1. Early Early Atomic ideas 400 BC, Democritus – said world was made up of two things air and a basic particle called an atom. Atom was the smallest piece of matter and could not be broken down further. Not supported by experimental evidence Trivia – word origin – Greek Atemnein – which means not cuttable ( a – non) (temnein – cuttable)

2. Aristotle Aristotle- He did not believe in the atom idea, he thought matter was continuous. Not supported by experimental evidence

3. Early 1700’s Knew about elements Elements could not be broken down. Knew elements combined to make a new substance with different properties. Did not know if a particular compound was always the same ratio or combination of elements.

4. Late 1700’s Quantitative analysis mass balances were improved, now they could accurately measure the masses of the elements and the compounds. This led to the discovery of certain laws

5. Law of the conservation of mass by Antoine Lavoisier Mass can neither be created nor destroyed during ordinary chemical reactions or physical changes. Used a closed system- a system that can not change matter with its surroundings Conducted chemical reactions and found that the mass of the reactants always equaled the mass of the products. –Reactant mass = product mass

6. Law of definite proportions By Joseph Proust A chemical compound always contains the same elements in the same proportions by mass regardless of the size of sample or source of the compound. Specific substances always contain elements in the same ratio by mass. NaCl is always 1 sodium atom and 1 chlorine atom

7. Early 1800’s Dalton, John Schoolteacher His work was based on the work of Proust and Lavoisier Developed the Atomic theory which has 5 main points The atomic theory provided an explanation of both laws and then developed his own Law of multiple Proportions Was supported by experimental evidence

8. Daltons Atomic Theory 1. All matter is composed of extremely small particles called atoms. 2. Atoms of a single element are all identical in size, mass,and other properties. Atoms of different elements would differ in size, mass, and properties

9. Dalton continued 3. Atoms can not be subdivided, created or destroyed 4. Atoms of different elements combine in simple whole number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.

10. Law of multiple proportions by John Dalton It was known that two elements combine to make more than one compound. –CO –CO 2

11. Multiple proportions continued If two or more different compounds are composed of the same two elements, then the ratio of the mass of the second element combined with a certain mass of the first element is always a ratio of small whole numbers.

12. Multiple proportions – now common sense You can not add half an atom to an atom, if you combine atoms you must add an entire atom. Atoms react as whole units, they can not be divided into smaller parts. Therefore, mass increases in whole number ratios.

13. Modern update Dalton’s theory has been modified since the early 1800’s We now know that –Atoms are divisible into electrons, protons and neutrons and even smaller yet into quarks and gluons etc. –We also now know that all atoms of a particular element may differ slightly in mass. These are called isotopes ( same # protons, different # neutrons)

14. Percent composition NaCl 1 atom of Na and 1 atom of Cl Look up atomic masses of each element Na = 22.98 amu and Cl = 35.45 amu Total mass of NaCl is 22.98 + 35.45=58.43 % composition of Na is (22.98 /58.43) x 100 or part over the whole Equals 39.32 %

15. Percent composition Fe(OH) 3 1 Fe atom; 3 – O atoms; 3- H atoms Total mass of one molecule is – 55.8 + 3 ( 15.99) + 3 ( 1.01) = 106.80 – To find the percent iron (55.8 / 106.8 ) x 100 = 52.2%

16. Percent composition CaSO 4 Ca= 40.07 amu;S= 32.06 amu;0 =15.99 amu Atom count?