Transcript Atoms: The Building Blocks of Matter

Atoms: The Building Blocks of
Matter
Ch. 3
Early atomic theory
• Democritus (400 B.C.)
▫ Atoms are nature’s basic particles
▫ “atom”  Greek word meaning “indivisible”
• Aristotle
▫ Disagreed, thought matter was continuous
▫ Aristotle's theory prevailed for next 2000 yrs.
1700s
• 1st experimentation
• Definition of element (pure sub. made of 1 kind
of atom)
• Elements combine  compounds
Late 1700s
• Quantitative studies of reactions
• law of conservation of mass
Dalton’s Atomic Theory
• All matter composed of atoms
Atoms…
• of same element are identical; atoms of diff.
elements are diff.
• cannot be divided, created, destroyed
• combine in simple whole # ratios to form
compounds (Law of Definite Proportions)
• are combined, separated or rearranged in chem.
rxns.
Modern Atomic Theory
• Not all of Dalton’s ideas were correct
▫ Atoms can be divisible
▫ Atoms of same element can have diff. masses
• Most still holds true
Sec. 2 - Structure of the Atom
• Atom
▫ nucleus – protons & neutrons
▫ electron cloud
Discovery of the e- (1897)
• Cathode ray tube – electric current passed
through gases, found neg. charged particles
• JJ Thomson
▫ Did more expts. and found particles identical (no
matter which gas used)
▫ Named them electrons
• Millikan
▫ Oil drop exp.
▫ Mass of e- extremely small, neg. charged
▫ Atom is neutral, therefore must contain + charge
also
Choc. chip cookie model
-
+ + - +
- - +
- + +
+ - - +
+
Discovery of Nucleus
• Rutherford
▫ 1900
▫ Gold foil exp.
▫ Confirmed dense
nucleus with + charge
• Chadwick discovered
neutrons ~30 yrs. Later
Modern atomic model
• nucleus: p+ and n0
▫ All the mass, very little space
• e- cloud
▫ No mass, lots of space
Nuclear forces
• Like forces repel, normally
• BUT when protons are really close there is a
strong attractive force
• Same with neutrons
• Called nuclear forces (energy, bombs)
Size of atom
• Tiny
• Radius  from center of nucleus to edge of ecloud
• Measured in pm (10-12)
• Atomic radii range from 40-270 pm
Sec. 3 - Counting Atoms
• Atomic #
▫ # of protons in nucleus
▫ Determines what element
Ex: oxygen is 8  8 p+ and 8 e-
• Mass #
▫ Total # of p+ and n0 in an isotope
Calculate # of neutrons:
Mass # - atomic #
(p + n) – (p) = n
• Isotopes
▫ Atoms of same element with diff. # neutrons
▫ Diff. masses
Ex: hydrogen
1
1H
protium
99.985%
2
H
1
deuterium
0.015%
3
1H
tritium
trace, synthetic
*Don’t differ much in chem. props.
• Isotope symbols
▫ Hyphen
▫ Nuclear symbol
uranium-235
235
92
U
Charges
Atomic #
Mass #
Protons
Neutrons
electrons
80
35
Br
35
80
35
45
35
80
35
Br-1
35
80
35
45
36
Ca+2
20
40
20
20
18
40
20
• amu’s
▫ atomic mass units
▫ masses so small, came up with scale that is
relative
▫ amu = mass of 1/12 mass of carbon-12 atom
• Average atomic masses
▫ # on periodic table
▫ Decimals  averages
Light
• The photoelectric effect refers to the
emission of electrons from a metal when
light shines on the metal.
Atomic Energy
• The lowest energy state of an atom is its ground state.
• A state in which an atom has a higher potential energy
than it has in its ground state is an excited state.
Hydrogen’s line emission spectrum
• Visual
Bohr Model
• According to the model, the electron can circle
the nucleus only in allowed paths, or orbits.
• The energy of the electron is higher when the
electron is in orbitals that are successively
farther from the nucleus.
• When an electron falls to a lower energy level, a
photon is emitted
• Energy must be added to an atom in order to
move an electron from a lower energy level to a
higher energy level.