How to determine the mass of an element using the periodic table. List of elements by atomic mass. Examples of problem solving
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 the 16O oxygen atom 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, a unified measurement system has been adopted in physics and chemistry. 1/12 of the mass of a 12C carbon atom was chosen as the standard unit of atomic mass.
1 amu = 1/12 m(12С) = 1.66057×10-27 kg = 1.66057×10-24 g.
When calculating relative atomic mass, the abundance of isotopes of elements in the earth's crust is taken into account. For example, chlorine has two isotopes 35Сl (75.5%) and 37Сl (24.5%). The relative atomic mass of chlorine is:
Ar(Cl) = (0.755×m(35Сl) + 0.245×m(37Сl)) / (1/12×m(12С) = 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
Relative atomic and molecular masses
This calculator is designed to calculate the atomic mass of elements.
Atomic mass(also called relative atomic mass) Is the value of the mass of one atom of a substance. Relative atomic mass is expressed in atomic mass units. Relative atomic mass distinctive(True) weight atom. At the same time, the actual mass of an atom is too small and therefore unsuitable for practical use.
The atomic mass of a substance affects the amount protons And neutrons in the nucleus of an atom.
The electron mass is ignored since it is very small.
To determine the atomic mass of a substance, you must enter the following information:
- Number of protons- how many protons are in the nucleus of the substance;
- Number of neutrons— how many neutrons are in the nucleus of a substance.
Based on this data, the calculator will calculate the atomic mass of the substance, expressed in atomic mass units.
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Table of chemical elements and their atomic mass
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Relative atomic mass of an elementTask status:Determine the mass of an oxygen molecule. Task no. 4.1.2 from the “Collection of problems in preparing upcoming exams in physics at USPTU” information:Solution:Consider a molecular oxygen molecule \(\nu\) (arbitrary number). Let us remember that the oxygen formula is O2. To find the mass (\m) of a given amount of oxygen, the molecular mass of oxygen\(M\) is multiplied by the number of moles\(\nu\). Using the periodic table, it is easy to establish that the molar mass of oxygen is \(M\) 32 g/mol or 0.032 kg/mol. In one mol, the number of avogadro molecules \(N_A\) and v\(\nu\) mol - v\(\nu\) is sometimes greater, i.e. To find the mass of one molecule \(m_0\), the total mass \(m\) must be divided by the number of molecules \(N\). \ [(m_0) = \frac (m) (N)\] \ [(m_0) = \frac ((\nu \cdot M)) ((\nu \cdot (N_A)))\] \ ((M_0) = \frac (M) (((N_A))) \] Avogadro's number (N_A1) is a tabular value equal to 6.022 1023 mol-1. We perform calculations: \[(M_0) = \frac ((0.032)) ((6.022\cdot ((10) * (23)))) = 5.3\cdot (10^(-26))\; = 5.3 kg\cdot(10^(-23))\; r\] Answer: 5.3 · 10-23 g.If you don't understand the solution and if you have any questions or found a bug, you can leave a comment below. Atoms are very small and very small. If we express the mass of an atom of a chemical element in grams, then it will be a number for which the decimal point is more than twenty zeros. Therefore, measuring the mass of atoms in grams is inappropriate. However, if we take a very small mass per unit, all other small masses can be expressed as a ratio between that unit. The unit of measurement for atomic mass is 1/12 of the mass of a carbon atom. It is called 1/12 of the mass of a carbon atom atomic mass(Ae. Atomic mass formulaRelative atomic mass the value is equal to the ratio of the actual mass of an atom of a particular chemical element to 1/12 of the actual mass of a carbon atom. This is an infinite value, since the two masses are separated. Ar = mathematics. / (1/12) mug. Nevertheless, absolute atomic mass equal to a relative value and has a measurement unit amu. This means that relative atomic mass shows how many times the mass of a given atom is greater than 1/12 of a carbon atom. If an Ar atom = 12, then its mass is 12 times greater than 1/12 the mass of a carbon atom or, in other words, 12 atomic mass units. This can only be for carbon (C). On the hydrogen atom (H) Ar = 1. This means that its mass is equal to the mass of 1/12 parts of the mass of the carbon atom. For oxygen (O), the relative atomic mass is 16 amu. This means that an oxygen atom is 16 times larger than a carbon atom, it has 16 atomic mass units. The lightest element is hydrogen. Its mass is about 1 amu. On the heaviest atoms the mass approaches 300 amu. Typically, for each chemical element, its value is the absolute mass of the atoms, expressed as a. For example. The meaning of atomic mass units is written in the periodic table. Concept used for molecules relative molecular weight (g). Relative molecular weight indicates how many times the mass of a molecule is greater than 1/12 the mass of a carbon atom. However, since the mass of a molecule is equal to the sum of the masses of its atomic atoms, the relative molecular mass can be found simply by adding the relative masses of those atoms. For example, a water molecule (H2O) contains two hydrogen atoms with Ar = 1 and one oxygen atom with Ar = 16. Therefore, gentleman (H2O) = 18. Many substances have a non-molecular structure, such as metals. In this case, their relative molecular mass is equal to their relative atomic mass. Chemistry is called a significant amount mass fraction of a chemical element in a molecule or substance. It shows the relative molecular weight of that element. For example, in water, hydrogen has 2 parts (as both atoms) and oxygen 16. This means that when hydrogen is mixed with 1 kg and 8 kg of oxygen, they react without a residue. The mass fraction of hydrogen is 2/18 = 1/9, and the oxygen content is 16/18 = 8/9. Microbalance otherwise support, atomic equilibrium(English microbial or English nanotubes) is a term referring to:
descriptionOne of the first references to the microglob is in 1910, when William Ramsay was informed of the extent to which it had developed, allowing the weight range of 0.1 mm3 of body to be determined to be 10-9 g (1 ng). The term microbial is now more commonly used to refer to devices that can measure and detect mass changes in the microgram range (10-6 grams). Microbiologists have become common practice in modern research and industrial laboratories and are available in different versions with varying sensitivities and associated costs. At the same time, measurement techniques are being developed in the nanogram field. chemistry. how to find relative atomic mass?When we talk about measuring mass at the nanogram level, which is important for measuring the mass of atoms, molecules or clusters, we first consider mass spectrometry. In this case, it should be borne in mind that measuring mass using this method implies the need to convert the weighed objects into ions, which is sometimes very undesirable. This is not necessary when using another practically important and widely used instrument for the accurate measurement of mass quartz microbes, the mechanism of action of which is described in the corresponding article. links
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Atomic mass is the sum of the masses of all protons, neutrons and electrons that make up an atom or molecule. Compared to protons and neutrons, the mass of electrons is very small, so it is not taken into account in calculations. Although this is not formally correct, the term is often used to refer to the average atomic mass of all isotopes of an element. This is actually relative atomic mass, also called atomic weight element. Atomic weight is the average of the atomic masses of all isotopes of an element found in nature. Chemists must differentiate between these two types of atomic mass when doing their work—an incorrect atomic mass value can, for example, result in an incorrect result for the yield of a reaction.
Steps
Finding atomic mass from the periodic table of elements
- The atomic mass unit characterizes the mass one mole of a given element in grams. This value is very useful in practical calculations, since it can be used to easily convert the mass of a given number of atoms or molecules of a given substance into moles, and vice versa.
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Find the atomic mass in the periodic table. Most standard periodic tables contain the atomic masses (atomic weights) of each element. Typically, they are listed as a number at the bottom of the element cell, below the letters representing the chemical element. Usually this is not a whole number, but a decimal fraction.
Remember that the periodic table gives the average atomic masses of elements. As noted earlier, the relative atomic masses given for each element in the periodic table are the average of the masses of all isotopes of the atom. This average value is valuable for many practical purposes: for example, it is used in calculating the molar mass of molecules consisting of several atoms. However, when you are dealing with individual atoms, this value is usually not enough.
- Since the average atomic mass is an average of several isotopes, the value shown in the periodic table is not accurate the value of the atomic mass of any single atom.
- The atomic masses of individual atoms must be calculated taking into account the exact number of protons and neutrons in a single atom.
Learn how atomic mass is written. Atomic mass, that is, the mass of a given atom or molecule, can be expressed in standard SI units - grams, kilograms, and so on. However, because atomic masses expressed in these units are extremely small, they are often written in unified atomic mass units, or amu for short. – atomic mass units. One atomic mass unit is equal to 1/12 the mass of the standard isotope carbon-12.
Calculation of the atomic mass of an individual atom
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Find the atomic number of a given element or its isotope. Atomic number is the number of protons in the atoms of an element and never changes. For example, all hydrogen atoms, and only they have one proton. The atomic number of sodium is 11 because it has eleven protons in its nucleus, while the atomic number of oxygen is eight because it has eight protons in its nucleus. You can find the atomic number of any element in the periodic table - in almost all its standard versions, this number is indicated above the letter designation of the chemical element. The atomic number is always a positive integer.
- Suppose we are interested in the carbon atom. Carbon atoms always have six protons, so we know that its atomic number is 6. In addition, we see that in the periodic table, at the top of the cell with carbon (C) is the number "6", indicating that the atomic carbon number is six.
- Note that the atomic number of an element is not uniquely related to its relative atomic mass in the periodic table. Although, especially for elements at the top of the table, an element's atomic mass may appear to be twice its atomic number, it is never calculated by multiplying the atomic number by two.
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Find the number of neutrons in the nucleus. The number of neutrons can be different for different atoms of the same element. When two atoms of the same element with the same number of protons have different numbers of neutrons, they are different isotopes of that element. Unlike the number of protons, which never changes, the number of neutrons in the atoms of a given element can often change, so the average atomic mass of an element is written as a decimal fraction with a value lying between two adjacent whole numbers.
Add up the number of protons and neutrons. This will be the atomic mass of this atom. Ignore the number of electrons that surround the nucleus - their total mass is extremely small, so they have virtually no effect on your calculations.
Calculating the relative atomic mass (atomic weight) of an element
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Determine which isotopes are contained in the sample. Chemists often determine the isotope ratios of a particular sample using a special instrument called a mass spectrometer. However, in training, this data will be provided to you in assignments, tests, and so on in the form of values taken from the scientific literature.
- In our case, let's say that we are dealing with two isotopes: carbon-12 and carbon-13.
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Determine the relative abundance of each isotope in the sample. For each element, different isotopes occur in different ratios. These ratios are almost always expressed as percentages. Some isotopes are very common, while others are very rare—sometimes so rare that they are difficult to detect. These values can be determined using mass spectrometry or found in a reference book.
- Let's assume that the concentration of carbon-12 is 99% and carbon-13 is 1%. Other isotopes of carbon really exist, but in quantities so small that in this case they can be neglected.
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Multiply the atomic mass of each isotope by its concentration in the sample. Multiply the atomic mass of each isotope by its percentage abundance (expressed as a decimal). To convert percentages to a decimal, simply divide them by 100. The resulting concentrations should always add up to 1.
- Our sample contains carbon-12 and carbon-13. If carbon-12 makes up 99% of the sample and carbon-13 makes up 1%, then multiply 12 (the atomic mass of carbon-12) by 0.99 and 13 (the atomic mass of carbon-13) by 0.01.
- The reference books give percentages based on the known quantities of all isotopes of a particular element. Most chemistry textbooks contain this information in a table at the end of the book. For the sample being studied, the relative concentrations of isotopes can also be determined using a mass spectrometer.
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Add up the results. Sum up the multiplication results you got in the previous step. As a result of this operation, you will find the relative atomic mass of your element - the average value of the atomic masses of the isotopes of the element in question. When an element as a whole is considered, rather than a specific isotope of a given element, this value is used.
- In our example, 12 x 0.99 = 11.88 for carbon-12, and 13 x 0.01 = 0.13 for carbon-13. The relative atomic mass in our case is 11.88 + 0.13 = 12,01 .
- Some isotopes are less stable than others: they break down into atoms of elements with fewer protons and neutrons in the nucleus, releasing particles that make up the atomic nucleus. Such isotopes are called radioactive.
Atoms are very small in size and have very little mass. If we express the mass of an atom of a chemical element in grams, then this will be a number preceded by more than twenty zeros after the decimal point. Therefore, measuring the mass of atoms in grams is inconvenient.
However, if we take any very small mass as a unit, then all other small masses can be expressed as a ratio to this unit. The unit of measurement for atomic mass was chosen to be 1/12 of the mass of a carbon atom.
1/12 of the mass of a carbon atom is called atomic mass unit(a.e.m.).
Relative atomic mass is a value equal to the ratio of the real mass of an atom of a particular chemical element to 1/12 of the real mass of a carbon atom. This is a dimensionless quantity, since two masses are divided.
A r = m at. / (1/12)m arc.
However absolute atomic mass equal to relative in value and has a unit of measurement a.m.u.
That is, the relative atomic mass shows how many times the mass of a particular atom is greater than 1/12 of a carbon atom. If an atom A has r = 12, then its mass is 12 times greater than 1/12 the mass of a carbon atom, or, in other words, it has 12 atomic mass units. This can only happen with carbon (C) itself. The hydrogen atom (H) has A r = 1. This means that its mass is equal to the mass of 1/12 of the mass of the carbon atom. Oxygen (O) has a relative atomic mass of 16 amu. This means that an oxygen atom is 16 times more massive than 1/12 a carbon atom, it has 16 atomic mass units.
The lightest element is hydrogen. Its mass is approximately equal to 1 amu. The heaviest atoms have a mass approaching 300 amu.
Usually for each chemical element its value is the absolute mass of atoms, expressed in terms of a. e.m. are rounded.
The values of atomic mass units are written in the periodic table.
For molecules the concept is used relative molecular mass (M r). Relative molecular weight shows how many times the mass of a molecule is greater than 1/12 the mass of a carbon atom. But since the mass of a molecule is equal to the sum of the masses of its constituent atoms, the relative molecular mass can be found by simply adding the relative masses of these atoms. For example, a water molecule (H 2 O) contains two hydrogen atoms with A r = 1 and one oxygen atom with A r = 16. Therefore, Mr(H 2 O) = 18.
A number of substances have a non-molecular structure, for example metals. In such a case, their relative molecular mass is considered equal to their relative atomic mass.
In chemistry, an important quantity is called mass fraction of a chemical element in a molecule or substance. It shows how much of the relative molecular weight is accounted for by a given element. For example, in water, hydrogen accounts for 2 parts (since there are two atoms), and oxygen 16. That is, if you mix hydrogen weighing 1 kg and oxygen weighing 8 kg, they will react without a residue. The mass fraction of hydrogen is 2/18 = 1/9, and the mass fraction of oxygen is 16/18 = 8/9.
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 included in the periodic table of chemical elements by D.I. Mendeleev is used.
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 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 a substance. One of the best solutions at that time was to bind 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 first created 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 became clear that it was possible to obtain a stable isotope from some of them under artificial conditions. 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.
