Atomic Structure-1

1
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

2
Daltons Atomic Theory

• Daltons 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.

3
Atomic Structure-2

• 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

4
Avogadro Number

• 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

5
Isotopes and Atomic Number

• 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

6
Nuclear Structure

• 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
• Leptonselementary particles electrons,
  neutrino, muon, tau
• Hadronsmade of quarks baryons3 different
  quarks neutrons and protons mesonsquarkantiqua
  rk
• Quarks are held together by exchanging gluons.
  Nucleons are held together by exchanging pions

7
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

8
The EM Spectrum and Spectroscopy

• 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

9
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

10
Quantum Theory

• 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

11
Atomic Mass

• 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