Download Basic Structure of the Atom

Basic Structure of
the Atom
Evidence Supporting the Atomic Theory
Building the Atomic Model
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Democritus (460-370 BC)
John Dalton (1803)
J. J. Thomson (1897)
Ernest Rutherford (1909)
Niels Bohr (1913)
James Chadwick (1932)
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
Dalton
 Proposed Atomic Theory in 1803
 Half a Century until well accepted
 100 years until proven
 Each element :
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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)
J. J. Thomson
 Discovered electrons in 1897
 Plum Pudding Model – Positive and
negative charges scattered randomly
throughout atom
 Instantly accepted
Rutherford
 Gold foil experiment 1909 –
Mass concentrated in very small core at
the atom’s center (nucleus)
 Nucleus positive, negative electrons moving
around it
 Contributions:
 Field of Nuclear Physics (1898, alpha and beta
particles)
 Radioactive decay
 Rectify Periodic Table
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
James Chadwick (1932)
 Identified neutron
Modern Atomic Model
 The atom consists of three main
(charged) particles:
Protons (positive)
Neutrons (neutral)
Electrons (negative)
 Two main parts:
 Nucleus
 Electron cloud
Atomic Model (cont.)
 Nucleus contains:
 Protons (+)
 Neutrons (0)
 Nucleus surrounded by:
 electron cloud
 Negative charge due to electrons
 Mass of an atom measured in amu’s, or
atomic mass units.
Atomic Structure
Atomic Structure
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
amu’s, or atomic mass units. It is a unit
of measurement of protons and neutrons
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
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
Atomic Symbol
Isotopes
 Isotopes are atoms of the same element
with differing numbers of neutrons.
 Isotopes have different masses
Isotopes
Isotopes of Carbon
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
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?
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.
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
Ions