Molecule: the mass of the molecule. Dimensions and mass of molecules

The composition of the substances is complex, although they are formed by tiny particles - atoms, molecules, ions. Molecular structure has many liquids and gases, as well as some solid bodies. Of the atoms and charged ions are composed of metals, many salts. All particles have a mass, even the tiniest molecule. The mass of the molecule, if expressed in kilograms, is very small. For example, m (H ₂ O) = 30 • 10 ^-27 kg. Such important characteristics of matter 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 microcosm have changed since then.

Lomonosov's views on "corpuscles"

The assumption of the discrete structure of matter was expressed by scholars of ancient Greece. At the same time, the name "atom" of the smallest indivisible particle of bodies, the "brick" of the universe, was given. The great Russian explorer, MV Lomonosov, wrote about an infinitesimal particle of the structure of matter-the corpuscle, indivisible by physical methods. Later, in the works of other scientists, it was called the "molecule".

The mass of a molecule, as well as its dimensions, are determined by the properties of its constituent atoms. For a long time, scientists have not been able to look deep into the microcosm, which hampered the development of chemistry and physics. Lomonosov repeatedly urged colleagues to study and in their work to rely on accurate quantitative data - "measure and weight." Thanks to the work of the Russian chemist and physicist, the foundations of the theory of the structure of matter were laid, which became an integral part of the harmonious atomic-molecular theory.

Atoms and molecules are the "bricks of the universe"

Even microscopically small bodies are complex, have different properties. Such particles, as atoms, are formed by the nucleus and electron layers, differ in the number of positive and negative charges, radius, mass. Atoms and molecules exist in the composition of substances not in isolation, they are attracted with different strengths. More noticeable effect of the forces of attraction in solids, weaker in liquids, are almost not felt in gaseous substances.

Chemical reactions are not accompanied by the destruction of atoms. Most often, their rearrangement takes place, another molecule appears. The mass of a molecule depends on what atoms it is formed by. But for all changes, the atoms remain chemically indivisible. But they can enter into the composition of different molecules. In this case, atoms retain the properties of the element to which they belong. The molecule preserves all the signs of matter before its breakup into atoms.

The microparticle of the structure of bodies is a molecule. Molecule weight

Instruments for measurement of macrobot mass are used, the oldest of which is a balance. The measurement result is conveniently obtained in kilograms, because this is the basic 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, atomic, was introduced. You can write it in the form of a letter abbreviation (amum). This unit corresponds to one twelfth part of the mass of the carbon nuclide ¹² C.

If we find the value found in standard units, we get 1.66 × 10 ^-27 kg. Such small indicators for the mass of bodies are operated, in the main, by physicists. The article contains a table from which it is possible to find out what the masses of atoms of some chemical elements are equal to. To find out what the mass of one molecule of hydrogen in kilograms is equal to, multiply by two atomic masses of this chemical element listed in the table. As a result, we get the value of the mass of a molecule consisting of two atoms.

Relative molecular weight

It is difficult to operate in calculations very small values, it is inconvenient, leads to time costs, to errors. As for the mass of microparticles, the use of relative values became the way out of the difficult situation . The term familiar to chemists consists of two words - "atomic mass", its designation is Ar. An identical concept was introduced for the molecular mass (the same as the mass of the molecule). The formula connecting the two quantities: Mr = m (in-va) / 1/12 m ( ¹² C).

It is often possible to hear what the "molecular weight" is saying. This outdated term is still used in relation to the mass of the molecule, but less often. The fact is that weight is another physical quantity - a force that depends on the geographical coordinates of the body. On the contrary, the mass serves as a constant characteristic of particles that participate in chemical processes and move at normal speed.

How to determine the mass of a molecule

The exact determination of the weight of the molecule is carried out using an instrument-mass spectrometer. To solve problems, you can use information from the periodic system. For example, the mass of the oxygen molecule is 16 • 2 = 32. We perform simple calculations and find the value of Mr (H ₂ O) - the relative molecular weight of water. According to Mendeleyev's table, we determine that the mass of the oxygen atom is 16, the hydrogen is 1. Let's perform simple calculations: M _r (H ₂ O) = 1 • 2 + 16 = 18, where M _r is the molecular weight, H ₂ O is the water molecule, H is the symbol of the hydrogen element, and O is the chemical sign of oxygen.

Weights of isotopes

Chemical elements in nature and technology exist in the form of several types of atoms - isotopes. Each of them has an individual mass, its magnitude can not have a fractional value. But the atomic mass of a chemical element is often a number with several signs after the decimal point. Counting takes into account the prevalence of each species in the Earth's crust. Therefore, the masses of atoms in a periodic system are not always integers. Using such values for calculations, we get masses of molecules that are also not integers. In some cases rounding of values is allowed.

Molecular mass of substances of a non-molecular structure

Most inorganic substances have no molecular structure. Metals consist of atoms, ions and free electrons, salts from cations and anions. For substances of non-molecular structure, the mass of the conditioned molecules is also counted according to the gross formula, which reflects the simplest composition. Let us find the value of Mr for the substance of ionic structure, sodium chloride, the formula of which is NaCl. M _r = 23 + 35.5 = 55.5. For some types of calculations, the molecular mass of the air - the mixture of gases - is required. Taking into account the percentage content of different substances in the atmosphere, the molecular weight of the 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 an ordinary microscope. The linear size of a particle of any substance, like the mass, is a constant characteristic. The diameter of a molecule depends on the radii of the atoms forming it, their mutual attraction. The particle size changes 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 larger than the hydrogen atom. The real "giants" of the microworld are molecules of organic substances. Thus, the linear size of one of the protein particles is 44 × 10 ^-8 cm.

Let's summarize: the mass of molecules is the sum of the masses of the atoms that make up their composition. The absolute value in kilograms can be obtained by multiplying the value of the molecular mass found in the periodic table by 1.66 × 10 ^-27 kg.

Molecules are negligibly small in comparison with macrobodies. For example, according to its size, the water molecule H ₂ O yields to the apple in the same amount, how many times this fruit is less than our planet.