Download Atoms are not the smallest thing

Atoms are not the
smallest thing
Growing evidence for the
divisibility of the indivisible
Learning Objectives





Describe the three particles in the atom
Define atomic number and mass number
Describe isotopes
Write symbols for elements
Determine the numbers of particles in any
atom from the element symbol
Electrostatics and electricity




Static electricity was observed by
Thales (300 BC). Some
“charged” objects repel and
others attract
The voltaic cell (Volta, 18th
century) generated electrical
current from chemical reactions
Mechanical electrical generation
was achieved in 1825
The point: Atoms are neutral. If
indivisible, where do electrical
charges come from?
Faraday’s prescience

“Although we know nothing of what an atom
is, we cannot resist forming some idea of a
small particle; and though we are in equal
ignorance of electricity, there is an immensity
of facts which justify us in believing that the
atoms of matter are associated with electrical
powers to which they owe their most striking
qualities, and amongst them their chemical
affinity.”
Ray of hope


1858
Discovery of cathode rays by Julius Plucker


Application of a large voltage across an evacuated
tube causes a current to flow. The current flow is
accompanied by radiation from the excited gas
molecules
How does the neutral and indivisible atom create a
charge?
Cathode rays are negatively charged particles
1897.J. Thomson demonstrates that cathode rays consist of negatively charged
particles. The first sighting of the electron: a particle much smaller than an atom.
The Thomson :Plum Pudding”
model
"I regard the atom as containing a large number of smaller bodies
which I will call corpuscles, these corpuscles are equal to each
other.... In the normal atom, this assemblage of corpuscles
forms a system which is electrically neutral. Though the
individual corpuscles behave
like negative ions, yet when they are
assembled in a neutral atom the negative
effect is balanced by something which
causes the space through which the corpuscles are spread to act as
if it had a charge of positive electricity equal in amount to the
sum of the negative charges of the corpuscles…”
X-rays and atoms’ invisible rays


1895
Wilhelm Conrad
Roentgen discovers Xrays while doing
experiments with
cathode rays
Radioactivity: invisible rays and
unstable atoms




1896
Henri Becquerel discovered radioactivity,
which suggested that some atoms were capable
of decomposing to give smaller particles.
1903
Frederick Soddy and William Ramsey
demonstrated that uranium decayed to give
helium. Direct proof that atoms were
divisible.
1909 Ernest Rutherford’s gold foil
experiment

The atom’s inner secrets exposed by its own
offspring
Hierarchy of discovery

The path to knowledge occurs in steps, each
depending on a previous advance
The battery and electricity: Volta 1799
Cathode rays: Plucker 1858
X-rays: Roentgen 1895
Radioactivity: Becquerel 1896
The nucleus: Rutherford 1909
The nucleus




Tiny
Incredibly dense – contains all the mass
of the atom
Positively charged
Contains protons (charged) and neutrons
(neutral) – not discovered until much
later
Summary of Atom Pictures



Dalton: Indivisible atom
Thomson: Electrons
Rutherford: Nucleus
Atoms are mostly nothing
Comparison of subatomic particles
Atoms are neutral:
# electrons = # protons

The number of protons in the nucleus of an atom
determines the atomic number (Z) and indicates the
element's identity. For a neutral atom, the atomic
number also describes the number of electrons around
the nucleus.
Variations on the number of neutrons in the
nucleus give rise to different isotopes of the
same element.
1
1
H
2
1
H
3
1
H
Element notation: Atomic number
and mass number
Mass number:
number of protons
+ neutrons
13
6
C
Element symbol
Atomic number:
number of protons

Counting particles:


Number of electrons = number of protons = 6
Number of neutrons = mass number – atomic number (13 –
6 = 7)
Ions: losing and gaining electrons




Atoms can lose or gain
electrons
Atomic number remains
the same
Loss: Positive ions have
fewer electrons than
protons
Gain: Negative ions
have more electrons
than protons
What of the electrons?



We now understand atom contains tiny
positively charged massive nucleus surrounded
by vast empty space containing electrons
When atoms combine the electrons must
interact
We need to understand the arrangement of
electrons in the atom