Download Atoms, Molecules and Ions Part 2

Atoms, Molecules and Ions
Chapter 2, Part 2
September 2nd, 2004
Exam #1 on Thursday, 16th of September
Chapters 1,2 and wherever we stop from
Chapter 3 on Tuesday the 14th of
September.
More History
• In 1909 Robert Millikan, at the University of
Chicago calculated the charge of the electron by
performing his famous oil-drop experiment.
• Henri Becquerel discovered accidentally, the
existence of radioactivity in 1896.
• In 1911 Ernest Rutherford performed his famous
gold foil experiment (which he later described as
shooting at a sheet of paper with a cannon and
having the shell return to you) leading to the
development of the idea of the nucleus and the
neutral atom.
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The Neutral Atom
• Neutral = No charge
• # of protons (positive charges) = # of electrons
(negative charges)
• Atoms of different elements have different
numbers of protons, neutrons and electrons.
• The number of protons determine the identity
of the element.
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Atomic Symbol
Atomic Mass Number
(A) = The sum of # of
protons and the # of
neutrons
Atomic number (Z) =
The # of protons
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C
In any neutral atom, the atomic number = # of electrons in
one atom of that element
Atomic Mass Number – Atomic number = Number of
neutrons.
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Isotopes
• Dalton, had proposed that all atoms of the same
element must have the same mass.
• One of the 2 flaws in Dalton’s theory was, atoms of
the same element can have DIFFERENT masses.
• The mass of an atom is due to the mass of the
protons and the neutrons in that atom.
• Isotopes are atoms of the same element which have
the same number of protons but different number
of neutrons.
The word isotopes is from the Greek words for “equal place”.
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Isotopes
• Isotopes have the exact same physical properties but
different chemical properties. (Some are
RADIOACTIVE and can emit particles and energy).
• All naturally occurring elements have 2 or more
isotopes.
• Isotopes exist in different amounts (called isotopic
abundance) and have different lifetimes.
• A mass spectrometer is the most direct and accurate
means of determining atomic weights and the
existence of isotopes.
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Example of Isotopes
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C
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C
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C
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Atomic Mass
• The mass of an atom is decided by the number of
protons and the number of neutrons present in the
atomic nucleus.
• Since atoms are so tiny, using an ordinary unit
such as gram or kilogram is inconvenient and the
atomic masses are usually reported in a special
unit called atomic mass unit (amu).
• One amu = 1.661 x 10-24 g
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How and Why of the amu
• Dalton had proposed an atomic scale based on the
mass of one Hydrogen atom, which he had
assigned the mass of 1. Several of his reported
masses were later proven wrong.
• The Hydrogen based scale was then replaced with
an Oxygen based scale and then by the now
accepted Carbon – 12 based scale.
• One amu = 1/12th of the mass of one carbon atom.
• The mass of one proton = 1 amu.
• The mass of one neutron = 1 amu.
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Atomic Masses and Isotopes
• The sum of the atomic masses of the protons and
the neutrons present in an atom should add up to
give an exact round number and be the same as
the atomic mass number “A”.
• The presence of isotopes makes matters slightly
complicated.
• Each isotope of an element is present in certain
percent abundance, which has to be accounted for
when it comes to calculating the atomic masses.
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Importance of % Abundance
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Using % abundance
• The % abundance is factored in the calculation
for finding the atomic mass of any element.
• Sample Exercise 2.4 shows how to make use of
this idea.
• Consider the following example:
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Isotopes of Neon
• Neon has 3 naturally occurring 1. Change each % value to a
isotopes. The masses and
fraction.
natural abundances of these
isotopes are as follows.
Calculate the atomic mass of
Neon.
1 19.99 amu 90.48 %
2 20.99 amu 0.27 %
3 21.99 amu 9.25 %
2. Multiply the isotopic masses by
these fractions and then add the
results together.
Atomic mass of Neon = (Mass of
isotope #1 x fraction of #1) +
(Mass of isotope #2 x fraction of
#2) + (Mass of isotope #3 x
fraction of #3)
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Thus, the average atomic mass of Neon is 20.18
amu
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Compounds, Molecules & Ions
Compounds
Molecular
Ionic
Molecules composed of 2
or more elements
Cations with
positive charges
Anions with
negative charges
There are more than 19 million registered compounds with no end
in sight as to how many more will be prepared.
Each compound can be identified and distinguished by its
characteristic properties.
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The Periodic Table
• A systematic arrangement of elements according to increasing atomic numbers.
Comic book periodic table (http://www.uky.edu/Projects/Chemcomics/)
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The Periodic Table
• A systematic arrangement of elements according to increasing atomic numbers.
• The vertical columns are known as groups.
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The Periodic Table
• The horizontal rows are known as periods.
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Metals, Metalloids and Non-metals
• Metals:
– Solids at room temperature
(except Hg).
– Hard, shiny.
– Good conductors of
electricity and heat.
– Malleable (from Latin for a
hammer).
– Ductile.
– High melting point and
density.
– Typically located on the left
side of the periodic table.
– Metals will typically
combine with non-metals to
form ionic compounds.
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Metalloids
• Have properties
that are in between
the metals and
non-metals.
• Certain metalloids
are used as the raw
materials for the
manufacture of
semiconductor
devices.
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Non-metals
• Solids, liquids (Br) or
gases.
• Dull, non-malleable
and non-ductile.
• Low melting points
and densities.
• Poor conductors of
heat and electricity.
• Located on the right
of the periodic table.
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Elements that exist as diatomic molecules
• There are 7 known elements that exist as diatomic
molecules:
H2, F2, O2, N2, Cl2, Br2, I2
• When any of these elements are mentioned it is
assumed that they exist as diatomic molecules
unless specified otherwise.
• 2 other elements that are commonly polyatomic are Phosphorus (P4) and Sulfur (S8)
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Chemical Formulas
• Show the symbols and the ratios in which the atoms of the
elements in a compound combine.
• Do not show the arrangement of atoms in the compound
(structural formulas).
• If the formula contains one atom of an element the symbol
of the element represents the atom. The number 1 is not
used as a subscript. The formula of water is H2O, not
H2O1.
• When the formula contains more than one atom of an
element the number of atoms is indicated by a subscript
written to the right of the atom. The number 2 to the right
of the H indicates 2 atoms of hydrogen present in H2O.
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