Early Ideas about Matter

atom u The smallest piece of the element with all the chemical properties of the element u an old and new idea

Greeks u Discontinuists –Democritus u Continuists –Aristotle

Discontinuists u Democritus u Believed there was a smallest piece u “atomos” = indivisible

Continuists u Continuists thought there was no “smallest piece” u they believed a bar of gold, for example, could be cut into smaller pieces forever

Aristotle’s ideas u There are four elements –earth, air, fire, and water u all matter is composed of these four elements in different proportions

The Alchemists u Tried to turn lead into gold –put more fire into it –the “medicine stone” would “cure” the Pb u developed many lab techniques and apparatus

The Renaissance u A new approach to science u experiment to test if the hypothesis is true u many of Aristotle’s ideas were shown to be wrong

Law of Conservation of Mass u Lavoisier> France, 1790’s u Matter (or mass) is u neither created nor u destroyed in an ordinary chemical reaction

Law of Definite Proportions u In a given compound, elements are always combined in the same proportion by mass. u Ex: H 2 O is always 11% H, 89% O by mass.

Dalton’s Atomic Theory u All matter is composed of atoms –atoms are indestructible –“billiard ball” model u In a chemical reaction, atoms are rearranged –conservation of mass

u All atoms of the same element are identical u Atoms of one element are different from the atoms of any other element –elements could be identified and distinguished from each other

u Atoms combine to form compounds in small whole number ratios. –Ex: could be XY, X 2 Y, XY 2, etc, but never X 1 1 / 2 Y

What came next? u 49 new elements were discovered between u new particles were discovered –alpha, beta, gamma - radiation –particles smaller than atoms

u 1897: JJ Thomson discovered electrons –smaller than atoms –negatively charged –problem with Dalton’s model u 1914 Rutherford’s Gold Foil Experiment –nuclear model of atom u

Modern atom u Nucleus –in geometric center of the atom –very small and dense –nearly all of the atom’s mass –gives the atom its identity –protons (+) and neutrons

u Electron cloud –gives the atom its size –mostly empty space –electrons –electrons do not “orbit” like planets; more like bees/beehive

Summary of the subatomic particles

Summary of the subatomic particles particle where? Mass who?

Atomic Number u the number of protons in the nucleus of an atom u gives the atom it’s identity u also, equal to the number of electrons in a neutral atom

Mass Number u Equal to the number of protons plus the number of neutrons in the nucleus of an atom u #n o = mass # - atomic #

Important note! u The atomic number of an element is on the periodic table, but the mass number is usually not! u The location of the numbers on the periodic table is at the printer’s discretion

u The good news: –all atoms of the same element have the same atomic number u The bad news: –not all atoms of the same element have the same mass number u What is different? –The number of neutrons

isotopes u Atoms of the same element with different mass numbers –different numbers of neutrons u different versions of the same element u no difference in chemical properties u only fundamental difference is in mass

Isotope symbols u The elements symbol, plus... u the mass number is written in the upper left corner u the atomic number is written in the lower left corner

Examples of isotope symbols u Compare carbon-12, carbon-13, and carbon-14 u each have the same number of protons, but a different number of neutrons

Examples of isotope symbols u 12 C6p + 6n o 6 u 13 C6p + 7n o 6 u 14 C6p + 8n o 6

elements u There are ~114 known u There are 92 naturally occuring u All have unique name and symbol –1 letter = capitol –2 letters = cap w/ lower case –ex: B, C, Ca, Na