(Mg) + Ar(O); Mr (MgO) = 24+ 16 = 40. It is known that M = Mr, which means M(MgO) = 60 g/mol. Then the mass fraction of oxygen in magnesium oxide will be equal to: ω (O) = Ar (O) / M (MgO) × 100%; ω(O) = 16 / 40 × 100% = 40%. Thus, the mass fraction of oxygen is greater in magnesium oxide, since 40 > 19.75. |
| Answer | The mass fraction of oxygen is greater in magnesium oxide. |
EXAMPLE 2
| Exercise | In which of the following compounds is the mass fraction of metal greater: a) in aluminum oxide (Al 2 O 3); b) in iron oxide (Fe 2 O 3)? |
| Solution | The mass fraction of element X in a molecule of the composition NX is calculated using the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let's calculate the mass fraction of each element of oxygen in each of the proposed compounds (we will round off the values of relative atomic masses taken from D.I. Mendeleev's Periodic Table to whole numbers). Let's find the molecular weight of aluminum oxide: Mr (Al 2 O 3) = 2×Ar(Al) + 3×Ar(O); Mr (Al 2 O 3) = 2 × 27 + 3 × 16 = 54 + 48 = 102. It is known that M = Mr, which means M(Al 2 O 3) = 102 g/mol. Then the mass fraction of aluminum in the oxide will be equal to: ω (Al) = 2×Ar(Al) / M (Al 2 O 3) × 100%; ω(Al) = 2×27 / 102 × 100% = 54 / 102 × 100% = 52.94%. Let's find the molecular weight of iron (III) oxide: Mr (Fe 2 O 3) = 2×Ar(Fe) + 3×Ar(O); Mr (Fe 2 O 3) = 2×56+ 3×16 = 112 + 48 = 160. It is known that M = Mr, which means M(Fe 2 O 3) = 160 g/mol. Then the mass fraction of iron in the oxide will be equal to: ω (O) = 3×Ar (O) / M (Fe 2 O 3) × 100%; ω(O) = 3×16 / 160×100% = 48 / 160×100% = 30%. Thus, the mass fraction of the metal is greater in aluminum oxide, since 52.94 > 30. |
| Answer | The mass fraction of metal is greater in aluminum oxide. |
>> Chemistry: Chemical formulas. Relative atomic and molecular masses
Chemists around the world reflect the composition of simple and complex substances very beautifully and concisely in the form of chemical formulas. Chemical formulas are analogues of words that are written using letters - symbols of chemical elements.
Let us express, using chemical symbols, the composition of the most common Substance on Earth - water. A water molecule contains two hydrogen atoms and one oxygen atom. Now let's translate this sentence into a chemical formula using chemical symbols (hydrogen - Nor oxygen - O). We write the number of atoms in the formula using index numbers located at the bottom right of the chemical symbol (index 1 is not written for oxygen): Н2Ш (read “ash-two-o”).
The formulas of simple substances hydrogen and oxygen, whose molecules consist of two identical atoms, are written as follows: H2 (read “ash-two”) and O2 (read “o-two”).
To reflect the number of molecules, use the coefficients that are written before chemical formulas, for example, the notation 2COg (read two-tse-o-dva") means that they mean two molecules carbon dioxide, each of which consists of one carbon atom and two oxygen atoms.
Coefficients are written similarly when indicating the number of free atoms of a chemical element.
The sizes of molecules, and especially atoms, are so small that they cannot be seen even in the best optical microscopes, which provide a magnification of 5-6 thousand times. It is impossible to consider them in electron microscopes, giving an increase of 40 thousand times. Naturally, the negligible size of molecules and atoms corresponds to their negligible masses.
Let's calculate how many times the mass of an oxygen atom is greater than the mass of a hydrogen atom, the lightest element: 
Similarly, the mass of a carbon atom is 12 times greater than the mass of a hydrogen atom. 
