CHAPTER 2
Atoms, Molecules, and Ions
History
400 -370 BC - Democritus thought that there must be
atoms, “invisible particles”.
384-322 BC – Aristotle refused this theory.
1700 – Isaac Newton again favored the idea of
smaller invisible particles.
1800 – John Dalton formed the atomic theory.
Dalton’s Atomic Theory
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Elements composed of small particles called atoms.
All atoms of the same element are identical in physical
properties but different from atoms of other elements.
Atoms of one element can not change into atoms of
different elements with chemical reactions.
Compounds are composed of atoms of different
elements and are consistent in number and type of
elements.
Dalton’s Atomic Theory
Vocabulary
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Atom – the smallest particle of an element that
retains the chemical identity of the element.
Compound – contains atoms of two or more
elements.
Conservation of matter – atoms can neither be
created nor destroyed in a chemical reaction.
Continuing History
1897 – J. J. Thomson – using a cathode ray determined the
presence of negative particles, electrons, and the “plum
pudding” model.
1911 –Ernest Rutherford – using alpha particles through gold
foil determined electrons were not evenly spaced and
determined the presence of a nucleus.
1919 – Rutherford- determined the presence of protons.
1932 – James Chadwick – determined the presence of
neutrons.
Modern Atomic Structure
Atoms consist of subatomic particles:
Particle
Charge
Mass (amu) Location
Proton
Positive
1.0073
Nucleus
Neutron
Neutral
1.0087
Nucleus
Electron
negative
5.486 x 10-4 Electron
cloud
Atomic Number
The number of protons in an atom of an element.
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Each element has a different atomic number or
number of protons.
Each element has no charge.
Each element has the same number of electrons as
protons to keep neutral.
Periodic Table of Elements
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1750 only 17 elements
1800 – 31 elements
1865 – 63 elements
Today – 117 elements
Antoine Lavoisier – categorized elements into metals,
nonmetals, gases, and earths
Dmitri Mendeleev (Russia)- 1865 – categorized 63
elements according to atomic weight along with Lothar
Meyer (Germany).
Mendeleev
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Modeled chart of elements after the
solitaire card game.
Arranged the elements into rows in the order
of increasing mass so that elements with
similar properties were in the same column
like suits in the card game.
Within columns, atomic masses increased
from top to bottom leading to the periodic
table.
Mendeleev
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Since many elements were still undiscovered, he
left gaps in the chart where he thought the
undiscovered elements should be.
The structure of the table lead to the
prediction and discovery of gallium which had
similar properties as aluminum.
1913 Rutherford’s nuclear model of the atom
lead to atomic numbers, verification of
Mendeleev’s table.
Periodic Table
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Organized by increasing atomic number.
Basic Info:
17
Cl
35.453

17 = atomic number
Cl = symbol
35.453 = atomic mass
Organization
Table is configured into:
 Periods – rows on the periodic table.
 Groups – columns on the table with elements in the
same group having similar physical properties.
Further organized into:
 Metals
 Metalloids
 Nonmetals
Organization
Metals
 good conductors of heat and electricity
 Malleable (hammered into thin sheets)
 Ductile (drawn into thin wires)
 Lustrous (shiny)
Nonmetals
 Poor conductors
 Mostly gases
 If metal then brittle
Metalloid
 Demonstrate both metal and nonmetal properties
Organization
Groups
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Group 1 – Alkali metals
Group 2 – Alkaline earth metals
Group 7 – Halogens
Group 8 – Noble gases
Group B – Transition metals
Organization
Alkali metals – very reactive with water and oxygen.
They have low densities and melting points. They all
have 1 valence electron so readily give away 1
electron in s orbital.
Ex. Sodium and potassium react violently with water
such that they will react with the water in human skin
igniting the hydrogen molecules and burn the skin.
Organization
Alkaline earth metals – have 2 valence electrons.
Differences in reactivity along these elements is
shown by the ways they react with water. More
dense and higher melting temperatures
Ex. Calcium, strontium and barium react easily with
water. Magnesium reacts with hot water. Beryllium
has no reaction in water.
Organization
Halogens – highly reactive with metals. They all
have 7 valence electrons.
Nobel gases – mostly nonreactive colorless, odorless
gas that give off different colors when excited.
Ex. Helium-pink
Neon-orange/red
Argon-lavender
Krypton-white
Xenon-blue
Atomic & Ionic Radii Trends
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Columns of periodic table – radius increases from top
to bottom.
Periods of periodic table – radius decreases from left
to right.
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Cations have smaller radii than parent elements.
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Anions have larger radii than parent elements.
Molecule
Assembly of two or more atoms tightly bound together
with no net charge.
Chemical formula - representation of the number and
type of elements in a compound.
H2O, CO2, CH4
Molecular formula– actual chemical formula of a molecule
indicating the actual number of molecules of each atom
Empirical formula – simplified formula of a molecule
indicating the smallest ratio of atoms of elements.
Writing Chemical Formulas
1.
2.
3.
Each atom (element) is represented by it
symbol.
The number of atoms of each element is
represented by a subscript.
When the number of atoms is 1 then the
subscript is not written and understood to be
one.
Chemical Formulas
Examples of formulas
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Ca3(PO4)3
Al(NO3)3
H2SO4
3CO2
Diatomic molecules
Molecules that exist is pairs
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Hydrogen
Nitrogen
Oxygen
Fluorine
Chlorine
Bromine
Iodine
Ions
A charged entity produced by taking a neutral atom
and adding or removing one or more electrons.
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Cation-positively charged particle due to the
removal of an electron.
K+ , Mg2+
Anion-negatively charged particle due to the
acceptance of an electron (tend to be nonmetals).
Cl- , S2-
Ions
How many electrons are contained in each of the
following ions?
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Ba2+
P3Sn2+
Cl-
Writing Formulas
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There must be both positive ions (cations) and
negative ions (anions) present.
The numbers of cations and anions must have a
net charge of zero or the sum of the oxidation
numbers is zero.
Cation is first then the anion.
Writing Formulas
Ex. Na and Cl
Na+
Cl→ NaCl
Charge: +1
Charge: -1
Net = 0
Oxidation number = charge
Ex. Mg and Cl
Mg2+
Charge: +2
Cl - Cl2 x (-1)
→ MgCl2
net =0
Writing Formulas
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Calcium and chlorine
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Sodium and sulfur
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Lithium and nitrogen
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Phosphorus and Calcium
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Barium and oxygen
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Sulfur and Aluminum
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Potassium and phosphorus
Writing Formulas
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Ionic Compound – a molecule that contains both a metal
and a nonmetallic elements.
NaCl
MgCl2
Molecular Compound – a molecule that contains only
nonmetals.
CO2
CH4
Polyatomic ions – atoms joined as a molecule with a
charge.
Writing Formulas
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Iron (II) and chlorine
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Cooper (I) and fluorine
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Calcium and hydroxide
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Chromium (II) and peroxide
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Hydrogen and oxygen
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Hydrogen and phosphate
Nomenclature
Naming cations:
 Metals with single charge – cation is metal name
with “ion” added
 Metal with multiple charge – cation is metal name
with charge indicated by Roman numeral and “ion”
added.
 Polyatomic – nonmetal name with “ium” replacing
end.
Nomenclature
Naming anions:
 Monatomic – element name with end replaced with
“ide” .
 Polyatomic –
 With
oxygen (oxyanion)– element name with end
replaced with “ate”.
 With hydrogen & oxyanion – polyatomic name prefix
“hydrogen” added.
Nomenclature
Naming ionic compounds: Cation is named first using
cation name then anion name.
Naming molecular compounds:
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1.
2.
3.
4.
Name element further to left first.
If same group, name higher atomic number first.
Anion is named using anion name (ide)
Atoms are indicated by following prefixes:
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One – mono
Two – di
Three – tri
Four – tetera
Five - penta
six - hexa
seven - hepta
eight - octa
nine - nona
ten - deca
Nomenclature
Naming acids:
 If anion name ends in “ide” change to “ic” and add
“hydrogen” to name. These are molecules with no
oxygen.
 If anion name ends in “ate” change to “ic” and add
“acid”.
 If anion name ends in “ite” change to “ous” and
add “acid”.
Naming Flow Chart
Is it Ionic or molecular?
Molecular
nonmetals
Ionic
Transition metal
With multiple charges?
Yes
Metal – roman numeral
Nonmetal – “ide” ending
Or polyatomic ion name
No
Metal – metal name
Nonmetal – “ide” ending
Or polyatomic ion name
Use prefixes to tell
number of each
atom present