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!!!!