Matter is made out of atoms, and there are over a hundred
different atoms.  Various atoms can bind together to
form molecules. Because of their tiny size (only 10^-10 meters)
a quantity of a given substance can be observable with unaided
eye when the quantity of the substance contains of the order
of 10²³ such atoms or molecules.

An atom has a nucleus and electrons which are moving in
Shells around the nucleus. The nucleus is made of
protons and neutrons. The protons have positive electric
charge and the electrons carry negative charge, while the
neutrons are electrically neutral (no charge).  An electrically
neutral atom, is more stable and it has equal number of electrons
and protons.  The electric attractive force between the
positively charged protons in the nucleus and the negatively
charged electrons binds the electrons and they move in shells
around the nucleus.

The simplest atom is the Hydrogen atom with one proton
and no neutron in the nucleus and one electron in a shell
around the nucleus. The next element is Helium and the
most abundant form of helium is the isotope helium-4
having two protons and two neutrons in the nucleus
and two electrons.

A rather simple molecule is the water molecule which is
made out of two hydrogen atoms and one oxygen atom.

The atoms or molecules can form three states of matter, gas,
liquid or solid. A gas is a system in which its atoms or molecules
have random positions and motions. In a liquid, however, the
atoms or molecules are bound together to form large groups of
molecules or atoms which are visualized as droplets. Also, in a
liquid, the atoms or molecules have a rather random motion but
they prefer to stay near other atoms or molecules because of
mutual attraction. In a solid, the atoms or molecules are grouped
together in large groups and the atoms or molecules have
ordered positions in space forming arrays. We are going to
see later in the class, that there are other states of matter,
such as neutron stars of which we have no experience of earth.

Atomic excitations

The shells in which the electrons move in an atom are
characterized by an energy level. The lowest energy level
is called ground state. If an electron in an atom receives
energy from outside the atom, the electron goes in a
outer shell (further away from the nucleus) which is
called an excited state. The electron stays in such an
excited state only for very short time (within 10^-6 to 10^-9
seconds) and eventually it returns to its ground state by
emitting a photon which carries the energy lost from the
electron. A photon is a particle of light and at the same
time an electromagnetic wave of some frequency f. The
energy carried by such a photon is given by

E = h f

where h is the so-called Planck's constant (h=6.626 10^-34 Joules seconds).

The important fact to remember is that these energy levels
in which the electrons can get excited have well defined
energy. In other words, the electrons cannot receive any amount
of energy one wishes to give. They only absorb the exact amount
of energy which is the energy difference between two such
shells. The deep reason underlying this effect can be
explained by the Quantum Theory one of the highest
theoretical achievements of the 20th Century Physics.
An analogy from the everyday experience is the following.
When one wants to go up a floor using stairs, one has to
step on each step and thus raise his energy level by a well
defined amount. In other words, one cannot go in a continous
way because he cannot stay in between steps.
Such a stair case of the electron level in the hydrogen atom
are shown in the following figure.
 

The emission and absortion spectrum of hydrogen in the visible
range is the following.

Generally an emission spectrum is obtained as follows

Generally the absorption spectrum looks as follows

Radiation from a heated object.

Back body radiation

The peak of the spectrum is at a wavelength L_max in nanometers given by

L_max   =  3,000,000/T

where T is the temperature in degrees Kelvin, the absolute temperature scale
which is given in term of degrees Celsius by T = 273 + C.

The Stefan-Boltzmann Law states that the total energy E radiated in 1 second
from a sourse which is held at temperature T is given by

E = s T⁴

where s=5.67 x 10^-8 Joules/m²/sec degree⁴ is the so-called Stefan-Boltzmann
constant.