16.1 The Atom

Atomic and nuclear physics is one of the most impressive achievements of this century.

The concepts and techniques developed in this field are used in almost every area of science or technology.

The life sciences have been aided by the theories and techniques of atomic and nuclear physics.

The techniques provided many tools for both experimental and clinical work, and the theories provided a solid foundation for understanding the structure and interaction of organic molecule.

It is difficult to do justice to contributions from this field in a single chapter.

Our discussion will be limited to a survey of the subject.
We will discuss the applications of atomic and nuclear physics to the life sciences with a brief description of the atom and nucleus.

By 1912, a number of important facts about atoms had been discovered.

Small negatively charged electrons and heavier positively charged protons are found in atoms.
The electron is 2000 times heavier than the protons, but the charge on the two is the same.
There are more positively charged protons in an atom than negatively charged electrons.
The atom is neutral.

For example, hydrogen has 1 protons, carbon has 6 protons, and silver has 47 protons.

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The nucleus also contains a second particle, the neutron, which is neutral and has the same mass as the protons.

The nucleus contains most of the atomic mass, but only a small part of the total atomic volume.

The nucleus has a diameter of 10-13 cm, while the whole atom has a diameter of 10-8 cm.

The electrons around the nucleus were not known at that time.

In 1913, a model for the atom was proposed by a physicist.

The subject was in a state of confusion when he first became aware of atomic physics.

There were a number of theories proposed for the structure of the atom.
Each element emits a spectrum of light.
This is in contrast to a light bulb that emits light over a long period of time.

Scientists couldn't explain why the colors were emitted by atoms.

The reason for the sharp spectrum was explained by the model of the atom.

The model of the atom was proposed by Rutherford.

The positive nucleus is located at the center of the atom.

As the planets travel around the sun, the electrons travel around the nucleus.

They are maintained by the nucleus.

The main feature of the model is that it would explain the emission of lines.

The electrons can only be found in certain areas.
Bohr was able to show that the lines are caused by the orbital restrictions.
The calculations are found in most physics texts.

The simplest atom is hydrogen, which has a single-proton nucleus and one electron around it.

The electron is closest to the nucleus if energy is added to the atom.
Light is often referred to as radiation at shorter wavelength and longer wavelength.

The electron can only occupy a small part of the nucleus with a certain number of radii.

Many of the observations for the simple hydrogen atom were explained by the Bohr model.

The maximum number of electrons is 2 x (1)2 2, 2 x (2)2 8 and so on.

The atoms are constructed according to these restric tions.

There are two electrons in Helium, and thus it has its first orbit filled.
Two of the three electrons must be in the first and second loops.
The way the elements are constructed is the same as the way this simple sequence is applicable to the very complex atoms.

There is a specific amount of energy associated with each allowed orbital con figuration of the electron.

The electron can be referred to as having a corresponding amount of energy instead of being in a certain position.

Every element has its own energy level structure.

But can't have an energy between the two values.
There are restrictions on the allowed electron orbital configurations.

The electron can be excited into a higher energy state by adding energy to the atom.

An atom can be excited in a number of different ways.

The two most common methods are absorption and electron impact.
In a gas discharge, emission by electron impact is the most common.
The electron in the atom is promoted to a higher energy configuration if a current is passed through a gas of atoms.
Excess energy is given off when excited atoms fall into the lower energy states.
Excess energy is released in a single photon.

The emission of light at a specific Frequency is called transition or resonance Frequency.

A group of highly excited atoms of a given element emit light at a number of well-defined frequencies.

An atom in a given energy level can be excited to a higher level by light.

Each photon has the right amount of energy to promote the atom to one of the higher allowed energy states.
The light is only absorbed at the specific transition frequencies.

If a beam of white light is passed through a group of atoms of a given species, the spectrum of the transmitted light shows gaps corresponding to the absorption of the specific frequencies by the atoms.

Most of the atoms are in the ground.
The absorption spectrum usually contains only lines associated with transitions from the ground state to higher allowed states.

The outer electrons of the atom produce the optical spectrum.

The electrons that are closer to the nucleus are more difficult to excite.
An inner electron may be excited in a collision with another particle.
Excess energy is released as a quantum of radiation when an electron returns to the atom.
The binding energy here is about a thousand times greater than the outer electrons.

The formation of chemical bonds was explained by the Bohr model.

The formation of chemical compounds and matter in bulk is due to the distribution of electrons.