ATOMIC THEORYUnit 3
Atom
smallest particle of an element that can exist alone or in combinations with other atoms
What is atomic theory?
The idea that all matter composed of atoms
The structure of the atom has changed overtime as scientific advances have been made.
Democritus
First proposed by early Greek philosophers like Democritus in approximately 400 B.C.
All matter is made of “atomos” which means in destructible.
John Dalton
First to develop atomic theory based on experimental evidence
Early 1800s Based his theory on
several laws known at the time
Law of conservation of Mass
Remember this one?
Mass cannot be created or destroyed in a chemical reaction
Law of Definite Proportions
Any sample of a compound always has the same composition.
NaCl
39.34% Sodium by mass
60.66% Chlorine by mass
Law of Multiple Proportions
The mass ratio for one of the elements in a compound that combines with a fixed mass of another element can be expressed in small whole numbers.
Elements always combine in “whole” number increments.
Water: H2O Hydrogen peroxide: H2O2
Law of Multiple Proportions
Applies to different compounds formed from the same elements.
Name of Compound
Description % Oxygen by mass
% Nitrogen by mass
Formula
Nitrogen dioxide
Poisonous brown gas in smog
69.56 30.44 NO2
Nitrogen monoxide
Colorless gas that reacts readily with oxygen
53.32 46.68 NO
Dinitrogen monoxide
“laughing gas” anesthetic
36.35 63.65 N2O
Major Concepts – Dalton’s Theory
1. Atoms are the smallest particle of matter and can’t be divided.
2. Atoms of a given element are identical in size, mass, and other properties
Major Concepts – Dalton’s Theory
3. Atoms can’t be created or destroyed in chemical reactions (Law of Conservation of Mass)
Major Concepts – Dalton’s Theory
4. Atoms can combine in small whole number ratios (law of definite/multiple proportions)
Major Concepts – Dalton’s Theory
JJ Thomson (1897)
Discovered the electron with the cathode ray tube experiment
http://www.physics.uq.edu.au/people/mcintyre/applets/cathoderaytube/crt.html
Plum pudding model
Ernest Rutherford
Was a student of Thomson Investigated the plum pudding model
with the famous gold foil experiment Bombarded gold foil with alpha
particles (large and positive)
Nuclear model
Rutherford concluded that an atom contains a nucleus
Nucleus: small, dense, positive region in atom.› Contains most of the mass, but takes up
little space
Discovery of the proton
The gold foil experiment eventually led Rutherford to the discovery of the proton
Rutherford’s nuclear model
James Chadwick (1932)
1. Chadwick (1932) proposed the idea of a neutral particle with the same mass a proton, the neutron
Theoretically and mathematically base
When nuclear chemistry became the “cool” thing to study, Chadwick knew there had to be something present to stabilize the nucleus!
Bohr
Suggested that electrons have to be in fixed energy levels
Theoretically
Electrons are attracted to protons, so why do electrons not collapse into the nucleus and implode?
Reason! Electrons are in fixed energy levels. THESE ARE NOT HOW THE ORBIT!!!!!!!!!!!!!
Still the nuclear model
Quantum Mechanical Model
Introduced quantum theory (mathematically based) to explain the probability of finding electrons, orbitals (not to get confused with orbits!)
QMM
Modern day view of atom
What a beauty!!!!
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