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Chem Notes
Chapter 5
Section 5.1 & 5.2
Early Models of the Atom

First proposed by Democritus in the
4th century.

Believed atoms were indivisible and
indestructible
Dalton’s Atomic Theory

Came 2000 years later, based on
experimentation

All elements composed of tiny
indivisible particles called atoms
Dalton’s Atomic Theory

Came 2000 years later, based on
experimentation


All elements composed of tiny
indivisible particles called atoms
Atoms of the same element are
identical; different from atoms of other
elements
Dalton’s Atomic Theory

Came 2000 years later, based on
experimentation



All elements composed of tiny
indivisible particles called atoms
Atoms of the same element are
identical; different from atoms of other
elements
Atoms of different elements can mix or
chemically combine in simple wholenumber ratios to form compounds.

Chemical rxns occur when atoms are
separated, joined, or rearranged.
Atoms of one element are never
changed into atoms of another
element.

Atom: fundamental building block
of all matter

Smallest particle of an element that
retains the properties of that element.

Very, very small. Can view images of
surface of groups of atoms with scanning
tunneling microscope.
Subatomic Particles

Much of Dalton’s theory still
accepted – one notable exception is
that atoms are now known to be
divisible.
Subatomic Particles

Electrons
Subatomic Particles

Electrons

Negatively charged
Subatomic Particles

Electrons


Negatively charged
Each electron carries one unit of
negative charge
Subatomic Particles

Electrons



Negatively charged
Each electron carries one unit of
negative charge
Mass is 1/1840 the mass of a hydrogen
atom (or one proton)
Subatomic Particles

Electrons




Negatively charged
Each electron carries one unit of
negative charge
Mass is 1/1840 the mass of a hydrogen
atom (or one proton)
Discovered by J.J. Thomson in 1897
using cathode ray tubes.
Subatomic Particles

Protons


One unit of positive charge
Mass is 1840x that of the electron
Subatomic Particles

Neutrons


No charge, but mass nearly equal to
that of protons
Existence confirmed by James
Chadwick in 1932
Atomic Nucleus

Discovered by Rutherford’s gold foil
experiments.
Atomic Nucleus


Discovered by Rutherford’s gold foil
experiments.
Proposed that an atom is mostly
empty space with the positive
charge and nearly all mass
concentrated in small region.

Nucleus – central core of atom;
composed of protons and neutrons