Mileposts on the road to the atom - cod.edu · The number of protons in the nucleus of an atom...
Transcript of Mileposts on the road to the atom - cod.edu · The number of protons in the nucleus of an atom...
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
Determine average atomic mass from isotope
distribution
Identify important groups in periodic table
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 model of the atom
"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 X-
rays while doing
experiments with
cathode rays
Radioactivity: the search for
invisible rays
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.
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
Comparison of subatomic particles
Particle Mass Charge
grams amu coulombs e
Electron 9.1094 x 10-28 5.486 x 10-4 -1.6022 x 10-19 -1
Proton 1.6726 x 10-24 1.0073 +1.6022 x 10-19 +1
Neutron 1.6749 x 10-24 1.0087 0 0
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
13
6C
Mass number:
number of protons
+ neutrons
Atomic number:
number of protons
Element symbol
Counting particles:
Number of electrons = number of protons = 6
Number of neutrons = mass number – atomic number (13 –
6 = 7)
Atomic mass
Atoms are very small and so normal units of
mass are inconvenient for describing atomic
mass
Atomic Mass Unit
Mass of one atom of carbon-12 = 12 amu
1 amu = 1.660 x 10-24 g
Similarity of atomic mass and mass
number
Almost all mass is
concentrated in the
nucleons
Means mass
number (integer)
and atomic mass are
very similar
Isotopes cause disparity
Carbon has two isotopes: C-12 and C-13
Atomic mass C-12 = 12 amu; C-13 = 13.0034
amu
Atomic mass C is weighted average of the
masses of the two isotopes (98.89 % C-12 +
1.11 % C-13
= 12.011 amu
What of the electrons?
We now understand the atom to contain a tiny
positively charged massive nucleus surrounded
by a comparatively vast empty space
containing the electrons
When atoms combine the electrons must
interact
We need to understand the arrangement of
electrons in the atom
Rudiments of elements All of matter is made from elements, in combination with other elements or
alone.
There are some ninety naturally occurring – have “always” been.
More than twenty have been created artificially
Element names and symbols
Aluminum Al Chlorine Cl Manganese Mn Copper (cuprium) Cu
Argon Ar Fluorine F Nitrogen N Iron (ferrum) Fe
Barium Ba Helium He Oxygen O Lead (plumbum) Pb
Boron B Hydrogen H Phosphorous P Mercury
(hydrargyrum)
Hg
Bromine Br Iodine I Silicon Si Potassium (karium) K
Calcium Ca Lithium Li Sulfur S Silver (argentum) Ag
Carbon C Magnesium Mg Zinc Zn Sodium (natrium) Na
Elements organized – a triumph of
chemistry The periodic table of the elements is organized into 18 groups and 7
periods. Elements are represented by one- or two-letter symbols and are
arranged according to atomic number.
Elements in groups have similar
properties One of the eight main groups of elements, Group 1A
is known as the alkali metals group. Elements in this group are highly reactive and form alkaline (basic) solutions with water.
All groups have characteristic
properties One of the eight main groups of elements, Group 8A is known as the noble
gas group. Elements in this family have such a low reactivity that they were formerly known as the inert gases.
Although their chemical properties are very uninteresting, their importance to understanding chemistry cannot be understated