List of Contents

• 1. The Nature of the Atom
• 2. Radioactivity

Atoms are the smallest units of ordinary matter that form a chemical element. All solids, liquids and gases consist of atoms. An atom is about 100 picometers in diameter.

Atoms are composed of a nucleus made of one or more protons and a number of neutrons, and one or more electrons bound to the nucleus. The protons are positively charged particles and the electrons are negatively charged. About 99% of an atom's weight is in the nucleus. Atoms are indivisible. Atoms of each element have different properties and combine in predictable ratios.

Atomic Models

Dalton's Model

This model is also described as the billiard balls model. In 1803, John Dalton conducted experiments with gases and used the results to propose the modern theory of the atom based on the following assumptions.

• Matter is made up of atoms that are indivisible and indestructible.
• All atoms of an element are identical.
• Atoms of different elements have different weights and different chemical properties.
• Atoms of different elements combine in simple whole numbers to form compounds.
• Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged.

Thomson's model

This model is also described as the plum pudding model. The positive charges fills the atom while the electrons were embedded throughout the atom. Thomson discovered the electron and since the electron was negative, but atoms neutral, there had to be positive charge inside atoms. Thomson used a beam of cathode rays in a CRT with both an electric field and a magnetic field perpendicular acting on the beam. With only the electric field on, the beam was deflected toward the positive plate. With only the magnetic field on, the cathode rays were deflected into a curved path. When both fields were on, and the field strengths equal, the cathode rays were not deflected.

Rutherford's Model

Around 1911 Rutherford, Marsden and Geiger performed experiments to test the Thomson model. They directed alpha particles from radioactive sources onto thin gold foils. The Thomson model predicted that most of the alpha particles would go straight through, and only a few would be deflected at small angles since the electrons in the atom have much less mass than alpha particles. Most of the particles went straight through undeflected, some were deflected at angles of more than 10^o and a few were deflected almost straight back. He concluded that most of the atom was empty space with most of the mass and all of the positive charge concentrated in a very small region (the nucleus). Scattering angles indicated the size of the nucleus was about 10¹⁵ to 10¹⁴m in radius.

In Rutherford's model, electrons could 'orbit' the nucleus at any energy level. The closer the alpha particle is to the nucleus the greater its potential energy.

The Bohr Atom

The study of emission and absorption spectra led to the development of Bohr postulates. Incandescent solids, liquids and high pressure gases emit a continuous spectrum. Low pressure gases emit a bright line spectrum when excited by heat or electricity. Each element has its own emission spectrum, which shows that the atoms of that element can emit only certain photon energies.

A cool low pressure gas will absorb some wavelengths when white light is passed through it, resulting in a dark line i.e. absorption spectrum. The wavelengths that are absorbed match the emission spectrum. Each element has its own absorption spectrum which shows that the atoms absorb only certain photon energies.

Johann Jacob Balmer discovered an empirical equation that predicted the wavelengths of the 4 visible lines of hydrogen. Bohr used Balmer's equation to explain that the spectral lines corresponded to differences between quantized energy levels in the hydrogen atom, which led to the Bohr model of the atom. The Bohr model has three distinct principles:

• Electrons orbit the nucleus only at specific distances from the nucleus. These distances are multiples of the radius of the smallest permitted orbit. Thus the orbits around an atom are quantized.
• Both the kinetic energy and the electric potential energy of an electron in orbit around a nucleus depend on the electrons distance from the nucleus. Each orbit then corresponds to an energy level for the electron.
• An electron can move from one energy level to another either by emitting or absorbing energy equal to the difference between the two energy levels. An electron that stays in the same energy level is not emitting any energy, therefore the orbits can be defined as stationary states

The most stable energy level of an atom is the Ground state. Atoms can absorb energy causing the electron to transition to a higher energy state. The atom is said to be excited. Excited electrons are not stable and they transition back to the ground state. The energy emitted by the electron as it transitions back to ground state is emitted as a photon called Florescence. When the electrons are at the infinite energy level, they have 0.00 energy.

In 1896, Henri Bequerel discovered that Uranium and other elements emitted invisible rays that can penetrate solid material. These materials are now called 'Radioactive'. Radioactivity is defined as the process by which atoms emit energy in the form of electromagnetic waves, charged particles or uncharged particles. The unit for radiation is counts per second, also known as a Becquerel (Bq). The Geiger counter is the device used to measure ionizing radiation.

Exposure to radiation is unavoidable because radioactive elements occur in nature. Some forms of carbon absorbed in our bodies are radioactive. Cosmic rays are high energy radiations from space and individuals who are flying at high altitudes get higher exposure. Radioactive elements like uranium and radium are found in soil and rocks.

Artificial sources of radioactivity include electricity, space probes and submarines. Medical equipment such as X-rays and gamma rays used in diagnosis and cancer treatment respectively.

Effects of Radiation

The energy carried by ionizing radiations can break chemical bonds. These can cause mutations in DNA and these mutations have more impact on cells that are dividing rapidly. Mutation can cause cell damage, which may either result in cell death (apoptosis) or the damaged cells may be transmitted from parents to offspring causing genetic defects and/or malformations.

The strength of radiations depend on three factors:

• The kind of particles emitted.
• The amount of radiation released.
• The rate of emission.