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1 Basic Chemical Terms
Elementary Particles of Matter, Substances, Elements, Compounds
Substance is a specific kind of matter, which exhibits stable physical characteristics at definite conditions.
All substances are built up by atoms.
Atom is the smallest electrically neutral particle, which still keeps the chemical characteristics (quality)
of the corresponding element. It consists from a positively charged nucleus and negatively charged
electron shell. The arrangement of electrons in the shell influences the chemical characteristics of an
atom.
Molecule is the smallest part of a pure substance, which still keeps the chemical characteristics of this
substance. It is electroneutral, characterized by a specific composition (nature and number of the present
atoms) and structure (their mutual arrangement in space, kind of bonds). Di- or polyatomic molecules
of one type of chemical element have homoatomic composition (they contain one kind of atoms
e.g. H2, O2, N2, Cl2). Molecules of compounds have heteroatomic composition, e.g. HCl, H2O etc. In
some substances (e.g. solid substances with crystal structure), the individual molecules are not identifiable.
In this case, we formally appoint the smallest part of the substance (the smallest numerical ratio
of atoms), the composition of which still describes the composition of the whole substance, and we
call it formula unit (e.g. a pair of ions Na+
and F−
in sodium fluoride, carbon atom in diamond, Si
atom and two O atoms in silicon dioxide etc.). From this formula unit (the repeating part) the name of
the substance is derived.
According to the composition are distinguished chemically pure substances and mixtures (homogeneous,
colloid, and heterogeneous). In the nature, substances occur usually in the form of mixtures.
A chemically pure substance (chemical individual) consists of identical particles with stable characteristic
properties (boiling point and melting point, density, refractive index, specific rotation etc.). We
distinguish two groups of chemically pure substances – elements and compounds.
An element is a chemically pure substance consisting of atoms with the same proton/atomic number.
If the element is formed by atoms with the same mass number, it is called nuclide. Atoms of the given
element can be non-combined (noble gases), form molecules (gaseous oxygen O2), or they are combined
altogether to form more complex structures (covalent bonds – diamond, metal bonds – metals,
covalent bonds and van der Waals forces – graphite).
A compound is a chemically pure substance consisting of two or more different atoms. The atoms can
be arranged in isolated molecules (gaseous carbon dioxide) or they can form more complex structures
(crystalline structures – sodium chloride).
A mixture (dispersion system) consists of several different, chemically pure substances. The mixture
can be homogeneous (formed by only one phase – air, solutions), colloid (e.g. egg white in water) or
heterogeneous (more phases physically separated from each other – water with sand).
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Expressing the Amount of Substance and Their Structural Elements
The amount of any substance (chemical element, compound, mixture) can be expressed in several
ways: mass (m), volume (V) or the number of basic structural particles (atoms, ions, molecules).
Atomic and Molecular Mass
Mass (m) is an additive quantity, which does not depend on the temperature and pressure. The mass of
the structural particles can be expressed in common mass units (kg, g). The absolute mass of an atom
(the value of its resting mass) is very small, about 10−27
–10−25
kg. Thus, the absolute quantities are
impractical for everyday use. Mass can be more conveniently expressed by comparing to standard
(object with a mass similar to the mass of these particles). This standard was stated during the last
correction in 1961 to be the atom of a carbon nuclide 12
C. Exactly 1/12 of the mass of this atom m(12
C)
was determined as an accessory unit. It is the unified atomic mass unit (symbol u); in biochemistry
and molecular biology, it is more often called dalton (symbol Da). The value of its mass is called the
atomic mass constant (symbol mu).
mu = 1/12 m(12
C) = 1.660 56 × 10−27
kg = 1 u
The absolute atomic masses are not as important from the chemical point of view as the ratio (the relative
amount), under which the atoms react with each other. Therefore, we most frequently use the relative
(proportional) expression of mass (related to the 1/12 of the mass of the carbon nuclide 12
C).
The relative atomic mass (Ar) is a dimensionless number representing how many times the mass of a
given atom (X) is higher than the atomic mass constant mu. It is given by the ratio between the (absolute)
mass of one atom m(X) in kg to the atomic mass constant mu in kg.
For the given element X it is accepted that:
u
r
(X)
X)(
m
m
A =
The relative molecular mass (Mr) is sum of the relative atomic masses of all atoms present in the molecule.
It is equal to the ratio of the molecular mass m(XY) in kg to the atomic mass constant mu in kg.
Volume
Volume, as the expression of the amount of substances, is most frequently used in case of liquids or
gases. It is additive quantity only in case of ideal systems in gaseous state, where the individual particles
do not affect each other. The effect of temperature and pressure on the volume is very significant.
Avogadro’s law is of great importance for calculations of ideal gas volume: at the same pressure and
temperature, equal volumes of different gases contain the same number of particles. In simplified calculations,
the gaseous substances will be assumed to be ideal.
Substance Amount
The substance amount (n) is the quantity related to the number of particles (N). If we used one molecule
(ion, atom) in calculations as the unit for quantity of a substance, we would deal with very large
numbers. Therefore in chemistry is used one mole as the basic unit of the substance amount. It contains
exactly the same number of elementary entities (molecules, atoms, ions, electrons) as there are
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atoms in exactly 12 g of the carbon nuclide 12
C. This number is expressed by the Avogadro’s constant
NA = 6.022 × 1023
mol−1
.
The substance amount (n) is then calculated as
AN
N
n =
The mole is one of the seven SI base units. When the mol is used, the kind of elementary entities must be
always specified.
Molar Quantities
Quantities related to the mole are called molar (mass, volume, charge).
The molar mass (M) is the (absolute) mass of 6.022 × 1023
particles, e.g. 1 mole of a given substance
expressed in g mol−1
. It can be calculated by multiplying the absolute mass of an atom or molecule by
the Avogadro’s constant:
M = mx NA
where mx is a mass of one atom or molecule.
Because the atomic mass can be expressed as the product of the relative atomic mass and atomic mass
unit, it follows that M = Ar × u × NA, which after substitution gives:
M = Ar × 1.66 × 10−27
× 6.022 × 1023
kg mol−1
= Ar × 1.66 × 10−24
× 6.022 × 1023
g mol−1
Because the product 1.66 × 10−24
× 6.022 × 1023
is approximately equal to one, it implies that the mass
of one mole of substance M is numerically equal to the relative atomic or molecular mass, but it is
expressed in grams.
If we know the mass of a substance containing a certain number of moles of this substance, the molar
mass is expressed by the equation:
n
m
M =
The molar volume of a gas (VM) is the volume of one mole of an ideal gas under given conditions. It
is equal to 22.4 l mol−1
at temperature 0 ºC and pressure 101.3 kPa (1 atm). It is calculated by dividing
the volume of a gas (V) by the quantity of a substance (n):
n
V
V =M
The molar charge is the electric charge of one mole of charged particles with one elementary charge
and it is the product of the elementary charge and the Avogadro's constant NA. The elementary
charge is the smallest possible charge of a particle (the absolute quantity of the charge of an electron
or proton) and it is equal to 1.602 × 10−19
C (coulombs). Then the charge of one mole of particles with
one elementary charge is Faraday’s constant F = 1.602 × 10–19
C × 6.022 × 1023
= 96 485 C mol−1
.