Lexter C. Supnet Dr. Natividad LorenzoMAED-Physics 1st Semester, S.Y. 2014-2015

THE ATOM

Beginning more than 2 000 years ago, ancient philosophers developed the first theories of atomic structure. After a brief survey of these early chemical discoveries, we will describe physical evidence leading to the modern picture of the atom.

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

The Atomic TheoryJohn Dalton (1766-1844)

English school teacher, John Dalton, presented his Atomic 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.

The Atomic Theory1. Each chemical element is composed of minute, indestructible particles called atoms.

Atoms can be neither created nor destroyed during chemical change.2. All 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.3. In 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 Structure500 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.

1911 – The Nuclear Model The Nuclear Atomic Model was developed by Ernest Rutherford, a

New Zealander, through his Gold-Foil Experiment.

Plum Pudding Model

Nuclear Model

Rutherford’s Gold Foil Experiment

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. 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.

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.

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

Planetary Model

Particle Symbol Atomic Charge Mass(g)

Mass(u)

Mass Relative to Proton

Electron e- 1- 9.109 x 10-28 .0005486 11835

Proton p 1+ 1.673 x 10 -24 1.0073 1Neutron 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.

The study of chemistry is anchored with the study of the composition of atoms as well as the changes they undergo, it is therefore vital to learn the structure of atoms in order to correlate them with the chemical properties of matter.

The theory of the structure and behaviour of atoms has taken more than two millennia to evolve, from the abstract musings of ancient Greek philosophers to high-tech experiments of modern scientists. However, how can scientists be certain that atoms exists where no one has really seen an atom? The answer is that models of the atom, like other scientific models, can be tested by experimentation. Those models that pass the test of experimentation survive, while those models that do not are abandoned. The model of atoms that scientists use today has survived and been modified by untold number of experiments and will be subjected to other tests in the future.

REFERENCES

BAYQUEN, 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 Ltd., Singapore

http://chemed.chem.purdue.edu/genchem/history/bohr.htmlhttp://chemed.chem.purdue.edu/genchem/history/schrodinger.htmlhttp://www.colorado.edu/physics/2000/quantumzone/schroedinger.html