4.1a Rutherford Atom - Lecture notes 4 PDF

Preview

Summary

Fundamenals Of The Physical World notes...

Description

Atom and Atomic Structure In the fifth century B.C., Leucippus (loo-KIP-us) suggested that all matter -- everything -- was made up of a few simple building blocks. He managed to convince a number of people including Democritus who believed that matter was made up of particles so small that they could not be further subdivided. Democritus postulated that different substances has different properties because of the differences in the nature of their "atoms." However, his theories were not supported by Aristotle, 300 BC, who thought that matter was continuous, not discrete. He advocated that every object on the earth was made up of some combination of only four substances: earth, water, fire, and air. It took over 2000 years for Aristotle's philosophy to be replaced by the concept that matter was not just composed of only those four general substances to its being comprised of unique elements - hydrogen, carbon, oxygen, sulfur, etc.

Dalton’s Atomic Theory In the early 1800s, the English Chemist John Dalton revived the idea of atom and summarized his theory in five statements. 1. Indivisible minute particles called atoms make up all matter. 2. All the atoms of an element are exactly alike in shape and mass. 3. The atoms of different elements differ from one another in their mass. 4. Atoms chemically combine in definite whole number ratios to form chemical compounds. 5. Atoms are neither created nor destroyed in chemical reactions. Many scientists at that time were skeptical of the concept of atom because it could not be observed directly. The discovery of electron by J. J. Thomson changed all this.

J J Thomson and the Electron At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with electric current inside evacuated glass tubes.

He discovered a mysterious radiation emanating from the negative end of the tube He suggested that these mysterious rays are streams of particles much smaller than atoms, particles that belong to atoms.

Thomason called these particles "corpuscles," and suggested that they might be the constituents of atoms. Later experiments showed that these are negatively charged particles called electrons and are part of the constitution of all matter. Studies by an American physicist Robert Millikan showed that electrons carry an negative charge of 1.6 × 10-19 coulomb and a mass of 9.11 × 10-31 kg

J J Thomson’s Atomic Model One of the first atomic models was that proposed by J.J. Thomson after he discovered the existence of the electron as a result of his work with cathode rays. Since each atom was electrically neutral, Thomson thought that it consisted of a relatively large, uniformly distributed, positive mass with negatively charged electrons embedded in it like "raisins in a plum pudding." Thomson's careful experiments and adventurous hypotheses were followed by crucial experimental and theoretical work by many others in the United Kingdom, Germany, France and elsewhere. These physicists opened for us a new perspective--a view from inside the atom.

Rutherford’s Gold Foil experiment In 1909 Rutherford arranged a sample of Polonium in a lead box emitting alpha particles towards a gold foil.

The foil was surrounded by a luminescent zinc sulfide screen to detect where the alpha particles went after contacting the gold atoms. Rutherford observed many of the alpha particles went through the gold foil without suffering much deviation, but a few got deflected almost 180 o

Rutherford said, "It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you." He argued that the atom has a positive core called nucleus which is responsible for deflecting the alpha particles.

http://micro.magnet.fsu.edu/electromag/java/rutherford/

Soon he came up with a new atomic model based on the results of this experiment.

Rutherford’s Model of the Atom He published his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons.

Later, when Rutherford bombarded nitrogen with alpha particles, a positively charged particle that was lighter than the alpha particle was emitted. He called these particles protons and suggested that these are fundamental particles in the nucleus.

Protons have a mass of 1.673 x 10 -27 kg, about 1,835 times larger than an electron! However, protons could not be the only particle in the nucleus because the number of protons in any given element (determined by the electrical charge) was less than the weight of the nucleus. He argued that a third, neutrally charged particle must exist! It was James Chadwick, a British physicist and co-worker of Rutherford, who discovered the third subatomic particle, the neutron. The neutron has a mass of 1.675 x 10 -27 kg.

The nuclear model of the atom developed by Rutherford suggests that most of the atom consists of empty space.

5 km

30 cm

If the nucleus of an atom and the sun are represented by spheres of radius 30 cm, the earth can be found 100 m from the sun, but the nearest electron will be found 5 km from the nucleus.

Atomic Theory The nucleus is now understood to be composed of protons and neutrons, particles of nearly equal mass Protons and neutrons together are called nucleons or nuclear particles. Elements differ in the number of protons a neutrons in the nucleus of their atoms. An element is identified by its atomic number (Z) and mass number (A) Z is the number of protons in the nucleus. A is the sum of the protons and neutrons in the nucleus. If N is the number of neutrons in the nucleus, N=A–Z

The nuclear symbol consists of three parts: the symbol (X)of the element, the atomic number (Z) of the element and its mass number (A).

A Z

X

A lithium (Li) nucleus that contains 3 protons and 4 neutrons is represented by:

7 3

Li

Isotopes Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. 1 1

H

2 1

H

3 1

H

There are "preferred" combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable. That's not to say that an element can't have more than one stable isotope. Ordinary hydrogen and deuterium are both stable; tin has ten stable isotopes, more than any other element. All the elements beyond bismuth (Z = 83) are unstable.

Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves. This process is called radioactivity

Unstable isotopes

There are three types of radiations emitted by a radioactive isotope. They are called alpha, beta and gamma radiations. Alpha and beta are particles, while gamma radiations are electromagnetic waves of high energy.

Problems with Rutherford Model According Maxwell’s law of electromagnetic radiation, an accelerating electric charge should radiate electromagnetic energy. An electron in orbit is an accelerating electric charge and it should lose energy and ultimately fall into the nucleus. Since this does not happen in a normal atom, there is a significant problem with Rutherford model of the atom. A Danish physicist Bohr tried to solve this problems by linking familiar phenomena to atomic models....