Dalton’s Atomic Theory
• Elements are composed of tiny particles called atoms.
• All atoms of a given element are identical.
– All carbon atoms have the same chemical and physical
properties.
• Atoms of a given element are different from those
of any other element.
– Carbon atoms have different chemical and physical
properties than sulfur atoms.
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Symbols for the Elements
• Symbolized with one or two letters
• First letter is capitalized, second is small
Carbon: C
Bromine: Br
• Common elements and symbols in Table 4.3
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Elements
• Robert Boyle (1627-1691)
• Elements: cannot be broken down into atwo or
more simpler substances
• 155 elements: 88 natural
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Dalton’s Atomic Theory
• Atoms of different elements combine to form
compounds.
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Dalton’s Atomic Theory
• Law of Constant Composition: all samples of a
compound contain the same proportions (by mass) of
the elements that form the compound.
• Atoms are indivisible by chemical processes.
– All atoms present at beginning are present at the end.
– Atoms are not created or destroyed, just rearranged in
chemical reactions.
– Atoms of one element cannot change into atoms of
another element.
• Cannot turn lead into gold by a chemical reaction
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Writing Formulas of Compounds
• Each element is represented by its symbol.
• The number of each type of atom is indicated by a
subscript written to the right of the element
symbol.
• If only one atom is present, do not include a
subscript.
• If polyatomic groups are present in the molecule,
they are written inside parentheses if more than
one group is present.
• Examples: H2O, NaCl, KNO3, Mg(NO3)2
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J. J. Thomson’s “plumb pudding” model of
the atom (1910)
• Atom contains positive particles that balance the
negative charge of the electrons.
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Rutherford’s Gold Foil Experiment
• www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf
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Rutherford’s Nuclear Model (1911)
• The atom contains a tiny dense center called
the nucleus.
• The nucleus is essentially the entire mass
of the atom.
• The nucleus is positively charged.
– It is composed of protons (positive charge) and
neutrons (no charge)
– balances the negative charge of the electrons.
• The electrons move around in the empty space
of the atom surrounding the nucleus.
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The Modern Atom (cont.)
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Components of an Atom
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The Modern Atom
• Atoms are composed of three main pieces:
protons, neutrons, and electrons.
• The nucleus contains protons and neutrons.
the hydrogen atom (H)
the helium atom (He)
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Isotopes
• All atoms of an element have the same
number of protons.
• The number of protons in an atom of a given
element is the same as the atomic number.
– Found on the periodic table
• Atoms of an element with different numbers
of neutrons are called isotopes.
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Isotopes (cont.)
• All isotopes of an element are chemically identical.
– Undergo the exact same chemical reactions
• Isotopes of an element have different masses.
• Isotopes are identified by their mass numbers.
– Mass number = # of protons + # of neutrons
• Isotope symbols:
A
ZX
X = the symbol of the element
A = the mass number
Z = the atomic number (number of protons)
A - Z = # of neutrons
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Isotope Examples
Symbol
1
1H
Number of
Protons
1
Number of
Electrons
1
Number of
Neutrons
0
2
1H
1
1
1
3
1H
1
1
2
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Other Examples
Symbol
34
16
54
26
14
7
Number of
Protons
Number of
Electrons
Number of
Neutrons
S
Cr
N
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The Modern Periodic Table
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Properties of Metals
•
•
•
•
•
Good conductors of heat and electricity
Malleability (thin sheets)
Ductility (wires)
Shiny
All are solids at room temperature except
mercury (Hg)
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Properties of Nonmetals
• Poor conductors
• Not malleable or ductile
• Exist in various physical states:
solids – carbon, phosphorus
liquids – bromine
gases – hydrogen, oxygen, helium
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Metalloids
• Show a mixture of metallic and nonmetallic
properties
• Examples:
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Problem
Give the symbol and classify as a metal,
nonmetal or metalloid:
• Silver
• Sulfur
• Silicon
• Barium
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Natural States of Elements
• Most elements are solids at room temperature.
• Group 8 Noble gases:
• Diatomic molecules:
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Diatomic Molecules
Element
Elemental State (25oC)
Molecule
hydrogen
colorless gas
H2
nitrogen
colorless gas
N2
oxygen
pale blue gas
O2
fluorine
pale yellow gas
F2
chlorine
pale green gas
Cl2
bromine
reddish brown liquid
Br2
iodine
purple solid
I2
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Allotropes of Carbon
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Ions
• Cations: ions that have a positive charge
– Form when an atom loses electrons
• Anions: ions that have a negative charge
– Form when an atom gains electrons
• Cations and anions attract each other.
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Atomic Structures of Ions
• Metals form cations.
• For each positive charge the ion has 1 less
electron than the neutral atom.
– Na = 11 e-, Na+ = 10 e– Ca = 20 e-, Ca+2 = 18 e-
• Cations are named the same as the metal.
sodium
Na  Na+ + 1e-
sodium ion
calcium
Ca  Ca+2 + 2e-
calcium ion
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Atomic Structures of Ions (cont.)
• Nonmetals form anions.
• For each negative charge the ion has 1 more
electron than the neutral atom.
– F = 9 e-, F- = 10 e– P = 15 e-, P3- = 18 e-
• Anions are named by changing the ending
of the name to –ide.
– Fluorine
– Oxygen
F + 1e-  FO + 2e-  O2-
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fluoride ion
oxide ion
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Ionic Charges and Compounds
• Charges on the ions of some elements can be predicted
from the periodic table.
• Cations and anions usually form simultaneously to yield an
ionic compound.
• When ions are formed the number of protons and neutrons
does not change.
• There must be a net charge of zero.
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Ionic Charges and Compounds (cont.)
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Formulas of Ionic Compounds
• The formula of an ionic compound can be
determined by balancing the positive charge of the
cation(s) with the negative charge of the anion(s)
to yield a net charge of zero.
•
•
•
•
Ba2+ and ClK+ and P3Ca2+ and O2Mg2+ and N3-
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