The atom is composed of protons, neutrons and electrons. The protons and neutrons are held together by strong nuclear forces (to be discussed in the next chapter). The chemical characteristics of an atom depend on the number of protons and electrons. Ordinary atoms are in neutral charge state, that means the number of protons and neutrons are equal to each other.

 The electrons occupy discreet energy levels inside the atom. In the past it was thought that the electrons had specific radii of orbits, but that is not the case. When an electron jumps from one discreet energy state to a lower energy state, the difference in energy is converted to a photon. These various discreet photon energies are called emission spectra.

Each kind of atom has its own energy levels, and so the emission spectra are unique to each atom. These discreet energy states are called quantum states.

 The energy of the photon is E = h f = h c /l ; where h is called Planck's constant. F is the frequency; c is the speed of light and l is the wavelength.

h = 6.63 ´ 10^-34 J.s

When an electron is completely knocked off an atom, the atom is ionized; i.e. it has a positive charge. If the electron from the outside now falls into the atom, it releases all it energy which can have any value and the spectra is now continuous. The continuous spectra shape and mean value is dependent on the temperature.

 Dual nature of particles and waves. Just like photons have momentum, moving particles have wavelength and exhibit all the wave properties, such as diffraction. The de Broglie wavelength is

l = h / p = h / mv.

Quantum mechanics is the study of particles in the atomic and subatomic level. The rules of quantum mechanics control the behavior of particles in an atom. E.g. each electron has a certain quantum value, and no two electrons can have the same, identical quantum number in the same atom.

 In quantum mechanics we cannot tell where a particle is at a given time, We can only calculate probabilities.

 The uncertainty principle of Heisenberg. This principle is often stated as a measurement disturbs the system. It is written mathematically as:

D p_y D y > h.

D ED t > h

where p is momentum, y is position, e is energy and t is time.

Laser: Light amplification by stimulated emission of photons. In a laser an emitted photon stimulates other atoms to emit photons in the same direction.

The nucleus of an atom is made of protons and neutrons. It is bound together by what is known as strong forces. This force overcomes the repulsive electric force as well as keeps the neutrons inside the nucleus. This force can be written as reduction of the mass of the nucleus and is called binding energy. In a nuclear reaction, if we combine 4 protons to form a helium nucleus, the binding energy is released as nuclear energy.

There is a fourth force called weak force that can force either the proton or neutron to be ejected. If this occurs, we call the phenomena radioactivity. If the binding energy is very strong then the proton or neutron cannot be emitted and we ave a stable nucleus.

 Radioactivity is inside the nucleus and cannot be influenced from the outside, such as temperature or pressure. Half-life is defined as the time it takes for half the nucleons to emit either a proton or neutron. After two half-lives we have one fourth of the original atoms left and after 3 half-lives, only one eight is left. This half-life of radioactive elements can be used to determine how old a sample is by measuring the ratio of the unchanged atoms to the changed ones. Radiation can be dangerous to humans or other animals, as the emitted proton or neutron can damage material along its path, by changing the nucleus, and thereby changing the atom.

 A curie is a unit of radioactivity which is equal to 3.7´ 10¹⁰ decays per second.

 Nuclear reactions can change one nucleus into another nucleus.

 Besides protons and neutron we can have electrons emitted by radioactive nucleus called beta rays. Energetic photons called gamma rays or even two protons and two neutrons called alpha particles (same as the nucleus of the helium atom).

Anytime we can change an element from one nucleus to another and the binding energy is reduced, energy is emitted. If this energy can be harness, we have nuclear energy. If the nucleus breaks up into lighter elements, it is called fission. If several nuclei are combined together to form a more massive nucleus, the process is called nuclear fusion.

Example of fission is reactors that use uranium, where the uranium breaks into lighter particles in a containment vessel. A chain reaction occurs whenever the decay product in a reactor causes more nuclear breakup. If this chain reaction occurs in an uncontrolled way, we have an atomic bomb.

 The Sun energy comes from fusion, where 4 protons, under high pressure and temperature are fused together to form an alpha particle and the excess mass is released as energy. If the process is repeated on Earth, we have a hydrogen bomb. Unfortunately we have not yet been able to fuse protons into alpha particles in a controlled way. If that occurs we will have plenty of cheap power.

 The release in energy is computed by means of determining the mass lost. The unit used is the amu, which is 1/16 of the oxygen nucleus. 1 amu is equal to 930 MeV, where 1eV is equal to 1.6´ 10^-19 joules. Of course it takes a lot of amu's to create a measurable amount of energy.