There were many theories on the structure of an atom. Here are some of the most important ones.
J.J. Thomson's Model
Using an ampoule similar to Crooke's but with the perforated anode and a set of equipment that formed an electric field and a magnetic field, Thomson managed to discover the relationship load/mass of the electron .
Thomson laid the fundamental concept of the structure of the atom. He referred to it as the plum pudding model, in which the nuts showed the negatively charged particles.
He proposed the following two key points:
- An atom consists of a positively charged sphere, and the negatively charged particles are embedded in it.
- The negatively charged particles and the positively charged particles are equal in magnitude, i.e. an atom is electrically neutral.
This model had serious drawbacks; it was proved that this was a wrong concept by the next model as it couldn't explain the results of the gold foil experiment carried out by Ernest Rutherford in 1911.
He conducted a very famous experiment, the gold foil experiment. In this experiment, he bombarded alpha particles on a thin gold foil. He expected that the α particles would be deflected by the gold atoms, as he believed that the atom was a positively charged sphere. Moreover, he thought that matter was composed of atoms, in which there was no space. Imagine it like a row of marbles, one next to the other. With this idea in mind, all alpha particles should bounce back.
Astonishingly, only one out of every -particles bounced back. And very few got deflected by some angles. So he concluded that
- The positively charged sphere, the nucleus , is very small and densely packed, situated in the center.
- The electrons (negatively charged particles) revolve around the center in circular paths.
- The size of the nucleus is very small, i.e. most of the space in the atom is empty. One Angstrom for the atom, m for the nucleus, which means the nucleus is 100000 times smaller than the atom
But later his second conclusion was not accepted, because if the electrons revolve in a circular motion, they must spend energy and finally will fall into the nucleus, which means the atoms will be unstable. After a few years, Niels Bohr came up with a new model explaining the stability correctly. Although this model got the basics right, there are fundamental problems with it. For one, it doesn't explain why there are only 2 electrons in the K-level.
He explained the drawbacks of the previous model. His proposed the following:
- The electrons revolve in discrete orbits, which help them in overcoming the energy loss.
- They don't radiate energy in these orbits. Hence they are stable.
- Angular momentum is quantized.
What are Orbitals?
Quantum mechanical models are the modern concept to explain the structure of atoms and describe the state of electrons in an atom in terms of probability of finding electrons in the space around the nucleus .This leads to the concept of orbitals.
It is defined as "the three-dimensional region in space around the nucleus where the probability of finding the electron is maximum."
The classical concept of orbits, like planets revolving a sun, for example, is given up in the quantum mechanical description of the atomic world. We only know where there is a high chance that the electron is to be found. We can no longer pinpoint the position of the electron (nor the proton nor the neutron...), but we can define a probability function which gives us a good idea of where it should be. Thus the orbitals are the region where the probability of finding an electron is high. An electron can move anywhere in an atom, even inside the nucleus, or theoretically even at the other side of the universe, but 90% probability is that it is found in a small specific region of space around the nucleus, and this region is nothing but the"orbital."
This idea radically changes our view, to the extent that even Einstein himself doubted the model. He was willing to consider it a correct, yet incomplete model. The fundamental notion that we can only statistically gain information about quantum mechanical properties is a radical thought changer from the deterministic view Newton used to uphold. Even the theory of relativity is utterly deterministic.
However, all experiments conducted so far strengthen the quantum mechanical and we even have strong reasons to believe the great master himself was wrong in his doubting.
Table of Contents
Atoms | Electrons and energy | Chemical Bonding | Chemical reactions and molecules
Learning Objectives| Terms | Review Questions | Links
Atoms | Back to Top
Electrons and energy | Back to Top
Chemical Bonding | Back to Top
Chemical reactions and molecules | Back to Top
Learning Objectives| Back to Top
- All forms of matter are composed of one or more elements. Be able to list the major elements in living things.
- Describe how protons, electrons, and neutrons are arranged into atoms and ions.
- Define the terms atomic number and atomic mass and be able to describe their sugnificance.
- Atoms with the same atomic number but a different mass number are isotopes. List the isotopes of hydrogen and of carbon.
- Be able to describe radioisotopes and list three ways they are used in biology.
- The union between the electron structures of atoms is known as the chemical bond. Be able to list and describe the three types of chemical bonds found in living things.
- Be able to describe the distribution of electrons in the space around the nucleus of an atom.
- An atom tends to react with other atoms when its outermost shell is only partly filled with electrons. Be able to discuss why this happens.
- Be able to define the two types of ions and describe thow ionic bonds form between positive and negative ions.
- In a covalent bond, atoms share electrons. List several elements that tend to form covalent bonds.
- Distinguish between a nonpolar covalent bond and a polar covalent bond and give an example of each.
- Define hydrogen bond and describe conditions under which hydrogen bonds form and cite one example.
- Explain what is meant by the polarity of the water molecule, and how the polarity of water molecules allows them to interact with one another.
Terms | Back to Top
Review Questions| Back to Top
- Which of these is not a subatomic particle? a) proton; b) ion; c) neutron; d) electron
- The outermost electron shell of every Noble Gas element (except Helium) has ___ electrons. a) 1; b) 2; c) 4; d) 6; e) 8
- An organic molecule is likely to contain all of these elements except ___. a) C; b) H; c) O; d) Ne; e) N
- The chemical bond between water molecules is a ___ bond. a) ionic; b) polar covalent; c) nonpolar covalent; d) hydrogen
- A solution with a pH of 7 has ___ times more H ions than a solutrion of pH 9. a) 2; b) 100; c) 1000; d) 9; e) 90
- The type of chemical bond formed when electrons are shared between atoms is a ___ bond. a) ionic; b) covalent; c) hydrogen
- The type of chemical bond formed when oppositely charged particles are attrached to each other is a ___ bond. a) ionic; b) covalent; c) hydrogen
- Electrons occupy volumes of space known as ___. a) nuclei; b) periods; c) wavelengths; d) orbitals
- Carbon has an atomic number of 6. This means it has ___. a) six protons; b) six neutrons; c) six protons plus six neutrons; d) six neuitrons and six electrons
- Each of the isotopes of hydrogen has ___ proton(s). a) 3; b) 1; c) 2; d) 92; e) 1/2
- A molecule is ___. a) a mixture of various components that can vary; b) a combination of many atoms that will have different ratios; c) a combination of one or more atoms that will have a fixed ratio of its components; d) more important in a chemistry class than in a biology class
Links | Back to Top
- ChemicoolA colorful and easy to use Periodic Table. More information about elements than most of us would want.
- WebElements Much more detailed Periodic Table. Even more information about those pesky elements! If this site is busy there are a series of mirror sites.
- The Periodic Table This site, developed by the Fossil Company, provides a version of the Periodic Table where moving the mouse cursor over an element on the table brings up information about electron shells, mass, etc. A very nice site.
- James K. Hardy's Chemistry Site (U of Akron). Powerpoint slides (that run over the Web) and a series of animations...a must see!
- Diamond Images of diamond and graphite crystal structure.
Text ©1992, 1994, 1997, 1998, 1999, 2000, 2001, 2002, 2007, by M.J. Farabee, all rights reserved. Use for educational purposes is encouraged.
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