TITLE The Development of Atomic Theory. Early Ideas Democritus—Greece 400 B.C. His idea: There was a limit to how far you could divide matter. You would.

TITLE

The Development of Atomic Theory

Early IdeasDemocritus—Greece 400 B.C.

His idea: There was a limit to how far

you could divide matter. You would eventually end up with a piece of matter that could not be cut. Called particles atoms--Greek

for “indivisible”

His theory: all atoms are small hard particles made of a single material formed into different shapes and sizesalways moving, and that they form different materials by joining together

Early Ideas• Aristotle--Greece 384-

322 B.C.– All matter was continuous,

that matter could be divided into smaller and smaller pieces forever —did not believe in atoms

– Opinion accepted for 2000 years

John Dalton 1803

He performed many experiments to study how elements join together to form new substances.

He found that they combine in specific ratios and he supposed it was because the elements are made of atoms.

2000 Years Later

His ideas: All matter is made of very small particles called atoms.

Atoms of one element are identical in size, mass, and properties; atoms of different elements differ in size, mass, and properties

Atoms cannot be subdivided, created, or destroyed.

Atoms of different elements combine in simple whole-number ratios to form compounds.

Atoms are combined, separated, or rearranged in chemical reactions.

J.J. Thomson 1897English chemist and physicist

discovered 1st subatomic particles

His theory:negatively charged particles called electrons and positively charged matter

created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in

referred to it as the plum pudding model

He proposed the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream.

His ideas

the atom was a sphere of positively charged material. Spread throughout the atom were the negatively charged electrons similar to plums in a pudding or chocolate chips in ice cream.

The Plum Pudding Model

Negatively charged

Positively charged

Ernest Rutherford1911

New Zealand physicist discovered the nucleus

Was a student of J.J. Thomson but disagreed with the “Plum Pudding Model”

Devised an experiment to investigate the structure of positive and negative charges in the atom.

Gold Foil Experiment

What did most of the particles shot at the gold foil do?

Most of the particles traveled straight through the gold foil

What was the surprising behavior of a few of the particles?

A few of the particles were deflected and some even bounced back

His theory:

small, dense, positively charged particle present in nucleus called a proton

electrons travel around the nucleus, but their exact places cannot be described

most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense

Niels Bohr 1913Danish physicist discovered energy levelsproposed that electrons

move in paths at certain distances around the nucleus.

Electrons can jump from a path on one level to a path on another level

Bohr’s Model

James Chadwick 1932

English physicist discovered neutrons

His theoryneutrons have no electrical charge

neutrons have a mass nearly equal to the mass of a proton

unit of measurement for subatomic particles is the atomic mass unit (amu)

Modern Theory Atoms are composed of three main Atoms are composed of three main

subatomic particles: electron, subatomic particles: electron, proton, and neutron. proton, and neutron.

Most of the mass of the atom is Most of the mass of the atom is concentrated in the nucleus of the concentrated in the nucleus of the atom. atom.

The protons and neutrons are The protons and neutrons are located within the nucleus while the located within the nucleus while the electrons exist outside of the electrons exist outside of the nucleus. nucleus.

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In In stable atomsstable atoms, the number of , the number of protons is equal to the number of protons is equal to the number of electrons. electrons.

The type of atom is determined by The type of atom is determined by the number of protons it has. the number of protons it has.

The number of protons in an atom is The number of protons in an atom is equal to the equal to the atomic number atomic number

The sum of the number of protons The sum of the number of protons and neutrons in a particular atom is and neutrons in a particular atom is called the called the atomic massatomic mass