Relative atomic mass of an element how to determine. Relative atomic mass of an element. The historical course of counting the substance
Currently, the atomic mass unit is taken equal to 1/12 of the mass of a neutral atom of the most common carbon isotope 12 C, so the atomic mass of this isotope is, by definition, exactly 12. The difference between the atomic mass of an isotope and its mass number is called the mass excess (usually expressed in MeV ). It can be both positive and 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 the 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.
History
Until the 1960s, atomic mass was determined 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 two different tables of atomic masses. Chemists used a scale based on the fact that a natural mixture of oxygen isotopes should 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 (having 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. The 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 (a.m.u.). For 1 amu 1/12 of the mass of a carbon isotope with an atomic mass of 12 is adopted. 1 amu = 1.6605655 10 27 kg. Atomic mass is made up of the masses of all protons and... Nuclear power terms
atomic mass- is the mass of the element's atoms, expressed in atomic mass units. The mass of that amount of an element that contains the same number of atoms as 12 g of the 12C isotope. General chemistry: textbook / A. V. Zholnin ... Chemical terms
ATOMIC MASS is a dimensionless quantity. A. m. atom mass chem. element, expressed in atomic units (see) ... Great Polytechnic Encyclopedia
- (obsolete term atomic weight), the relative value of the mass of an atom, expressed in atomic mass units (amu). A. m. is less than the sum of the masses of the constituent atom h q per mass defect. A. m. was taken by D. I. Mendeleev for the main. characteristic of the element at ... ... Physical Encyclopedia
atomic mass- — [Ya.N. Luginsky, M.S. Fezi Zhilinskaya, Yu.S. Kabirov. English-Russian Dictionary of Electrical Engineering and Power Industry, Moscow, 1999] Electrical engineering topics, basic concepts EN atomic weight ... Technical Translator's Handbook
The mass of an atom, expressed in atomic mass units. For the atomic mass of a chemical element consisting of a mixture of isotopes, take the average value of the atomic mass of isotopes, taking into account their percentage (this value is given in the periodic ... ... encyclopedic Dictionary
The concept of this quantity underwent long-term changes in accordance with the change in the idea 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 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. Atomasse …
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. Atomasse, f;… … Penkiakalbis aiskinamasis metrologijos terminų žodynas
In the process of developing science, chemistry faced the problem of calculating the amount of a substance for carrying out reactions and the substances obtained in their course.
Today, for such calculations of a chemical reaction between substances and mixtures, the value of the relative atomic mass included in the periodic table of chemical elements of D. I. Mendeleev is used.
Chemical processes and the influence of the proportion of an element in substances on the course of a reaction
Modern science under 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 more than one twelfth of a carbon atom.
With the advent of the era of chemistry, the need for accurate determinations of the course of a chemical reaction and its results grew.
Therefore, chemists constantly tried to solve the problem of the exact masses of interacting elements in matter. One of the best solutions at the time was snapping to the lightest element. And the weight of its atom was taken as one.
The historical course of counting the substance
Initially, hydrogen was used, then oxygen. But this method of calculation turned out to be inaccurate. The reason for this was the presence of isotopes with a mass of 17 and 18 in oxygen.
Therefore, having a mixture of isotopes technically gave 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 the basis, in the ratio 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 calculate 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 these data, Dalton calculated that the relative atomic mass of the element that makes up water, in this case oxygen, is 5.67. The erroneousness of his calculations is due to the fact that he believed incorrectly regarding the number of hydrogen atoms in a water molecule.
In his opinion, there was one hydrogen atom per oxygen atom. Using the chemist Austin's data that ammonia contains 20 percent hydrogen and 80 percent nitrogen, he calculated what the relative atomic mass of nitrogen is. With this result, he came to an interesting conclusion. It turned out that the relative atomic mass (the ammonia formula was erroneously taken with one molecule of hydrogen and nitrogen) is four. In his calculations, the scientist relied on the periodic system of Mendeleev. From analysis, he calculated that the relative atomic mass of carbon was 4.4, instead of the previously accepted twelve.
Despite his serious blunders, it was Dalton who first created a table of some elements. It has undergone numerous changes during the lifetime of the scientist.
The isotopic component of a substance affects the relative atomic weight accuracy value
When considering the atomic masses of the elements, one can 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 variable. For example, ordinary water contains three types of hydrogen isotope. In addition to ordinary hydrogen, they include deuterium and tritium.
The relative atomic masses of hydrogen isotopes are two and three, respectively. "Heavy" water (formed by deuterium and tritium) evaporates worse. 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 in relation to carbon. Carbon with a relative atomic mass equal to twelve is used to build organic molecules. 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 the sulfur isotope is 34, that is, much higher. It is on the basis of the ratio of sulfur in the soil rocks that geologists come to the conclusion about the nature of the origin of the layer - whether it has a magmatic nature or a sedimentary one.
Of all the chemical elements, only one has no isotopes - fluorine. Therefore, its relative atomic mass is more accurate than other elements.
The existence of unstable substances in nature
For some elements, the relative mass is given in square brackets. As you can see, these are 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 brackets.
Over time, it turned out that it is possible to obtain a stable isotope from some of them under artificial conditions. I had to change the atomic masses of some transuranium elements in the periodic table of Mendeleev.
In the process of synthesizing new isotopes and measuring their lifetimes, it has sometimes been possible to find nuclides with half-lives millions of times longer.
Science does not stand still, new elements, laws, relationships of various processes in chemistry and nature are constantly being discovered. Therefore, in what form the chemistry and the periodic system of chemical elements of Mendeleev will turn out in the future, in a hundred years, is vague and uncertain. But I would like to believe that the works of chemists accumulated over the past centuries will serve a new, more perfect knowledge of our descendants.
To measure the mass of an atom, the relative atomic mass is used, which is expressed in atomic mass units (a.m.u.). The relative molecular mass is the sum of the relative atomic masses of substances.
Concepts
To understand what relative atomic mass is in chemistry, it should be understood that the absolute mass of an atom is too small to be expressed in grams, and even more so in kilograms. Therefore, in modern chemistry, 1/12 of the mass of carbon is taken as an atomic mass unit (amu). The relative atomic mass is equal to the ratio of the absolute mass to 1/12 of the absolute mass of carbon. In other words, the relative mass reflects how many times the mass of an atom of a particular substance exceeds 1/12 of 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 amu. eat.
Molecular weight - a value showing 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 the masses of the atoms of a 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 denoted A r , molecular mass - M r ;
- the unit of measurement is the same in both cases - a. eat.
The molar and molecular masses coincide 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 is equal to Avogadro's number, i.e. 6.02 ⋅ 10 23 . For example, 1 mol of water weighs 18 g / mol, and M r (H 2 O) \u003d 18 a. e.m. (18 times heavier than one atomic mass unit).
How to calculate
To express the 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 the 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 integers. 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. The 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 an oxygen cell.
Rice. 3. Periodic table.
Relative molecular weight - the sum of the masses of the atoms of a substance:
Symbol indices are taken into account when determining the relative molecular weight value. For example, the calculation of the mass of H 2 CO 3 is as follows:
M r \u003d 1 ∙ 2 + 12 + 16 ∙ 3 \u003d 62 a. eat.
Knowing the relative molecular weight, one 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. For this, the equation D (y) x \u003d M r (x) / M r (y) is used.
What have we learned?
From the 8th grade lesson, we learned about the relative atomic and molecular mass. The unit of relative atomic mass is 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 a substance by the atomic mass unit (a.m.u.). The value of the relative atomic mass is indicated in the periodic system of Mendeleev in each cell of the element. The molecular weight 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 the atomic theory Dalton did, who compiled a table of atomic masses, taking the mass of a hydrogen atom as a unit.
Until 1961, in physics, 1/16 of the mass of an oxygen atom 16 O was taken as an atomic mass unit (abbreviated amu), and in chemistry - 1/16 of the average atomic mass of natural oxygen, which is a mixture of three isotopes. The chemical mass unit was 0.03% larger than the physical one.
At present, a unified measurement system has been adopted in physics and chemistry. 1/12 of the mass of the carbon atom 12 C is chosen as the standard unit of atomic mass.
1 amu \u003d 1/12 m (12 C) \u003d 1.66057 × 10 -27 kg \u003d 1.66057 × 10 -24 g.
DEFINITION
Relative atomic mass of an element (A r)- this is a dimensionless quantity equal to the ratio of the average mass of an element atom to 1/12 of the mass of an atom 12 C.
When calculating the relative atomic mass, the abundance of isotopes of elements in the earth's crust is taken into account. 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) \u003d (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 times the amu:
m(Cl) = 35.5 × 1.66057 × 10 -24 = 5.89 × 10 -23 g.
Examples of problem solving
EXAMPLE 1
| The task | In which of the following substances is the mass fraction of the oxygen element greater: a) in zinc oxide (ZnO); b) in magnesium oxide (MgO)? |
| Decision |
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%. 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
| The task | In which of the following compounds, the mass fraction of metal is greater: a) in aluminum oxide (Al 2 O 3); b) in iron oxide (Fe 2 O 3)? |
| Decision | The mass fraction of the element X in the molecule of the HX composition is calculated by the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us calculate the mass fraction of each element of oxygen in each of the proposed compounds (the values of the relative atomic masses taken from the Periodic Table of D.I. Mendeleev will be rounded to integers). Find the molecular weight of aluminum oxide: Mr (Al 2 O 3) = 2×Ar(Al) + 3×Ar(O); Mr (Al 2 O 3) \u003d 2 × 27 + 3 × 16 \u003d 54 + 48 \u003d 102. It is known that M \u003d Mr, which means M (Al 2 O 3) \u003d 102 g / mol. Then the mass fraction of aluminum in the oxide will be equal to: ω (Al) \u003d 2 × Ar (Al) / M (Al 2 O 3) × 100%; ω (Al) \u003d 2 × 27 / 102 × 100% \u003d 54 / 102 × 100% \u003d 52.94%. Find the molecular weight of iron oxide (III): Mr (Fe 2 O 3) = 2×Ar(Fe) + 3×Ar(O); Mr (Fe 2 O 3) \u003d 2 × 56 + 3 × 16 \u003d 112 + 48 \u003d 160. It is known that M \u003d Mr, which means M (Fe 2 O 3) \u003d 160 g / mol. Then the mass fraction of iron in the oxide will be equal to: ω (O) \u003d 3 × Ar (O) / M (Fe 2 O 3) × 100%; ω (O) = 3×16 / 160 × 100% = 48 / 160× 100% = 30%. Thus, the mass fraction of 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 - signs of chemical elements.
Let's use chemical symbols to express 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 - Ni oxygen - O). We write the number of atoms in the formula with the help of indices-digits to the lower right of the chemical symbol (index 1 for oxygen is not written): H2Sch (read "ash-two-o").
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, coefficients are used that are written in front of chemical formulas, for example, the notation 2COg (read two-ce-o-two) means that they mean two molecules of carbon dioxide, each of which consists of one carbon atom and two atoms oxygen.
Coefficients are written similarly when the number of free atoms of a chemical element is indicated.
The sizes of molecules, and even more so atoms, are so small that they cannot be seen even in the best optical microscopes, which give an increase of 5-6 thousand times. They can not be seen in electron microscopes, giving an increase of 40 thousand times. Naturally, the negligibly small 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. 
