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Atomic Theory of Matter
The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.
Dalton’s Postulates
2. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
* He was a little wrong on this one…isotopes!
Dalton’s Postulates
3. Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
Law of Conservation of Mass
Dalton’s Postulates
4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.
Law of Definite Proportions(a.k.a. Law of Constant Composition)See Chapter 1 Notes
Dalton also deduced theLaw of Multiple Proportions
CO2 CO
2.671.33
=2.00 *This is always a whole number!
The Electron• J. J. Thompson is credited with their
discovery (1897).• Streams of negatively charged particles were
found to emanate from cathode tubes.
Discovery of the Nucleus
Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.
The Nuclear Atom
• Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.
• Most of the volume of the atom is empty space.
Other Subatomic Particles
• Protons were discovered by Rutherford in 1919.
• Neutrons were discovered by James Chadwick in 1932.
Isotopes:
• Atoms of the same element with different masses.
• Isotopes have different numbers of neutrons.116C 12
6C13
6C14
6C
Atomic Mass (Weight) of an Atom
The actual mass can be calculated for any atom:• Actual Mass (g) = (Mass#)(1.67x10-24g)
• This can be converted to an atomic mass by relating the mass to carbon-12
Mass of one atom in amu = Mass of a mole of atoms in grams!
*Scientists defined the mole so this would work!
Atomic Mass (Weight) of an Element
• A weighted average of atomic masses of all the isotopes of an element.
Use the following data to calculate the atomic mass for the element Magnesium
Isotope Atomic Mass of Isotope Abundance
Mg - 24 23.982628 µ 78.600 %Mg - 25 24.963745 µ 10.11 %Mg - 26 25.960802 µ 11.29 %
(.78600) (23.982628 g) + (.1011) (24.963745 g) + (.1129) (25.960802
g)
18.850 g + 2.524 g + 2.931 g 24.305 g/mol
Periodic Table:
• A systematic catalog of elements.
• Elements are arranged in order of atomic number.
Periodic Table
• The rows on the periodic chart are periods.
• Columns are groups.• Elements in the same
group have similar chemical properties.
Chemical FormulasThe subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.
Types of Formulas• Empirical formulas give the lowest
whole-number ratio of atoms of each element in a compound.
• Molecular formulas give the exact number of atoms of each element in a compound.
• Molecular Formula: C6H12O6
• Empirical Formula: CH2O
Ionic BondsIonic compounds (such as NaCl) are generally formed between metals and nonmetals.
*See handout on naming and writing formulas!
Molecular CompoundsMolecular compounds are composed of molecules and almost always contain only nonmetals.
*See handout on naming and writing formulas!
Diatomic Molecules
These seven elements occur naturally as molecules containing two atoms.
HONClBrIF
Acids
• Compounds containing hydrogen with something that looks like a negative ion
(for now at least…..)
*See handout on naming and writing formulas!
Organic Molecules (Alkanes)
• Organic molecules contain carbon.
• Alkanes are the most simple organic compounds. They have chains of carbon with only single bonds surrounded by hydrogen.
• They are named by the number of carbons in the chain + the suffix -ane
Alkanes# of carbons Name of alkane
1 Methane
2 Ethane
3 Propane
4 Butane
5 Pentane
6 Hexane
7 Heptane
8 Octane
9 Nonane
10 Decane
Alcohol• Other classifications of organic
compounds have multiple bonds or other atoms or groups of atoms replacing a hydrogen.
• Alcohol has an –OH group instead of a –H off one of the carbons in an alkane
• They are named with the same root as the alkanes + the suffix -ol Starting with propanol they include a
number in front of the name indicating which carbon the –OH is bonded to
Atomic Mass (Weight) of an Element
• A weighted average of atomic masses of all the isotopes of an element.
Use the following data to calculate the atomic mass for the element Magnesium
Isotope Atomic Mass of Isotope Abundance
Mg - 24 23.982628 µ 78.600 %Mg - 25 24.963745 µ 10.11 %Mg - 26 25.960802 µ 11.29 %
(.78600) (23.982628 g) + (.1011) (24.963745 g) + (.1129) (25.960802
g)
18.850 g + 2.524 g + 2.931 g 24.305 g/mol
Where does that data come from?
• Mass SpectroscopyAtoms that go into the mass spectrometer
are organized by mass and the relative amounts are measured.
This is how we found isotopes and disproved part (the only part so far) of Dalton’s Atomic Theory.
Mass Spec for Molybdenium
- we will assume the charge is +1…so this is the atomic mass
http://www.chemguide.co.uk/analysis/masspec/howitworks.html#top
Estimating Atomic Mass of an Element from its Mass Spectrum
This scale is usually made by setting the tallest line at 100
100
23
All we really care about is this ratio…if we have 123 atoms, 23 would be B-10 and 100 would be B-11