for a model and theory to remain valid it…
- MUST explain ALL known facts
- MUST enable the scientists to make correct predictions
chemist models
helps us understand the unseen
* can be visual, verbal, or mathematical
Democritus
- 460-370 BC
- Proposed the existence of an atom; if a substance is cute into smaller piece eventually you will have the smallest possible piece
- Called the tiniest piece the atom
*Comes from “atomis “ meaning not to cut
Aristotle
- 384-322 BC
- Rejected the idea of the atom
- Said matter could be cut continually
- Everyone believed him; idea of atom disappeared for 2,000 years
Three laws of how chemical compounds are formed
- Later 1700s
*Law of Conservation of Mass
*Law of Definite Properties
*Law of Multiple Properties
Johns Daltons theory prosper that atoms:
- Are the building blocks of life
- Are indivisible
- Of the same elements are identical
- Of different elements are different
- Unite in small, whole number ratios to form compounds
John Dalton
- Early 1800s
- Model of atom was introduced by him
- Developed first model in 1803
- Saw atoms as indivisible/indestructible
1) Believed all atoms of a given element were identical in: shape and mass.
2) Believed the atoms of different elements were different with a distinct shape, size, and mass
3) Believed that compounds were formed by combining two or more elements
What 2 parts of Daltons theory were proven incorrect?
- Atoms in the same element are exactly the same
- Atoms are indivisible and cannot be broken down
J.J. Thomson
- late 1800s
- Credited with discovery of the electron
- Saw the atoms as a solid mass
- Made the first discovery of a subtonic particle from using cathod-ray tubes
- Results found that the cathode-ray particles(electrons) are fundamental particles that are presents in the atoms of all elements
Plum Pudding model:
- Created by Thomson
- Negatively charged particles enclosed in a substance of positive charge
Lord Rutherford
- Early 1900s
- In 1901, indicated that atoms are largely made up of empty space
- Conducted the gold-foil experiment
Gold-foil Experiment
- Aimed alpha particles at foil
- MOST passed through
- A FEW particles were deflected
- SOME particles bounced back
- Results: Most of the atom is empty space, an atom has a dense(positively charged) core surrounded by electrons
Planetary Model
The model that Rutherford suggested that electrons orbit the positively charged nucleus, just like how planets orbit the sun
model
a familiar idea used to explain unfamiliar facts observed in nature
theory
an explanation of observable facts and phenomena
Law of Conservation of Mass
- Matter is neither created or destroyed during ordinary chemical reactions or physical changes
- Happens because the atoms are all still there but separate to create the new product
Law of Definite Proportions
- A chemical compound will have the same elements in the same proportions by mass regardless of sample size or where the sample came from
- EX. No matter what sample of water you have, you will always have 11% hydrogen and 89% oxygen
% composition
(mass of elements / mass of compounds) x 100%
Law of Multiple Proportions
- If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always the ratio of small whole numbers
- EX. when you mix oxygen and hydrgen sometimes it makes water and other times it makes hydrogen peroxide
Always a relationship between the outcomes like a ratio of 1:2 between H20 and H202
Bohr
- Early 1900s
- Builds on Rutherfords model and goes after him
electron mass
9.1 x 10*-28 g
proton mass
1.673 x 10*-24 g
neutron mass
1.675 x 10*-24 g
an atoms radius can range from…
40 - 270 pm
