The composition of substances is complex, although they are formed by tiny particles - atoms, molecules, ions. Many liquids and gases, as well as some solids, have a molecular structure . Metals and many salts are composed of atoms and charged ions. All particles have mass, even the smallest molecule. The mass of a molecule, if expressed in kilograms, gets a very small value. For example, m ( 2 ) = 30 • 10 -27 kg. The most important characteristics of a substance, such as the mass and size of microparticles, have long been studied by physicists and chemists. The foundations were laid in the writings of Mikhail Lomonosov and John Dalton. Let us consider how the views on the microworld have changed since then.
Lomonosov’s notions of “corpuscles”
The assumption of the discrete structure of matter was expressed by scientists of Ancient Greece. Then the name "atom" was given to the smallest indivisible particle of bodies, the "brick" of the universe. The great Russian researcher M.V. Lomonosov wrote about the insignificantly small, indivisible by physical methods particle of the structure of matter - the corpuscle. Later, in the writings of other scientists, it was called the "molecule".
The mass of a molecule, as well as its size, are determined by the properties of its constituent atoms. For a long time, scientists were not able to look deep into the microworld, which hindered the development of chemistry and physics. Lomonosov repeatedly called on his colleagues to study and rely on accurate quantitative data - “measure and weight” in their work. Thanks to the work of the Russian chemist and physicist, the foundations of the doctrine of the structure of matter were laid, which became an integral part of a harmonious atomic-molecular theory.
Atoms and molecules - "bricks of the universe"
Even microscopically small bodies are complex, have various properties. Particles such as atoms are formed by the nucleus and electron layers, differ in the number of positive and negative charges, radius, and mass. Atoms and molecules do not exist in the composition of substances in isolation, they are attracted with different strengths. The action of attractive forces in solids is more noticeable, weaker in liquids, they are almost not felt in gaseous substances.
Chemical reactions are not accompanied by the destruction of atoms. Most often, their rearrangement occurs, another molecule appears. The mass of a molecule depends on what atoms it is formed. But with all changes, atoms remain chemically indivisible. But they can be part of different molecules. At the same time, the atoms retain the properties of the element to which they relate. The molecule until its decay into atoms retains all the signs of matter.
A microparticle of the structure of bodies is a molecule. Molecule mass
To measure the mass of macrobodies, instruments are used, the oldest of which is scales. The measurement result is conveniently obtained in kilograms, because this is the main unit of the international system of physical quantities (SI). To determine the mass of a molecule in kilograms, it is necessary to add the atomic masses taking into account the number of particles. For convenience, a special unit of mass was introduced - atomic. You can write it in the form of a literal abbreviation (amu). This unit corresponds to one twelfth of the mass of carbon nuclide 12 C.
If we express the found value in standard units, we get 1.66 • 10 -27 kg. Such small indicators for the mass of bodies are operated mainly by physicists. The article provides a table from which you can find out what the masses of atoms of some chemical elements are equal to. To find out what is the mass of one hydrogen molecule in kilograms, we multiply by the atomic mass of this chemical element shown in the table. As a result, we obtain the mass value of a molecule consisting of two atoms.
Relative molecular weight
It is difficult to operate with very small values in the calculations, this is inconvenient, leads to time wasted, to errors. As for the mass of microparticles, the way out of the difficult situation was the use of relative values. The term familiar to chemists consists of two words - “atomic mass”, its designation is Ar. An identical concept was introduced for molecular mass (the same as molecular mass). A formula connecting two quantities: Mr = m (in-va) / 1/12 m ( 12 C).
You can often hear what they say "molecular weight." This obsolete term is still used in relation to the mass of the molecule, but less and less. The fact is that weight is another physical quantity - a force that depends on the geographical coordinates of the body. On the contrary, mass serves as a constant characteristic of particles that participate in chemical processes and move at ordinary speed.
How to determine the mass of a molecule
The exact determination of the weight of the molecule is carried out using a device - a mass spectrometer. To solve problems, you can use information from the periodic system. For example, the mass of an oxygen molecule is 16 • 2 = 32. We perform simple calculations and find the value of Mr (H 2 O) - the relative molecular mass of water. According to the periodic table, we determine that the mass of the oxygen atom is 16, the hydrogen is 1. We carry out simple calculations: M r (H 2 O) = 1 • 2 + 16 = 18, where M r is the molecular mass, H 2 O is the water molecule, H is the symbol of the hydrogen element, O is the chemical sign of oxygen.
Isotope masses
Chemical elements in nature and technology exist in the form of several types of atoms - isotopes. Each of them has an individual mass, its value cannot have a fractional value. But the atomic mass of a chemical element is most often a number with several decimal places. When calculating, the prevalence of each species in the earth's crust is taken into account. Therefore, the masses of atoms in the periodic system are not always integers. Using such quantities for calculations, we obtain the masses of molecules, which are also not integers. In some cases, rounding is allowed.
Molecular mass of non-molecular substances
Most inorganic substances have no molecular structure. Metals are made up of atoms, ions and free electrons, and salts are made up of cations and anions. For substances of non-molecular structure, the mass of conditional molecules is also calculated according to a gross formula that reflects the simplest composition. Find the value of Mr for the ionic substance - sodium chloride, whose formula is NaCl. M r = 23 + 35.5 = 55.5. Some types of calculations require a molecular mass of air - a mixture of gases. Given the percentage of different substances in the atmosphere, the molecular mass of air is 29.
Dimensions and mass of molecules
On electron micrographs of large molecules, individual atoms can be considered, but they are so small that they are not visible in a conventional microscope. The linear particle size of any substance, like mass, is a constant characteristic. The diameter of a molecule depends on the radii of the atoms that form it, their mutual attraction. Particle sizes change with increasing number of protons and energy levels. The hydrogen atom is the smallest in size, its radius is only 0.5 • 10 -8 cm. The uranium atom is three times the size of the hydrogen atom. The real "giants" of the microworld are molecules of organic substances. So, the linear size of one of the protein particles is 44 • 10 -8 cm.
To summarize: the mass of molecules is the sum of the masses of atoms that make up their composition. The absolute value in kilograms can be obtained by multiplying the molecular weight found in the periodic table by 1.66 • 10 -27 kg.
Molecules are negligible compared to macrobodies. For example, the water molecule H 2 O is inferior in size to an apple as many times as many times as much as this fruit is smaller than our planet.