CHM 1020 Chapter 2: Chapter 2 Summary

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7 Feb 2017
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Chapter 2- Atoms and Elements Summary
Scanning Tunneling Microscope
Binnig and Rohrer found that as you pass a sharp metal tip over a flat metal surface, the
amount of current that flows varies according to the distance between the tip and surface.
Measuring this
tunneling
current allowed them to scan the surface on an atomic scale
essentially by taking pictures of the atoms on the surface. Later scientists, found that
individual atoms were also able to move across the surface.
Law of Conservation of Mass
“In a chemical reaction, matter is neither created nor destroyed” – Antoine Lavoisier.
Basically, the total mass of materials there are before a reaction should equal the total
mass of the final result of the reaction. (Total mass of reactants= total mass of products).
Law of Definite Proportions
“All samples of a given compound, regardless of their source or preparation have the same
proportions of their constituent elements” – Joseph Proust.
E.g. A 100.0g sample of sodium chloride contains 39.3 g of Na and 60.7g of Cl

 
  
A 58.44g sample of sodium chloride contains 22.99g of Na and 35.44g of Cl

 
 
Law of Multiple Proportions
“When two elements “A” and “B” form two different compounds, the masses of B that
combine with 1g of A can be expressed as a ratio of small, whole numbers” – John Dalton.
Dalton’s Atomic Theory
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Dalton proposed a theory of matter based on it having ultimate, inseparable particles to
explain these laws.
1) Each element is composed by tiny, indestructible particles called atoms.
2) All atoms of a given element have the same mass and other properties that distinguish
them from atoms of another element.
3) Atoms combine in simple, whole-number ratios to form molecules of compounds.
4) In a chemical reaction, atoms of one element cannot change into atoms of another
element. (They simply rearrange the way they are attached).
Notes on Charge
There are two kinds of charge: + and
Like charges repel each other: + repels + and repels
Opposite charges attract: + attracts to
To be neutral, something must have no charge or equal amounts of opposite charge.
Cathode Ray Tubes
A glass tube containing metal electrodes from which almost all the air has been evacuated.
When connected to a high voltage power supply, a glowing area is seen emanating from the
cathode. (Cathode= - & anode= +).
J.J. Thomson believed that cathode ray was composed of tiny particles with an electrical
charge. He designed an experiment to demonstrate that there were particles by measuring
the amount of force it takes to deflect their path a given amount. (Like measuring the
amount of force it takes to make a car turn).
Thomson investigated the effect of placing an electric field around tube (1) charged matter
is attracted to an electric field (2) light’s path is not deflected by an electric field.
Thomson’s Results:
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Cathode rays are made of tiny particles that have a negative charge (because the beam
always deflected toward the anode plate). The amount of deflection was related to two
factors: charge and mass of particles. Every material tested contained same particles. The
charge/mass of these particles was -1.76 x 10 C/g.
Quick fact: hydrogen ions have a +9.58
x 10^4 C/g.
Thomson’s conclusions:
If the particle has the same amount of charge as a hydrogen ion, then it must have a mass
almost 2000x smaller than hydrogen atoms!!! (WRONG). Later experiments by Millikan
showed that the particle did have the same amount of charge as the hydrogen ion and the
only way for this to be true is if these particles were pieces of atoms (apparently, the atom
is not unbreakable). Finally, Thomson believed that these particles were therefore ultimate
building blocks of matter. These cathode ray particles became known as electrons.
Electrons
Tiny, negatively charged particles found in all atoms. Cathode rays are made of streams of
electrons. The electron has a charge of -1.60 x 10¹⁹ C and a mass of 9.1 x g.
Electrons are held in the atom by their attraction for a positively charged electric field within
the atom.
New Theory of the Atom
Thompson had to correct Dalton’s Atomic Theory regarding an atom being indivisible.
Thomson proposes the idea of instead an atom being a hard, marble-like unbreakable
sphere (how Dalton describes it), the atom actually has an inner structure.
Predictions of the Plum Pudding Atom
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