In the early twentieth century Rutherford was experimenting with one of the newly discovered radioactive substances, one that emitted alpha particles. He knew that these particles had a mass much larger than the electron and had a net positive electric charge; now we know that these particles are identical to the nucleus of the helium atom. He was directing a beam of these alpha particles onto a very thin piece of gold foil. If Thomson's plum pudding model was correct, the experiments would be sort of similar to firing BB's from a BB gun into a thin slab of cream cheese with chives. And in fact the results of Rutherford's experiments usually followed this model: almost all of the alpha particles emerged on the other side slightly deflected by their interaction with the gold. However, once in a while he observed an alpha particle that was scattered right back towards the radioactive source.
If you were doing the experiment involving the BB's and the cream cheese and occasionally had a BB scattered back towards you, you would probably conclude that there was something fairly small and very massive inside the slab. Similarly, Rutherford concluded that inside the gold foil there must be something fairly small, very massive, and positively charged. Thus the plum pudding model of the atom collapsed: most of the mass and the positive charge of the atom were concentrated into a very small volume. This small massive positively charged object is called the nucleus.
Soon, people proposed a planetary model of the atom. The electrons were in orbits around the nucleus, held in their orbits by the electric force that attracts negatively charged electrons to the positively charged nucleus.
Thence people began to research the atom further. The development of our ideas about the electron has been improved.
- The development of people’s ideas about the electron through scientists’ investigation.
After the electron was discovered, chemists regarded this particle. Some of important theories that influenced the whole chemistry field were set up. For instance, the Bohr Model, Planck photoelectric effect and so on. Undoubted, these theories made people develop their ideas about electron. Our understanding has changed.
German physicist Max Plank proposed that the exchange of energy between matter and radiation occurs in quanta, or packets of energy. His central idea was that an atom oscillating at a frequencyυcan exchange energy with its surrounding only in packets of magnitude
E= hυ (the constant h, now called the Plank constant, has value 6.63×10 J·s.)
Photoelectric effect which is the ejection of electrons from a metal when its surface is exposed to light. So we can explain some electrical properties in photoelectric effect.
<1> No electrons are ejected unless the radiation has a frequency above a certain value characteristic of the metal.
<2> Electrons are ejected immediately, however low the intensity of the radiation.
<3> The kinetic energy, Ek, of the ejected electrons varies linearly with the frequency of the incident radiation.
Meanwhile, he proposed that electromagnetic radiation consists of particles, which were called photons, which was the new particle.
Then in 1913 Bohr, by accident, stumbled across Balmer's numerology for the hydrogen spectrum, and in a flash came up with a workable model of the atom. The model asserts that:
- The planetary model is correct.
- When an electron is in an "allowed" orbit it does not radiate. Thus the model simply throws out classical electromagnetic theory. Technical note: an allowed orbit is one in which the electron mass times its speed times the radius of the orbit is equal to a positive integer n times Planck's constant divided by 2 pi. The integer n can be 1, 2, 3, 17, 108, etc. In fact, there are an infinite number of allowed orbits corresponding to the infinite number of positive integers.
- When an electron absorbs energy from incident electromagnetic radiation, it "quantum jumps" into a higher energy allowed state. This higher energy state corresponds to an allowed orbit with a higher value of the integer n.
- When an electron is in a higher energy state, it can quantum jump into a lower energy state, one with a smaller value of n, emitting all of its energy as a single photon of electromagnetic energy.
As Bohr fully realized, this model is largely ad hoc, if not downright ugly. It does, however, "explain" the line spectra of the elements.
Stern commented, "If that nonsense which Bohr has published is correct, then I will give up to be a physicist."
Recall Balmer's formula:
Here R is a constant equal to about 10,970,000 and n is any integer greater than 2, such as 3, 4, 5, 108, etc. Each different value of n gives the wavelength of a different line in the Hydrogen spectrum. The constant R is usually called the Rydberg constant.
The wavelengths described by this formula correspond exactly to the radiation emitted when an electron in the Bohr model quantum jumps from a high energy orbit, described by some n greater than 2, to an orbit whose value of n is equal to 2. In the model transitions to other "final" states such as n equal to 1 or 3, 4, 5, etc. are also predicted and the wavelengths in the spectrum for these are found experimentally to exist. Further, the constant R turns out to be expressed in terms of fundamental physical constants. So we have pushed the numerology of Balmer's work a bit back.
The lowest energy allowed orbit, the one with n equal to one, is the ground state of the atom. An unexcited hydrogen atom will have its electron in this state.
We can explain some reactions by Bohr’s theory currently. Moreover people had developed their ideas about electron and atomic structure.
French scientist Louis de Broglie suggested that all particles should be regarded as having wavelike properties, which showed that electrons have both wavelike and particlelike; their wavelike properties must be taken into account when describing the structure of atoms.
The wave-particle duality not only changes our understanding of electromagnetic radiation and matter, but it also sweeps away the foundations of classical physics. The Heisenber uncertainty principle express the complementarity quantitatively: it states that if the location of a particle is known to within an uncertainty Δx; then the linear momentum parallel to the x-axis can be known only to within an uncertainty Δp, where ΔpΔx ≥ 0.5h (the symbol h means h/2π, an useful combination that occurs widely in quantum mechanics.).
The location and momentum of a particle are complementary; that is both the location and momentum cannot be known simultaneously with arbitrary precision. The quantitative relation between the precision of each measurement is described by the Heisenberg uncertainty principle.
Twentieth-century scientists had to rebuild their description of matter to take into account wave-particle duality. One of the first people to succeed was the Austrian scientist Erwin Schrödinger. He proposed that the probability density for a particle at a location is proportional to the square of the wavefunction at that point. The wavefunction is found by solving the Schrödinger equation for the particle. When the equation is solved, it is found that the particle can possess only certain discrete energies. The Born interpretation of the wavefunction is the base of Schrödinger equation.
The location of an electron in an atom is described by a wavefunction known as an atomic orbital; atomic orbitals are designated by the quantum numbers n, l. and ml and fall into shells and subshells.
Through deep understanding for electron, people have already mastered some properties of electron and begun to research all kinds of elements.
In 1925, the Austrian scientist Wolfgang Pauli discovered a principle that states that “ no more than two electrons may occupy any given orbital, when two electron do occupy one orbital, their spins must be paired, no two electrons in an atom can have the same set of four quantum numbers”. This is also called Pauli exclusion principle. This principle made people know the elements are periodic.
Due to the fact that people have understood the properties of electron, the chemical bonds have been explained easily. The chemical bonds play an essential role in chemistry. The scientists have discovered some types of chemical bonds, and got plenty of properties, which depends on understanding of electron.
During this century people have developed their ideas about the electron but we need to insight into the electron, which will spend a time. Our understanding of electrons was far from complete. The octet and hypervalence have not been solved, they are two misunderstood concepts. We must use electrical properties to research them. However, we have not enough evidence to explain them.
Electron is an essential factor for studying chemistry. Although people have developed their understanding of ideas about the electron during this century, yet we need to insight into it. Because electron is a clew through the whole chemistry. In other wards, we should study hard to investigate new production, and make use of the theories that the scientists researched. The new century is coming, the new history of the electron is developing. Chemical milestone will be created.
Group: (First year) 4 Date: 12th 12 2001
