THE ATOMATOMIC THEORY AND ATOMIC STRUCTURE

LEXTER C. SUPNETMAED-PhysicsMariano Marcos State UniversityGraduate SchoolLaoag CityScn Ed 214

ATOM noun

\ˈa-təm\

: the smallest particle of a substance that can exist by itself or be combined with other atoms to form a molecule.

Miriam-Webster Dictionary

SIZE

RELATIVE SIZE

“If an apple is magnified to the size of the earth, then the atoms in the apple are approximately the size of the original

apple.” –Richard Feyman

ATOMIC THEORYJOHN DALTON (1766-1844)

English school teacher, John Dalton, presented his theory in 1803 using the two fundamental laws of chemical combination (Law of Conservation of Mass and Mass of Constant Composition) as the basis of the atomic theory.

Considered not a good experimenter because of his color blindness (daltonism). However, he did skilfully use the data of others in formulating his atomic theory.

Dalton’s theory led to prediction – Law of Multiple Proportions.

ATOMIC THEORY 1Each chemical element is composed of minute, indestructible

particles called atoms. Atoms can be neither created nor destroyed during chemical change.

ATOMIC THEORY 2All atoms of an element are alike in mass in mass (weight) and

other properties, but the atoms of one element are different from those of all other elements.

ATOMIC THEORY 3In each of their compounds, different elements combine in a

simple numerical ratio: for example, one atom of A to one of B (AB), or one atom of A to two of B (AB2)

ATOMIC STRUCTURE THE SEARCH FOR THE PORTRAIT

500 B.C. - Atomos Greek philosopher Leucipus and his pupil Democritus first made the proposal that matter is composed of tiny particles that cannot be subdivided.

Democritus gave these ultimate particle name atomos, which literally means “uncuttable” in Greek.

1803 – ATOM

John Dalton proposed the atomic theory based on the original thoughts of Democritus. Modern-day highly sophisticated instrument called scanning tunnelling microscope reveals Dalton’s theory is true.

1897 – PLUM PUDDING MODEL New discoveries led to the belief that atoms were not indivisible but were made up of smaller subatomic particles.

J.J. Thomson discovered electron, an almost weightless particle and has a negative electrical charge (assigned value of 1-), common to the atoms of all elements.

J.J. Thomson proposed the Plum Pudding Model.

CATHODE RAY TUBE EXPERIMENT

PLUM PUDDING MODEL

1911 – THE NUCLEAR MODEL The Nuclear Atomic Model was developed by Ernest Rutherford, a New Zealander, through his Gold-Foil Experiment.

GOLD FOIL EXPERIMENT

GOLD FOIL EXPERIMENT

RUTHERFORD’S EXPLANATION1. Most of the mass and all the positive charge of an atom are

centered in a very small region called the nucleus. The atom is mostly empty space.

2. The magnitude of the positive charge is different for different atoms and is approximately one-half the atomic weight of the element.

3. 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.

“Rutherford concluded that an atom was made up of very dense, positively charged nucleus surrounded primarily by an empty space

in which the electron could be found.”

RUTHERFORD ATOMIC MODEL

The nucleus is 1/100,000 of the size of an atom.

RELATIVE SIZE OF NUCLEUSA grain of

sand!

1912-1913 – PLANETARY MODEL While studying the line spectra of hydrogen, Neils Bohr (1885-1962) led to believe that electrons in an atom exist in specific regions at various distances from the nucleus. He visualized the electrons as rotating in orbits around the nucleus like planets rotating around the sun.

Bohr’s model explained a great number of facts during his time. However, Bohr’s method for calculation did not succeed for heavier atoms.

BOHR’S ATOM

Bohr-Rutherford Model

THE OTHER SUBATOMIC PARTICLES In 1919, Ernest Rutherford himself suggested the existence of positively

charged fundamental particles of matter in the nuclei of atoms called protons through his studies involving the scattering of α particles by nitrogen atoms in the air. At about the same time, Rutherford predicted the existence in the nucleus of electrically neutral fundamental particles called neutrons. However, in 1932, it was James Chadwick who was able to prove the existence of neutral particles.

Other Claimants:

Eugene Goldstein (1850-1930), a German scientist, was the first to observe the existence of protons in 1886 using a modified cathode-ray tube made of metal disk filled with holes. However, J.J. Thomson was the one who discovered the nature of protons.

PROPERTIES OF 3 SUBATOMIC PARTICLES

Particle Symbol

AtomicCharge

Mass(g)

Mass(u)

Mass Relative to Proton

Electron e- 1- 9.109 x 10-28 .0005486

Proton p 1+ 1.673 x 10 -24 1.0073 1

Neutron n 0 1.675 x 10 -24 1.0087 1

1926 – QUANTUM MECHANICAL MODEL Erwin Schrödinger combined the equations of the behaviour of

waves with the de Broglie equation to generate a mathematical model for the distribution of electrons in an atom.

The model assumes that the electron is a wave and tries to describe the regions in space, or orbitals, where electrons are most likely to be found. Instead of trying to tell us where the electron is at any given time, the Schrödinger model describes the probability that an electron can be found in a given region of space at a given time. This model no longer tell us where the electron is; it only tells us where it might be.

ELECTRON CLOUDS

QUANTUM MECHANICAL MODEL

ATOMIC PORTRAIT TIMELINE

“Nuclear Model”Ernest Rutherford

(1911)

“Quantum Mechanical Model”

Erwin Schrödinger(1926)

“Bohr Model”Neils Bohr

(1913)

“Plum Pudding”J.J. Thomson

(1897)

“Atom”John Dalton

(1803)

“Atomos”Democritus & Leucipus

(500 BCE)

KENNETH SNELSON

“The Portrait of an Atom”

Kenneth Snelson(2003)

Thank you!

REFERENCESBAYQUEN, A.V., 2007. Exploring Life Through Science: Chemistry. Phoenix Publishing House Inc., Quezon City, Philippines.

PETRUCCI, R.H., HARDWOOD, W.S., HERRING F.G. 2002. General Chemistry: Principles and Modern Application-8th Edition. Pearson Education South Asia pte Ltd., Singapore

http://chemed.chem.purdue.edu/genchem/history/bohr.html

http://chemed.chem.purdue.edu/genchem/history/schrodinger.html

http://www.colorado.edu/physics/2000/quantumzone/schroedinger.html