Atomic Theory and Structure
Democritus:
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Greek Philosopher (~480 BC)
named the basic particle the “atom” (means “indivisible”)
Foundations of Atomic Theory
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developed 1700s – 1800s
Elements combine to form different compounds
- Do they always combine in the same ratio?
Law of Conservation of Mass
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Mass is neither created nor destroyed
Law of Definite Proportions
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Chemical compound contains same elements in same proportions by mass
regardless of size of the sample or source
- Ex. NaCl (table salt) … always 39.34% Na, 60.66% Cl by mass
(same combination of atoms)
Law of Multiple Proportions
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If two or more different compounds are composed of the same 2 elements (or
more) than the ratio of masses of the second element combined with certain
mass of the 1st element is always a ratio of small whole #s
CO2 always has twice as much O2 as C (1:2 ratio)… CO always has 1:1 ratio, C to O.
Dalton’s Atomic Theory
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1808 English schoolteacher
Explains laws (from above, ↑) with a few statements
1. All matter is composed of atoms (very) small
2. Atoms of a given element are identical in size, mass, and other properties –
atoms of different elements differ in properties
3. Atoms cannot be subdivided, created, or destroyed
4. Atoms of different elements combine in simple whole-number ratios to form
chemical compounds
5. In chemical reactions, atoms are combined, separated, or rearranged.
(3, 5 explain law of conservation of mass)
Modern Atomic Theory
Now we know [atoms can be divided into subatomic particles]
[elements can have atoms with different masses… Isotopes]
1. All matter is composed of atoms
2. Atoms of any one element differ in properties from atoms of another element
**An atom: the smallest particle of an element that retains the chemical properties of that
element**
Atoms are made up of 2 regions: nucleus (protons and neutrons) & electron cloud
(general area where the electrons are located)
Discovery of the Electron
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Came about with experiments dealing with electricity
Cathode Rays and Electrons
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Sent an electrical current through a cathode
2 obsrvations:
1. Rays deflected by a magnetic field (just like current in wire, known to be
negative)
2. Rays deflected away from a negatively charged object
Thomson measured the ratio of charge of particles to their mass- the ratio is
ALWAYS the same!
Therefore, cathode rays composed of electrons
From this information, Thomson proposed the “Plum Pudding Model” (electrons
spread evenly throughout the atom)- This was DISPROVED shortly later
Discovery of the Atomic Nucleus
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Rutherford’s gold foil experiment
Bombarded foil with alpha particles (4x mass of hydrogen atom)
Most passed through, but some bounced back! Caused by a very densely packed
bundle of matter with a positive electric charge- called it the nucleus.
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Volume of the nucleus is very small compared to the total volume of the atom
Where were electrons?
Composition of the Atomic Nucleus
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All nucleus are made of protons and neutrons (except hydrogen, which is only a
proton)
Proton is positively charged (equal in magnitude to the electron’s negative
charge), Neutrons are neutral- they have no charge.
Atoms of different elements differ in their number of protons- Number of
protons determine atom’s identity (determines which element it is)
Forces in the Nucleus
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Like charges repel each other, but when extremely close together they have a
very strong attraction
Short range proton-neutron, proton-proton, and neutron-neutron forces hold the
nuclear particles together and are referred to as nuclear forces
The Sizes of Atoms
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Electrons exist in the electron “cloud” (no exact position at any one moment)
Atomic radii- from the center of the nucleus to the outer portion of the electron
cloud
SMALL- measured in picometers (pm)
Atomic nuclei are very dense as well