Atomic structure 2 Flashcards
Evidence for sub-atomic particles
Atoms are the fundamental building blocks of matter. However, even atoms are constructed of smaller, sub-atomic particles. It was originally thought that the atoms were small indivisible particles (atomos = indivisible), however a paradigm shift occured with the discovery that the atoms themselves have a sub-structure.
Evidence for this emerged though the experiments conducted in the nineteenth century on cathode rays, culminating in the experiments of Rutherford (the nuclear atom) and Chadwick (neutrons) in the early 20th century.
It is now known that even these sub atomic particles themselves have an even deeper sub-structure - the quarks!
The sub-atomic family
The fundamental sub-atomic particles are protons, neutrons and electrons. The protons and neutrons are held together by strong nuclear binding forces in the nucleus of the atom. The electrons may be considered to be tiny particles that exist in regions of space known as orbitals around the atom.
This model of the atom is precisely that, a model. It is impossible to see atoms and, in order to be able to describe their properties, we use models representing this microscopic world that is invisible to us.
Similarly, the sub-atomic world is a strange place with unusual forces acting over infintesimally small distances. The rules of behaviour that govern the macroscopic world often break down in this strange environment, and it is important to understand that our representations and models are necessarily limited here.
The picture of the atom above is easy to discuss and is comfortably familiar. However, if you consider the actual dimensions of an atom compared to the nucleus you can see just how inaccurate even this simple picture is.
Hydrogen atomic radius: 3.7 x 10-11 m
Hydrogen nucleus radius: 8 x 10-16 m
You should appreciate that the nuclear radius is much smaller than the atomic radius by a factor of about 100,000. This means that the atom is mostly empty space with a very solid and tiny nucleus. This was originally demonstrated by the scattering experiments of Ernst Rutherford.
Electrical charges
Summary of fundamental particle charge and location
Particle
Proton 1+ (positive) , nucleus
Neutron
nucleus
none
Electron
energy shells
1- (negative)
Mass number
The atomic mass number is represented by the symbol (letter) ‘A’. This is not to be confused with the relative atomic mass Ar.
The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.
The relative mass is a value that is not necessarily integral that compares a mass to the mass of a carbon isotope, assigned a value of exactly 12.0000 units.
Atomic number
This is represented by the symbol (letter) ‘Z’. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.)
Ions
The system can be extended to cover ions simply by adding the charge onto the element symbol. It is important to remember that a positive ion has LOST electrons.
Example: Determine the number of electrons in the following ion:
The atomic number is 12 therefore in a neutral atom there would be 12 protons and 12 electrons.
However, the charge is 2+ therefore the atom has LOST 2 electrons
The remaining electrons then = 10 electrons
Uses of isotopes
Isotopes are used in medicine, industry, and in many other applications. The danger of radioisotopes revolves around their ability to cause cell damage by ionising the atoms that are involved in molecules and hence, breaking bonds. Radioisotopes may emit three different common types of radiation, alpha, beta and gamma radiation, depending on the specific atom.
Alpha radiation consists of particles containing two protons and two neutrons (equivalent to a helium nucleus); is highly destructive to living tissue, but has very low penetration power and is stopped by a few centimetres of air. It is only seriously dangerous if ingested in some way.
Beta radiation consists of highly energetic electrons. It has poorer ionising characteristics than alpha radiation, but has greater penetrating power.
Gamma radiation is electromagnetic in nature and has the lowest ionising ability, but extremely great penetrating power.
14C - radiocarbon dating
Living organisms respire. Plants breathe in carbon dioxide and animals eat plants (and other animals!). The consequence is that all living things take up carbon throughout their lives. The percentage of the isotope carbon 14 remains fairly constant in our atmosphere, as it is produced in the upper atmosphere by cosmic bombardment of naturally occurring carbon-12 in the form of carbon dioxide. At the same time the carbon 14 nuclei are decaying. There is an equilibrium between these two processes:
carbon-12 carbon-14
This means that the proportion of carbon-14 compared to carbon-12 found in all living organisms is also constant. However, when a living organism dies it stops taking up both forms of carbon. The carbon -14 isotope decays naturally with a half life of about 5,600 years. So, a simple procedure involving counting the radioemissions due to carbon-14 from a sample of material that was once alive, can be used to estimate how long ago it died.
Therapeutic applications
Cobalt-60 is used in hospitals as a beta emission source in the treatment of cancer
Beta rays are fast moving electrons. They can be focussed onto cancerous tissue to destroy it using a cobalt 60 source. This form of treatment is known as radiotherapy.
Iodine-131 and Iodine-125 are used as medical tracers and for treating certain cancers.
In several conditions the body can be scanned for problems using iodine, which is easily taken up by the body and transported through the lymphatic system. The isotopes 131I and 125I are easy to detect and short lived in the body.
Use is made of the destructive effect on cellular tissue to destroy cancer cells in treatment with radioisotopes. Radioactive sources are used that have a short lifetime in the body, but which can be focussed in their effects on tissues.
Technetium-99, for example is used in gammagraphy, a technique where a sample of the radioisotope is injected into the body. After a few hours the technetium circulates around the body and binds to areas of bone damage. By detection of areas of unusually high concentration of radiation, it is possible to identify bone injuries that do not show up on X-rays.
The bone scans of radiation emissions are called gammagrams.
Industrial applications
Detection of leaks in gas pipes by injection of a radioisotope into the pipeline and detecting where the radiation emerges. Beta emitters are used in measurement of thickness in the paper industry.
Nuclear energy generation
Both uranium-235 and plutonium-239 are neutron emitting radioactive isotopes. The neutrons emitted cause further events in neighbouring nuclei leading to chain reactions, which release large amounts of energy as the nuclei break apart (fission). This energy is used to heat up water into steam to drive turbines for electricity production.
Nuclear energy remains controvertial and there are strong arguments both for and against its use.
Other applications
Americium-241 is a man-made isotope used in smoke detectors.
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