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S. 8.1 – The Structure of the Atom
A. Early Models of the Atom.
a) The Atomic models of the Early Greeks and Medieval Alchemists.
The Greeks.
- Philosophy ruled and experimenters did not. Experimental results were ambiguous and
misleading.
- Thus, the idea that the four major elements were: Air, Water, Fire, and Earth – everything
was made of some sort of combination of these four things.
Middle Ages (Lets go club the enemy with a shank of lamb ---- good fun).
- Practical chemistry came from investigations of Arabs and European alchemists.
- Separation of metals from ors and different distillations.
- Ideas that matter existed as various attractions and repulsions were created.
b) Dalton’s Atomic Theory.
- Founded on experimental data and is follows.
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Elements are made up of small atoms.
Atoms of the same elements are the same and different atoms have different properties.
Compounds have distinctive combinations and types of atoms.
Chemical rx involves remixing of atoms to form new compounds with same atoms as to
the original compound.
Three laws founded from Dalton’s Theories.
1. Law of Definite Proportions (Law of Constant Composition).
2. The Law of Multiple Proportions.
3. The Law of Conservation of Mass.
c) The Thomson Model of the Atom.
- Atoms must comprise of a ball of positive charge with negative charges distributed throughout
(like raisins in a raisin bun).
B. The Rutherford-Bohr Model of the Atom.
Rutherford.
1. Discovered alpha (), beta (), and gamma () particles.
2. Correctly interpreted the nature of radioactivity.
3. Discovered the true nature of the alpha particles.
4. Discovered the atomic nucleus.
- Most alpha particles shot through a thin gold foil, but a few alpha particles did not
penetrate the foil and shot back towards the alpha particle source).
5. Predicted the existence of the neutron.
- Helium had a amu of 4 but only contained 2 protons within the nucleus; thus, the
nucleus must contain neutral particles having a amu of 1 (Later discovered by
Chadwick in 1932).
Rutherford’s atomic Theory: An atom consists of a nucleus containing most of the atoms
mass with protons and neutrons that are surrounded by a
negatively charged cloud containing electrons that equal in
number to the number of protons within the nucleus. Thus,
creating a neutral charge.
Bohr.
- Tried to describe electron activities by looking at the activity of Hydrogen’s electron.
- Idea of electrons orbiting the nucleus was proven wrong, but helps with understanding of
future ideas.
Bohr Model of the Atom: Electrons within an atom are restricted to specific energy levels
and movement around a nucleus called orbits. Orbits are a fixed
distance from the nucleus and electrons can only absorb and emit
energy when moved between orbits.
C. Atomic Number and Atomic Mass.
Atomic number:
Subatomic particles:
Protons: Positively charged subatomic particles with an amu of 1 located within the nucleus.
Neutrons: Neutrally charged subatomic particles with an amu of 1 located within the nucleus.
Electrons:
Neutral Atom: Atoms # of protons and electrons are equal.
Charged atom: Atoms # of protons and electrons are not equal.
Ion: General term used to indicate an atom with a charge.
Cation:
Ex.
Anion:
Ex.
Determining charge of a non neutral atom.
- Assign positive value to protons and negative value to electrons.
- Add them up and you have your charge.
Ex.
Ex.
Why atoms form ions.
- Atoms aspire to be like the noble gases, have full outer orbital’s.
- Electrons are gained or lost to acquire full outer orbital’s.
Ex. Bohr models for Neutral and ionic fluorine, neon, and sodium
Fluorine (F and F-)
Neon (Ne)
Sodium (Na and Na+)
Neutral
Ionic
***Notice***: Fluorine will accept electrons to have a valence orbital the same as neon.
Sodium will lose electrons to have a valence orbital the same as neon.
Electrons are lost and gained (Donated and accepted) so atoms can obtain full outer orbitals.
Questions:
- Pg. 146, # 13 – 17.
Atomic mass: Number of protons and neutrons within the nucleus.
Atomic mass = # of Protons + # of Neutrons
Atomic mass = Atomic # + # of Neutrons
(Atomic # = # of protons)
Ex. How many neutrons does Mg – 24 have.
# of neutrons = Atomic mass – Atomic #
= 24 – 12
= 12 neutrons
Standard notation method of indicating element atomic mass and atomic number
Another method of indicating element and atomic mass.
- Write the elements name or symbol followed by a dash and then the atoms atomic mass.
Ex. Chlorine–35 or Cl–35
Using standard notation to find # of protons, neutrons, and electrons.
Ex. 1: Find the number of protons, neutrons, and electrons possessed by:
Neutral chlorine  # of protons = # of electrons = Atomic number = 17
# of neutrons = Atomic mass – Atomic #
= 35 – 17
= 18 neutrons
Ex. 2: Find the number of protons, neutrons, and electrons posses by:
Charged chlorine  # of protons = atomic number = 17.
# of electrons = Charge - # of protons
= -1 – 17
= -18  there are 18 electrons
# of neutrons = Atomic mass – Atomic #
= 35 – 17
= 18 neutrons
Questions:
- Pg. 147, # 18 – 19.