Transcript Atomic Structure 2

Atomic Structure-1
Democritus is credited with
proposing the first atomic
theory
 Proust stated that the
elements in substances always
combine in definite proportions
by mass
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Dalton’s Atomic Theory
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Dalton’s atomic theory:
1. All matter is composed of atoms;
2. All atoms of the same element are
identical;
3. Atoms of different elements are
not alike;
4. Atoms unite in simple ratios to
form compounds.
Atomic Structure-2
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Atoms of the same element may
differ in mass. Average mass is used
instead of whole-numbered mass
The law of multiple proportions
states that the ratio of masses of
one element that combine with a
constant amount of another element
can be expressed in small whole
numbers
Avogadro Number
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Avogadro stated that equal
volumes of gases have the same
number of molecules under the
same conditions (22.4 L/mole)
The Avogadro number is
6.02 x 1023
molecules/mole of ANY
substance
Isotopes and Atomic Number
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Isotopes contain the same number of
protons but a different number of
neutrons
The atomic number (Z) of an
element equals to the number of
protons in the nucleus
The mass number (A) of an atom is
the total number of its nucleons
Number of neutrons = A - Z
Nuclear Structure
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Protons and neutrons are held together by a
nuclear force
The particles composing atoms are called
subatomic particles
A mirror-image antiparticle exists for every
particle
Leptons—elementary particles: electrons,
neutrino, muon, tau
Hadrons—made of quarks: baryons—3 different
quarks; neutrons and protons; mesons—
quark+antiquark
Quarks are held together by exchanging gluons.
Nucleons are held together by exchanging
pions
Radiation
Three forms of radiation: alpha
particles, beta particles, and
gamma rays
 The Rutherford-Bohr model of
the atom is called the planetary
atomic model because it
describes electrons in “orbit”
around the nucleus
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The EM Spectrum and Spectroscopy
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Heat, AM FM radio, infrared, visible light,
ultraviolet, microwaves, X rays, gamma rays
are all forms of electromagnetic energy
The energy emitted by gaseous atoms can be
spread into an emission spectrum
The lines missing in an absorption spectrum will
be the same as the bright lines in the emission
spectrum
Both ultraviolet and visible spectroscopy must
be used to describe the electronic structure of a
substance
The Hydrogen Atom and Quantum Theory
Planck proposed light was
radiated in small packets called
quanta or photons
 Electrons absorb or emit only
whole numbers of quanta
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Quantum Theory
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When an electron absorbs a quantum of energy
it moves to a higher energy level. When it
releases a quantum of energy it drops back to
a lower energy level
The smallest orbit an electron can occupy is its
ground state
Modern atomic theory describes electron cloud
around the nucleus, and not planets round the
nucleus
Nearly all the mass of an atom is in the nucleus
Chemists work with moles of atoms rather than
individual atoms
Atomic Mass
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1 atomic mass unit, u, is defined as 1/12
the mass of a C-12 nuclide
The average atomic mass of an element
is used in calculations
A mass spectrometer measures the
masses and amounts of the nuclides of
elements
To account for the varying masses of
isotopes, a weighted average is used to
find the average atomic mass of each
element