Download Atomic Theory Summary Sheet Answers

The Story Behind Atomic Theory
Summary Sheet
Model
Diagram of the Atom
Main Ideas and Explanation
All matter is made up of indivisible units
called atomos (uncuttable in Greek).
Greek
Democritus
i) All elements are made of atoms that
are indivisible.
ii) All atoms of one element are
identical. Atoms of different
elements are different.
iii) Atoms cannot be created,
destroyed, or subdivided.
iv) Atoms of one element combine with
atoms of another element in
simple, whole number ratios to
form compounds. (Law of Definite
Proportions)
Dalton
The atom has a positively charged
electron cloud with negatively charged
electrons flowing freely throughout.
Thomson
Plum Pudding Model
Rutherford
This model introduced the nucleus. The
center of the atom is where the majority
of the mass and charge is located. The
nucleus contains protons and neutrons
and has an overall positive charge. The
remaining area is an electron cloud which
is negatively charged making up the
majority of the atom’s volume.
Based on the activity today, and the associated reading, complete the following questions.
1. What particles are found in the nucleus of the atom? Describe these particles.
Protons and neutrons. Protons are positively charged particles and
neutrons are neutral particles and they contribute to the mass of the
atom.
2. What is the third particle found in the atom? Where is it located?
The electron is the third particle. It is located outside the nucleus.
3. How was it determined that the charge of an electron is negative?
By using magnetic or electric fields placed near the electron beam in
the cathode-ray tube, the beams bend. Depending on the direction the
beam bends dictated the charges.
4. What observation led to the discovery of protons? How did we know that they were positively
charged?
The gold foil experiment showed that not all alpha particles passed
through the foil in a straight line path. Some were deflected which lead
to the belief that there was a center in the atom where the mass was
concentrated and it had an overall positive charge.
5. Describe the experiment which caused the Thomson model to be replaced by the Rutherford
model of the atom.
Using a very thin foil of gold, a few atoms thick, bombard it with
positively charged alpha particles. According to Thomson’s model, the
particles should pass straight through.
6. Contrast the Thomson model with the Rutherford model of the atom. What did Rutherford’s
experiment indicate about the structure of an atom?
Thomson’s model has the positive and negative charges balanced
throughout the whole atom. Rutherford’s model indicates there is a
central mass that is positively charged with the negatives around it.
7. Suppose that in a hypothetical scattering experiment all of the alpha particles went through
the gold foil. Propose a model of an atom that would account for this observation.
If all the alpha particles went through the foil, this would indicate that
there were no positive charges in the atom.
8. What were the four main postulates of Dalton’s atomic theory?
The four main postulates are:
i)All elements are made of tiny particles, called atoms (indivisible).
ii) All atoms of one element are identical, though atoms of different
elements are different, specifically in terms of mass.
iii) Atoms can’t be created, destroyed, or subdivided.
iv) Atoms of one element combine with atoms of another element to
form compounds. The molecules of these compounds always have the
same proportions of elemental atoms.
9. What parts of Dalton’s atomic theory no longer apply to our view of the atom?
Atoms can be subdivided.
All atoms of one element can differ in atomic mass (the number of
neutrons in the nucleus – isotopes).