Download Atomic Theory

Atomic Theory
The Modern Periodic Table
Atomic Theory
 How did we come up with the way the Periodic Table is set
up?
 Russian Scientist Mendelev started to weigh elements,
calculate their density and he noticed a pattern.
 He noticed that Hydrogen had a weight of 1 g, Lithium had
a weight of 7 g, Sodium had a weight of 23 g and
Potassium had a weight of 39 g.
 He then noticed Beryllium had a weight of 9 g, Magnesium
had a weight of 24 g, and Calcium had a weight of 40 g.
 He was then able to see the following pattern.
Mendeleev’s Periodic Table
Atomic Theory
 But what was causing the elements to have the
weights that they had and the density that they had?
Why was this pattern so noticeable?
 John Dalton, in the 1800’s was able to put some
context into what was happening.
 He envisioned an atom – and he said that the atom
was was the smallest piece of any element, it was s
smooth round solid sphere and did not have an
electrical charge.
Dalton’s First Theory
 Dalton stated:
 All matter is made of atoms, which are particles that are too
small to see.
 Each element has its own kind of atom, with its own
particular mass.
 All atoms of any one element are identical.
 Compounds are created when atoms of different elements
are combined in a specific ratio.
 Atoms cannot be created, destroyed, or subdivided during
chemical changes.
Dalton’s first theroy
 Basically, Dalton was able to indicate what an atom
looks like, and how elements interact with other
elements to create a chemical change and a chemical
compound.
 BUT…………. What about electric charges, how do
they come into play?
Dalton’s revised theory
 In 1830, scientist Michael Faraday said that electrons must
be present in an atom, hence this is why we get an electric
shock when we have a built up charge and then touch a
metal object.
 Dalton agreed and added three more parts to his theory:
 Matter must contain positive and negative charges
 Opposite charges attract, like charges repel.
 Atoms combine to form the particles of a compound because
of the electrical attraction between charged atoms.
Electron’s?
 So what were
these electron’s
that Dalton and
Faraday were
talking about?
 J.J. Thompson, an
English Scientist,
hooked a power
supply to a
cathode ray tube.
Electron’s?
 Cathode ray tube has a fluorescent dye on the inside,
and when an electron shots from one side to the
other you can see it.
 Thompson noticed that it was
very very very very very easy to
manipulate the electron, and
move it inside the tube.
So what is the mass of an electron?
 American R.A. Millikan decided to find out how the mass of
an electron.
 What he did was set up a box, spray oil into the box. He
then had two horizontal plates that were charged. In the
top plate he had a tiny hole for the charged electrons to
drop through. He could then observe how rapidly the oil
drops fell through the first charged plate (+) to the second
charged plate (-).
 He was able to deduce that one electron’s mass was
9.109382 X 10-28. I.E. VERY VERY VERY VERY VERY light.
Millikan’s Experiment
Atomic Theory: The electron
 Because of Thompson and Millikan, we are able to add to
the Atomic Theory:




Atom’s contain electrons.
The electrons have a negative charge and a very small mass.
The rest of the atom has a positive charge.
The electrons are embedded randomly into the positive parts
of the atom.
 Electrons can be removed from, or added to, atoms to create
a charged atom.
Atoms – what do they look like
 So far we have deduced why chemical reactions occur
based on Dalton’s theory, and we have been able
understand more about electron’s, but if electrons
are so light, what was it that Mendeleev was
weighing then, and how did electrons and protons
make up the arrangements of an atom?
Rutherford’s Experiment
 What Rutherford did was shoot an alpha particle from
an alpha particle emitter.
 He shot the alpha particle into a piece of gold foil.
 Surrounding the gold foil was a screen to see where
the alpha particles hit.
 Since alpha particles are positive charged, the original
theory is that all shots would go straight through the
gold foil and hit the back of the screen… but not all
did.
Rutherford’s Experiment
 To the amazement of Rutherford,
while about 80% or so did go
straight though the gold foil, some
of the alpha particles started to
bounce all over the place.
 They would hit the gold foil and
bounce left, and bounce right, and
even bounce back towards the
alpha particle emitter.
Rutherford’s Experiment
 British Physicist Ernest Rutherford famous gold foil
experiment.
So what happened?
 What Rutherford was observing was the positively
charged alpha particle was hitting another positively
charged particle and hence the alpha particle
bounced around. Every once in a while an alpha
particle would hit the gold foil and not bounce
anywhere.
 Rutherford deduced that there were three subatomic
particles in an atom: an electron, a proton, and
neutron.
Rutherford’s Atomic Theory
 Rutherford’s Atomic Theory:
 The nucleus contains all of the positive charge and most of the
mass of the atom.
 The nucleus contains positively charged protons and uncharged
neutrons.
 Neutrons have the same mass as protons.
 The nucleus is very small, compared with the size of the atom.
 The electrons orbit the nucleus, like satellites orbit the planet.
 The mass of an electron is 1/1800 of a proton.
 The size of the atom is determined by the size of the orbit of the
electrons.
 There is only empty space between the electrons and the
nucleus.