Basic Structure of the Atom

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Basic Structure of the Atom

• Evidence Supporting the Atomic Theory

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Building the Atomic Model

• Democritus (460-370 BC)
• John Dalton (1803)
• J. J. Thomson (1897)
• Ernest Rutherford (1909)
• Niels Bohr (1913)
• James Chadwick (1932)

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Democritus

• Ancient Greek
• First to propose matter was made up of tiny
  particles called atomos
• Stated atoms could not be
  created, destroyed, or further
  divided

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Dalton

• Proposed Atomic Theory in 1803
• Half a Century until well accepted
• 100 years until proven
• Each element
• Composed of indivisible and indestructible atoms
• Atoms of different elements are different
• atoms of the same element are the same
• Atoms of different elements combine to form
  compound atoms (molecules)

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J. J. Thomson

• Discovered electrons in 1897
• Plum Pudding Model Positive and negative
  charges scattered randomly throughout atom
• Instantly accepted

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Rutherford

• Gold foil experiment 1909
• Mass concentrated in very small core at
• the atoms center (nucleus)
• Nucleus positive, negative electrons moving
  around it
• Contributions
• Field of Nuclear Physics (1898, alpha and beta
  particles)
• Radioactive decay
• Rectify Periodic Table

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Bohr

• Doctoral Thesis 1911 Theory of
• Electrons
• In 1913, theorized electrons revolve
• around the nucleus in energy levels (Einstein
  and Planck)
• Energy levels closest to nucleus have low energy
• Energy levels increase in energy with distance
  from the nucleus
• Electrons gain and lose energy by moving between
  energy levels (quantum)
• This is an enormous achievement Einstein

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James Chadwick (1932)

• Identified neutron

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Modern Atomic Model

• The atom consists of three main (charged)
  particles
• Protons (positive)
• Neutrons (neutral)
• Electrons (negative)
• Two main parts
• Nucleus
• Electron cloud

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Atomic Model (cont.)

• Nucleus contains
• Protons ()
• Neutrons (0)
• Nucleus surrounded by
• electron cloud
• Negative charge due to electrons
• Mass of an atom measured in amus, or atomic mass
  units.

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Atomic Structure
12
Atomic Structure
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Atomic Model (cont.)

• Protons and neutrons are found in the nucleus, so
  it has a positive charge
• Electrons are found in the electron cloud, so it
  has a negative charge
• The mass of an atom is measured in amus, or
  atomic mass units. It is a unit of measurement
  of protons and neutrons

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Atomic Mass Units

• 1 amu is defined as 1/12 the mass of a carbon
  atom containing 6 protons and 6 neutrons
• 1 amu is also the mass of 1 proton or 1 neutron
• An electron has a mass of 1/2000 amu

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Key Terms

• Atomic number the number of protons in the
  nucleus of an atom.
• Mass number the sum of the number of protons
  AND the number of neutrons in the nucleus.
• Mass protons neutrons

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Atomic Symbol
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Isotopes

• Isotopes are atoms of the same element with
  differing numbers of neutrons.
• Isotopes have different masses

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Isotopes
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Isotopes of Carbon
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How do you record the mass of a group of isotopes?

• Because most elements have more than one isotope,
  each element is given an average atomic mass
• The average atomic mass is the average mass of
  the mixtures of its isotopes

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How do you calculate the average atomic mass of
an atom?

• The number of naturally occurring isotopes, their
  masses, and their percent abundances must be
  known.
• Example Lithium has 2 isotopes Li-6 (mass
  6.015 amu and 7.5 abundance), and Li-7 (mass
  7.017 amu and 92.5 abundance). What is its
  average atomic mass?

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How do you calculate the average atomic mass of
an atom?

• Calculate the average atomic mass of silicon.
  The three silicon atoms have masses of 27.98 amu,
  28.98 amu, and 29.97 amu with relative abundances
  of 92.23, 4.67, and 3.10, respectively.

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Charged Atoms

• In a neutral atom, the number of protons equals
  the number of electrons. The positive and
  negative charges balance out, leaving the atom
  with 0 net charge
• In a charged atom or ion, there is an uneven
  number of protons and electrons, so the atom will
  have either a positive or negative net charge

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Ions