(greek for indivisible) HISTORY OF THE ATOM Democritus develops the idea of atoms 460 BC he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA (greek for indivisible)

Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms. He believed that atoms were indivisible and indestructible This is called the discontinous theory of matter. His ideas did agree with later scientific theory, but did not explain chemical behavior, and was not based on the scientific method – but just philosophy

Aristotle – did not believe in atoms. Aristotle (b. 346BC) dismissed the atomic idea as worthless. People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also. Aristotle believed in the continuous theory of matter – that it could be continually divided into smaller and smaller pieces.

Development of the Modern Atomic Theory 2000 Years later … Development of the Modern Atomic Theory In 1782, a French chemist, Antoine Lavoisier (1743-1794), made measurements of chemical change in a sealed container. He observed that the mass of reactants in the container before a chemical reaction was equal to the mass of the products after the reaction.

Click box to view movie clip. Lavoisier concluded that when a chemical reaction occurs, matter is neither created nor destroyed but only changed. Lavoisier’s conclusion became known as the law of conservation of matter. Click Link Below: Click box to view movie clip.

In 1799, another French chemist, Joseph Proust, observed that the composition of water is always 11 percent hydrogen and 89 percent oxygen by mass. Regardless of the source of the water, it always contains these same percentages of hydrogen and oxygen. Proust studied many other compounds and observed that the elements that composed the compounds were always in a certain proportion by mass. This principle is now referred to as the law of definite proportions.

John Dalton discovered the law of multiple proportions: When a pair of elements form two or more different compounds, the masses of the one element that combines with a fixed mass of the other element form simple whole number ratios.

Topic 2 John Dalton (1766-1844), an English schoolteacher and chemist, studied the results of experiments by Lavoisier, Proust, and many other scientists.

John Dalton 1808 suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them ATOMS

Dalton’s Atomic Theory (experiment based) All elements are composed of tiny indivisible and indestructible particles called atoms Atoms of the same element are identical. Atoms of any one element are different from those of any other element. John Dalton (1766 – 1844) Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

3 modern modifications to Dalton’s Atomic Theory Atoms are divisible Atoms are destructuble Atoms of the same element don’t always have the same mass - isotopes

Dalton Model of the Atom:

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

Modern Cathode Ray Tubes Television Computer Monitor Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

CRT Animation CRT Video #1 CRT Video #2 Review Observations and Conclusions

Thomson’s Atomic Model J. J. Thomson Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Discovery of the Nucleus - Rutherford Rutherford Animation #1 Rutherford Animation #2

Ernest Rutherford’s Gold Foil Experiment - 1911 Discovery of the Nucleus - Rutherford Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded

Rutherford’s Gold Foil Experiment (Observations) Most of the -particles passed through the foil. Many of the -particles deflected at various angles. Surprisingly, a few particles were deflected back from the Au foil. V.Montgomery & R.Smith

Rutherford’s Gold Foil Experiment (Conclusions) Rutherford concluded that most of the mass of an atom is concentrated in a core, called the atomic nucleus. The nucleus is positively charged. Most of the volume of the atom is empty space. V.Montgomery & R.Smith

Rutherford Model of the Atom:

Shortcomings of Rutherford’s Model Did not explain where the atom’s negatively charged electrons are located in the space surrounding its positively charged nucleus. What prevents the negative electrons from being drawn into the positive nucleus?

Bohr Model (1913) Niels Bohr (1885-1962), Danish scientist working with Rutherford Proposed that electrons must have enough energy to keep them in constant motion around the nucleus Analogous to the motion of the planets orbiting the sun

Shortcomings of Bohr Model: It didn’t explain why electrons could only be located on specific orbits. It only worked for atoms that had just one electron.

Quantum Model of Atom