Dev Elopement Of Atomic Theory

The Developmen t of Atomic Theory

Unit 1 Exam Results

Remember our Atomic History Time Line Started with the Greeks…then nothing for a long… 450 BCE - Democritus First posed idea that matter is made of indivisible particles called atoms

long,

long time…

until…

1780s Antoine Lavoisier proposed the law of conservation of mass: matter is neither created nor destroyed

1770s Ben Franklin Experimented with electricity and labeled the charges + and -

1790s Joseph Louis Proust found that a compound always has the same composition regardless of where it is found

1800s John Dalton Combined Democritus, Lavoisier and Proust’s theories into a unified atomic theory of matter

1890s Henri Becquerel, Marie and Pierre Curie Discovered radioactivity and proved that the atom is not the smallest unit of matter

1890s JJ Thomson Discovered that cathode rays were actually electrons, the negative particles Franklin saw

1910s Robert Millikan Measured the charge on the electron using the “oil can” experiment.

1900s Ernest Rutherford shot a particles at a thin gold foil to discover that most of the mass of an atom (the nucleus) takes up very little space

1910s Henry Moseley Found that each element has a unique number of positive charges (protons) and therefore atomic number

Law of Conservation of Mass

•

Law of Definite Discovered byProportions a Frenchman, Joseph Proust (1754 – 1826). Sometimes known as Proust’s Law.

• Each compound has a specific ratio of elements • It is a ratio by mass. • Water is always 8 grams of oxygen for each 1 gram of hydrogen

Law of Multiple • The discovery by John Dalton of the Proportions Law of Multiple Proportions helped, in part, lead him to the formation of his Atomic Theory. • Dalton discovered that if two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each is a simple whole number.

The Law of Multiple Proportions

Say What?

• Water is 8 grams of oxygen per 1 gram of hydrogen. • Hydrogen Peroxide is 16 grams of oxygen per gram of 1 hydrogen. • The ratio of Oxygen to hydrogen is 16 to 8 or a 2 to 1 ratio • True because you have to add a whole atom, you can’t add a piece of an atom.

Dalton’s Atomic Thus, armed with the Theory Law of Conservation of Mass, The Law of Definite & Multiple Proportions…Dalton outlined five fundamental postulates about atoms: 1. All matter consists of tiny, indivisible particles, which Dalton called atoms that cannot be broken down or destroyed. 2. All atoms of a particular element are exactly alike, but atoms of different elements are different. 3. All atoms of different elements are unchangeable and have different physical and chemical properties.

Dalton’s Atomic Theory

4. Atoms of elements combine to form "compound atoms" (i.e., molecules) of compounds in simple whole-number ratios. 5. In chemical reactions, atoms are neither created nor destroyed, but are only Oh, thank rearranged. Hurray! We’re Saved!

you Dalton!

“Random” Thoughts

• Using similar reasoning techniques for other compounds, Dalton prepared the first table of atomic masses (sometimes called atomic weights by chemists since masses are obtained by weighing). • His work paved the way for other scientists such as Joseph Gay-Lussac (1778 – 1850) and Amadeo Avogadro (1776 – 1856) to continue research that will be presented in class when we reach the Gas Laws Unit.

Percent Composition • The percentage by mass of each element in a compound. • If a sample of water contains 2 grams of hydrogen to 16 grams of oxygen, the percent composition of oxygen will always be approximately 89%, or 16 grams oxygen/18 grams total. Hydrogen would account for about 11%.

The End!