Molecular Physics
Introduction
The subjects of molecular physics:
The structure and properties of matter;
The physical laws and principles in this area of knowledge.
Material body model
The substance consists of atoms and molecules that interact in accordance with the quantum mechanics laws.
Atoms – Mendeleev's periodic table.
There are about 100 sorts of atoms.
All atoms of the same type in the Universe are absolutely identical.
There are enormous kinds of molecules.
Masses and sizes of atoms and molecules
The atomic
mass unit
mu
is defined in such a way that the mass of one atom of the isotope 12C
is exactly 12 mu,
where mu
is equal to
.
According to this definition, the proton and neutron each have a mass
of approximately mu
and the electron has a mass that is only a small fraction of this
value.
The relative molecular and atomic masses are (they are dimensionless):
So
and
For DNA
for example
One mole
of any substance is that amount of the substance that contains
Avogadro’s number
of constituent particles (atoms or molecules).
The number
of moles
of a substance is related to its mass m
through the expression
,
where M is the molar mass of the substance. The molar mass of each chemical element is the atomic mass (from the periodic table) expressed in grams per mole. For example, the mass of one He atom is 4.00 mu (atomic mass units), so the molar mass of He is 4.00 g/mol.
So 
Molar mass
The size of molecules
Let's consider the water molecule H2O:
It is known that 1g of water takes a volume 1 sm3 and therefore one mol of molecules of water has a volume 18 sm3. The volume per one molecule of water is:
And the average size of molecule is:
Aggregative states of matter
The characteristic's of gas, liquid, solid and plasma.
For gas (and rare plasma):
,
where
- is the average distance between the molecules.
For solids and liquids:
Plasma – it is ionized gas.
Thermal motion of molecules – M.V.Lomonosov
Ideal gas model
It is a simplest model of many body systems.
Ideal gas consists of chaotically moved molecules that interact as elastic balls.
By means of energy description the ideal gas has only kinetic energy. The potential energy of molecular interactions is neglected compare with it because of large average distances between molecules.
Therefore the internal energy of ideal gas depends only on kinetic energy of its molecules.
This model is valid for low density of matter.
Three methods in molecular physics
Dynamical method – it is a many body problem. The investigation of the system provides with numerical experiments by the computers technique.
Statistical method – it is an investigation by means of average quantities, statistics and probabilities. The theory of probability is the basic language for this description.
Thermodynamical method – it doesn't interesting the internal structure of matter, it deals with the general physical laws and principles such as conservation laws that are valid for any kinds of physical system.
Thermodynamical systems
Thermodynamical system (TDS) – it is a set of material bodies that interchange the energy between each other and external bodies.
TDS may have different states that characterized by the parameters of state or state variables such as temperature, volume, pressure, density and so on.
The state and process.
TD process – it is a transition from one state to another. The process it is always the disturbance of steady state.
Equilibrium state and non-equilibrium state.
Isolated TDS. The process of relaxation.
Reversible and irreversible processes.
Temperature
Let’s assume two objects are in thermal contact with each other if energy can be exchanged between them by electromagnetic radiation and heat due to a temperature difference.
Thermal equilibrium is a situation in which two objects would not exchange energy by heat or electromagnetic radiation if they were placed in thermal contact.
Let’s consider two objects A and B, which are not in thermal contact, and a third object C, which is our thermometer. We wish to determine whether A and B are in thermal equilibrium with each other. The thermometer (object C) is first placed in thermal contact with object A until thermal equilibrium is reached. From that moment on, the thermometer’s reading remains constant and we record this reading. The thermometer is then removed from object A and placed in thermal contact with object B. The reading is again recorded after thermal equilibrium is reached. If the two readings are the same, we can conclude that object A and object B are in thermal equilibrium with each other. If they are placed in contact with each other, there is no exchange of energy between them.
We can summarize these results in a statement known as the zeroth law of thermodynamics (the law of equilibrium):