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Physical Structure
Main Concepts
Atomic Structure: 2

Atomic Structure:
part Two

Rutherford's Model Atom

Thompson's and Rutherford's classic experiments showed that most of an atom was empty space. Concentrated in the atomic center is a region of mass sometimes called the nucleus. (In biology, the word nucleus has other meanings, so we will call this region the atomic center). In this central region are the protons and neutrons.

For every positively charged proton in the center, each atom also has a negatively charged electron. The number of protons and electrons are always equal.

Electrons, however, occupy well defined volumes of space around the atomic center. Electrons exist in a universe we would find it hard to imagine. For example, at one and the same time, electrons behave as if they were waves and particles (like bullets). This is called particle-wave duality, also you can never know exactly where they are (technically, this is know as the "uncertainity principle"). You can only speak of the probability of finding an electron within a certain region.

Because the true nature of electrons is hard to understand (without a lot of math!), we use simplified models and pictures to visualize their place in atoms. The most famous of these models was a refinement of the Rutherford model, and is now known as the ...

Bohr Shell Model

In 1913 the Danish theoretical physicist Niels Bohr published an new model to explain how electrons can have stable orbits around the atomic center.

The problem with the Rutherford model was the unstable orbits proposed for the electrons. According to classical theory, any electron moving on a curved path emits energy in the form of electromagnetic radiation. The orbiting electrons would therefore lose energy, move inwards and eventually spiral into the collection of protons and neutrons in the atomic center.

Bohr thought about this problem during his visit to Manchester. Soon he modified the Rutherford model by insisting that the electrons move around the center in orbitals that were fixed in size and energy.

Electrons in lower electron shells (or energy levels) would have lower levels of energy, and electrons in higher electron shells (and hence higher orbitals) would have higher levels of energy .

The modern picture of electrons around atomic centers, therefore, shows volumes of space (often drawn as circles, or other shapes) in which the mysterious electron can be found, most often. As these orbitals are found in electrons shells further and further away from the atomic center, they hold electrons with higher and higher amounts of energy.

Atomic Orbitals: the Rules
    According to the Bohr model, electrons arrange themselves around the atomic center of an atom by following a set of rules:
  1. Negatively charged electrons are attracted to the positively charged atomic center; so they will always try to get as close as they can to the center.

  2. Negatively charged electrons are repelled by other negatively charged electrons; so they will always try to get as far away from other electrons as possible.

  3. Electrons move so fast there is no way we can tell where they are at any one moment; so, we can only define a volume of space where an electron can be found most of the time. These volumes of space are called orbitals.

  4. A maximum of only two electrons can occupy any one orbital at any one time.

  5. According to Bohr, electrons have fixed levels of energy; so, all electrons with the same amount of energy must occupy the same zone, or energy level, or electron shell around the atomic center.

  6. The first energy level (closest to the atomic center) consists of a single orbital (holding two electrons), which is the shape of a sphere. It is called the 1s orbital.

  7. The second energy level consists of 4 orbitals (each with two electrons) a sphere shaped 2s orbital and three dumbbell shaped orbitals called 2p1, 2p2 and 2p3.

  8. When ever possible, electrons occupy the lowest energy level.

Mass and Number

The number of protons in the center of an atom is called its atomic number. The element hydrogen, for example, has one proton (and one electron) and is therefore given the atomic number of one (1). The element deuterium also has one proton in its center and thus shares the same atomic number as hydrogen (1). The element tritium also has an atomic number of one and has one proton in its center. These three elements share the same atomic number, but they differ in other ways.

Hydrogen atoms only have one proton and one electron, whereas deuterium atoms have one proton, one electron and one neutron. Tritium atoms have one proton, one electron and two neutrons. Each proton and neutron have an approximate mass of 1 amu (atomic mass unit) so hydrogen has a mass of one, deuterium has a mass of two units and tritium has a mass of three units. The mass of the atomic center (electrons are not counted in the mass, they are too small) is called the atomic mass number or more often the atomic mass.

Isotopes are groups of atoms, like hydrogen, deuterium and tritium, that share an atomic number (1 in this case) but have different atomic masses (1, 2 and 3).

Symbolic Representation All atoms are commonly represented in chemical formulas (and elsewhere) using one or two letters. Hydrogen is written H, carbon is written C, nitrogen N, oxygen O, whereas sodium is written Na, magnesium Mg and calcium Ca.

... now you explore
and build an atom
for yourself.
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© 2003, Professor John Blamire