Chapter 2
Atoms, Molecules,
and Ions
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Dalton's Postulates
Atoms of an element are
not changed into atoms of
a different element by
chemical reactions; atoms
are neither created nor
destroyed in chemical
reactions.
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Dalton's Postulates
Compounds are formed
when atoms of more than
one element combine; a
given compound always
has the same relative
number and kind of
atoms.
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Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
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Symbols of Elements
Elements are symbolized by one or two letters.
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Symbols of Elements
All atoms of the same element have the same
number of protons, which is called the atomic
number, Z.
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Symbols of Elements
The mass of an atom in atomic mass units (amu)
is the total number of protons and neutrons in
the atom.
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Isotopes
• Isotopes are atoms of the same element with
different masses.
• Isotopes have different numbers of neutrons.
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Key Points
• The elements of The Periodic Table are
classified into three groups – metals,
nonmetals, and metalloids
• Elements in a group have similar chemical
properties
• Elements in a in a period have different
chemical properties
Metals, metalloids, and nonmetals.
Figure 2.11
Copper
Cadmium
Lead
Chromium
Bismuth
Arsenic
Silicon
Antimony
Chlorine
Bromine
Sulfur
Iodine
Boron
Tellurium
Carbon
(graphite)
Noble
Gases
The modern periodic table.
Alkali
metals
Alkaline earth metals
Transition Metals
Halogens
Figure 2.10
The modern periodic table.
0
+1
+2
+3 NC -3 -2 -1
Chemical Formulas
The subscript to the right of
the symbol of an element
tells the number of atoms
of that element in one
molecule of the compound.
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Diatomic Molecules
• These seven elements occur naturally as
molecules containing two atoms:
– Hydrogen
– Nitrogen
– Oxygen
– Fluorine
– Chlorine
– Bromine
– Iodine
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Chemical Formulas
• Molecular Formula
– It only shows the exact number of atoms of each element
in the smallest possible units of a substance
– Ex. H2O, O2, N2
• Empirical Formula
– Tells us what elements are present and the simplest whole
number ratio of their atoms
– Not necessarily the actual number of atoms in a given
molecule.
N2H4 molecular formula
NH2 empirical formula
Ionic compounds (such as NaCl) are generally
formed between metals and nonmetals.
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Chemical Formulas
Molecular (covalent or
binary) compounds are
composed of molecules and
almost always contain only
nonmetals.
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Ions
• When atoms lose or gain electrons, they become
ions.
– Cations are positive and are formed by elements on the
left side of the periodic chart (donate electrons to nonmetals)
– Anions are negative and are formed by elements on the
right side of the periodic chart (gain electrons from
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metals)
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
2.5
Writing Formulas
• Because compounds are electrically neutral, one
can determine the formula of a compound this
way:
– The charge on the cation becomes the subscript on the
anion.
– The charge on the anion becomes the subscript on the
cation.
– If these subscripts are not in the lowest whole-number
ratio, divide them by the greatest common factor.
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• The sum of the charges on the cation and the
anion have to add up to zero, thus the
compound is electronically neutral
– Ex Al2O3
• Predicting Ionic Charge
– The number of electrons an atom loses or gains is
related to its position on the periodic table.
Naming Ionic Compounds cont..
• Certain Metals especially transition metals can form
more than one type of cation
• We use the Stock System to name them
– Mn 2+
– Mn 3+
– Mn 4+
MnO (manganese (II) oxide)
Mn2O3 (manganese (III) oxide)
MnO2 (manganese (IV) oxide)
Naming Ionic Compounds cont..
• Binary compounds: made from just 2 elements
– Cation (electron donating number fixed) is named
first and the name of the element stays the same
– Cation (electron donating more than one) follow
the mentioned above rules
– Anion (electron accepting) is named second and
you add -ide
• Ex. Oxygen turns to Oxide
• Ex Nitrogen turns to Nitride
Sample Problem 2.5
PROBLEM:
SOLUTION:
Naming Binary Ionic Compounds
Name the ionic compound formed from the following pairs of
elements:
(a) magnesium and nitrogen
(b) iodine and cadmium
(c) strontium and fluorine
(d) sulfur and cesium
(a) magnesium nitride
(b) cadmium iodide
(c) strontium fluoride
(d) cesium sulfide
Sample Problem 2.6
PROBLEM:
Determining Formulas of Binary Ionic Compounds
Write empirical formulas for the compounds named in Sample
Problem 2.5.
SOLUTION:
(a) magnesium nitride
Mg2+ and N3-; three Mg2+(6+) and two N3-(6-); Mg3N2
(b) cadmium iodide
Cd2+ and I-; one Cd2+(2+) and two I-(2-); CdI2
(c) strontium fluoride
Sr2+ and F-; one Sr2+(2+) and two F-(2-); SrF2
(d) cesium sulfide
Cs+ and S2-; two Cs+(2+) and one S2- (2-); Cs2S
Naming Ionic Compounds cont..
• Ternary Compounds: made up three elements
– If cation (electron donating) has a fixed number of
electrons to give the name will stay the same and it is
named first
– If the cation (electron donating) different number of
electrons to give the name will stay the same but you will
mention the number of electrons (roman numerals) that
are being donating and it is still named first
– If the anion (electron accepting) 1 element then you will
add –ide
– If the anion (electron accepting) more than one element
refer to table 2.5 page 63, it is mentioned after the cation
Sample Problem 2.8
Determining Names and Formulas of Ionic
Compounds Containing Polyatomic Ions
PROBLEM:
Give the systematic names or the formula or the formulas for the
names of the following compounds:
(a) Fe(ClO4)2
SOLUTION:
(b) sodium sulfite
(c) Ba(OH)2 8H2O
(a) ClO4- is perchlorate; iron must have a 2+ charge. This is iron(II)
perchlorate.
(b) The anion sulfite is SO32- therefore you need 2 sodiums per sulfite.
The formula is Na2SO3.
(c) Hydroxide is OH- and barium is a 2+ ion. When water is included in
the formula, we use the term “hydrate” and a prefix which indicates the
number of waters. So it is barium hydroxide octahydrate.
Molecular Compounds
• They are usually composed of nonmetal
elements
• The chemical bond that forms between the
elements is called “covalent bond”
– Most of them are binary compounds
– Named in the same manner as ionic compounds
Naming Molecular Compounds
• The first name the element that is first in the
formula, its name does not change
• The second element in the formula is named
second and –ide is added
– HCl (hydrogen chloride)
– HBr (hydrogen bromide)
Nomenclature of Binary Compounds
• The less electronegative atom
is usually listed first.
• A prefix is used to denote the
number of atoms of each
element in the compound
(mono- is not used on the first
element listed, however) .
• The ending on the more
electronegative element is
changed to -ide.
– CO2: carbon dioxide
– CCl4: carbon tetrachloride
Nomenclature of Binary Compounds
• If the prefix ends with a or
o and the name of the
element begins with a
vowel, the two successive
vowels are often elided
into one.
N2O5: dinitrogen pentoxide
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Patterns in Oxyanion
Nomenclature
• When there are two oxyanions involving the same
element:
– The one with fewer oxygens ends in -ite.
– The one with more oxygens ends in -ate.
• NO2− : nitrite; SO32− : sulfite
• NO3− : nitrate; SO42− : sulfate
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Sample Problem 2.11
Determining Names and Formulas of Binary Covalent
Compounds
PROBLEM:
(a) What is the formula of carbon disulfide?
(b) What is the name of PCl5?
(c) Give the name and formula of the compound whose molecules
each consist of two N atoms and four O atoms.
SOLUTION:
(a) Carbon is C, sulfide is sulfur S and di-means 2 - CS2.
(b) P is phosphorous, Cl is chloride, the prefix for 5 is penta-.
Phosphorous pentachloride.
(c) N is nitrogen and is in a lower group number than O (oxygen).
Therefore the formula is N2O4 - dinitrogen tetraoxide.
Acids
• Acid: a substance that yields hydrogen ions (H +,
equivalent to a proton) when dissolved in water
– Formulas for acids contain one or more hydrogen as well
as an anionic group
– The name assigned to the compound depends on its
physical state
• Anions whose names end in “ide” form acids with a
“hydro” prefix and an “ic” ending
– HCl Hydrochloric acid
Naming Acids
Sample Problem 2.10
Determining Names and Formulas of Anions and
Acids
PROBLEM:
Name the following anions and give the names and formulas of the
acids derived from them:
(a) Br -
(b) IO3 -
(c) CN -
(d) SO4 2-
(e) NO2 -
SOLUTION:
(a) The anion is bromide; the acid is hydrobromic acid, HBr.
(b) The anion is iodate; the acid is iodic acid, HIO3.
(c) The anion is cyanide; the acid is hydrocyanic acid, HCN.
(d) The anion is sulfate; the acid is sulfuric acid, H2SO4.
(e) The anion is nitrite; the acid is nitrous acid, HNO2.