Презентация на тему Atomic theory and structure of an atom

Leucippus originated the atom concept.
One of his students, Democritus

(460BC-371BC) extended it.
There are five major points in their atomic concept:
All matter is composed of atoms, which are too small to be seen. These atoms CANNOT be further split into smaller portions.
There is empty space between atoms.
Atoms are completely solid.
Atoms are homogeneous同質, with no internal
structure.
Atoms can differ in size, shape, and weight.

small particles called atoms原子.
2. All atoms of a given

element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements.
3. Compounds化合物are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction.
4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

form more than a single compound, the masses of

one element combined with a fixed mass of the second are in the ratio of small whole numbers.

Example 1: The mass of carbon is the same in the two molecules, but the mass of oxygen in CO2 is twice the mass of oxygen in CO. Thus, in accordance with the law of multiple proportions, the masses of oxygen in the two compounds, relative to a fixed mass of carbon, are in a ratio of small whole numbers, 2:1.

Oxides I & II
Nitrogen Oxide I: 46.68% Nitrogen and

53.32% Oxygen
Nitrogen Oxide II: 30.45% Nitrogen and 69.55% Oxygen

in 100 g of each Compound: m(O) = 53.32 g & 69.55 g
m(N) = 46.68 g & 30.45 g

Cmpd I Cmpd II

m(O) /m(N) = 1.142 & 2.284

Cmpd II 2.284 2

Cmpd I 1.142 1

=

neither created nor destroyed during physical or chemical processes

a compound have the same composition—the same proportions by

mass of the constituent elements.
Example: Water is made up of two atoms of hydrogen (H) for every atom of oxygen (O), with chemical formula H2O.

of smaller parts, which can only be detected in

experiments with special instruments.

There are sub-atomic particles!

rays experiment
In 1897, J. J. Thomson did an experiment

to confirm the presence of the tiny negative particles – electrons.
Thomson experiment (1906 Nobel Prize in Physics)

rays experiment
CRT, the abbreviation for cathode-ray tube, is a

hollow vessel with an electrode at either end. A high voltage is applied across the electrodes.
The cathode rays produced in the CRT are invisible, and they can be detected only by the light emitted by materials that they strike.

rays experiment
When gases are subjected to very high voltages

at very low gas pressure, they ‘break down’ and conduct electricity.

The voltage causes negative particles to move from the negative electrode to the positive electrode – cathode rays are produced.

Although the rays themselves could not be seen, their movement could be detected only by the light emitted by materials.

rays experiment
Conclusion:
The atom consists of positively and

negatively charged entities. However, Thomson did not know how the electrons in an atom were arranged. He proposed that the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons.

Right?

Right?

Right?

Prize in Physics)
2.2

Nobel Prize in Chemistry

very thin foils of gold are bombarded with α

particles, following phenomena is observed :
• The majority of particles penetrated the foil undeflected.
• Some particles experienced slight deflections.
• A few (about 1 in every 20,000) suffered rather serious
deflections as they penetrated the foil.
• A similar number did not pass through the foil at all, but
bounced back in the direction from which they had come.

explanation:
• Most of the mass and all

of the positive charge of an atom are centered in a very small region called the nucleus. The remainder of the atom is mostly empty space.
• The magnitude of the positive charge is different for different atoms and is approximately one-half the atomic weight of the element.
• There are as many electrons outside the nucleus as there are units of positive charge on the nucleus. The atom as a whole is electrically neutral.

undeflected straight-line paths exhibited by most of the particles
2.

slight deflections of particles passing close to electrons
3. severe deflections of particles passing close to a nucleus
4. reflections from the foil of a particles approaching a nucleus head-on

http://www.youtube.com/watch?v=NlDPPANJZXM

pm = 1 x 10-10 m
nuclear radius ~ 5

x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of
the Atom

Prize in Physics




Discovery of neutron !!!

are projected towards beryllium target. The emitted particles are

allowed to fall on paraffin wax, which in turn releases another type of particles (protons). From the energy calculations, Chadwick showed that the particles released from beryllium are uncharged and have the same mass as protons. He called them neutrons.

and neutrons are located in the nucleus of the

atom, which is small. Most of the mass of the atom is due to the nucleus. (There can be a variable number of neutrons for the same number of protons. Isotopes have the same number of protons but different numbers of neutrons.)
Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons.