Download Chapter 4: Atomic Structure

Chapter 4: Atomic Structure
Addison Wesley Chemistry
4.1 - Atoms
Lead is a soft metal. Suppose we take a
one gram cube of lead and cut it into the
smallest pieces we can.
 Eventually we will get to a particle that
cannot be cut any further and still retain
its physical properties.
 This would be one atom of lead

4.1 – The Atom

An atom is the smallest particle of an
element that retains the properties of that
element.

The ideas of atoms was first brought up by
Democritus, an ancient Greek. In the 4th
century, BCE.
 However at the time they were not useful b/c
they were not based on empirical evidence.
 The great philosopher Aristotle theorized there
were only four elements: Fire, Water, Air, &
Earth. That theory held until the late 1700s

In the late 1700s John Dalton brought back
the idea of atoms and first stated: Dalton’s
Atomic Theory:
1.
2.
3.
4.
All elements are composed of atoms.
Atoms of the same elements are the same.
Atoms of different elements can combine together
to make compounds.
Chemical reactions occur when atoms are
separated, joined, or rearranged.
4.2 – Atomic Theories

Dalton’s Theory – The “Billiard Ball”
model

Thomson’s Theory (1893) – the “raisin
bun” model
First suggested that electrons are
subatomic particles.
 Electrons are embedded on a positive
sphere
 Discovered electrons by using a cathode
ray


Thomson’s Model

Rutherford’s Theory (1907) – “beehive”
or “nuclear” model.
Performed the Gold foil experiment
 Atoms contain a tiny dense positive core
called the nucleus, and the first suggested
the proton
 Electrons occupy the empty space around
the nucleus


Bohr Theory – the ‘planet’ model
The nucleus contains both protons and
neutrons
 The electrons move around in fixed energy
levels

4.2 – Electrons, Protons, & Neutrons
Most of Dalton’s atomic theory is still
accepted.
 Now we know that atoms are made up of
subatomic particles called electrons,
protons, and neutrons.
 Protons – Have a positive charge
 Electrons – Have a negative charge
 Neutrons – Have no charge

Electrons
Electrons are negatively charged
subatomic particles.
 They were discovered by physicist Sir
Joseph Thomson in 1897.
 They were discovered by using a device
called a cathode ray.
 Electrons are extremely small and light
about 1/2000 of a hydrogen atom

Protons

Hydrogen is the simplest element consisting of
one electron and a proton.
 After the discovery of the electrons scientists
wondered about what was left over when
hydrogen lost it’s sole electron
 Since atoms were electrically neutral scientists
reasoned that if the negatively charged
electrons were removed from a hydrogen atom
then the resulting particle should have a
positive charge
Experimental evidence for this was
found.
 Researchers named the positively
charged particles protons.
 It is 1840 times heavier than an electron.
 A proton is what remains when a
hydrogen atom loses its’ electron

Hydrogen Atom
Neutrons
In 1932 physicist Sir James Chadwick
confirmed the existence of the neutron.
 Neutrons are subatomic particles with no
charge
 They have a mass close to that of a
proton
