Chapter 2 ATOMS, MOLECULES, AND IONS
ATOMIC STRUCTURE AND SYMBOLISM
• The nucleus contains the majority of an atom’s mass.
• It’s made of protons and neutrons which are much heavier
than electrons.
• Protons are positive and neutrons have no charge
• Electrons are negative
• Electrons occupy almost all of an atom’s volume.
• Diameter of an atom ~ 10–10 m
• Diameter of a nucleus is 100,000 times smaller ~ 10–15 m
ATOM VS. NUCLEUS SIZE
If an atom could be expanded to the size of a football stadium,
the nucleus would be the size of a single blueberry.
(Credit middle: modification of work by “babyknight”/wikimedia commons; credit right: modification of
work by paxson woelber)
UNITS
• Atoms and subatomic particles are very small.
• Example: a carbon atom weighs less than 2 × 10–23 g.
• Small units are needed.
• Atomic mass unit (amu).
• 1 amu = 1.6605 x 10-24 g.
• Mass of a carbon-12 atom = 12 amu
PROPERTIES OF SUBATOMIC PARTICLES
• Proton
• Mass = 1.0073 amu ~ 1 amu
• Charge = +1
• Neutron
• Mass = 1.0087 amu ~ 1 amu
• Charge = 0
• Electron
• Mass = 0.00055 amu
• Charge = –1
ATOMIC NUMBER (Z)
• The number of protons in the nucleus of an atom is its
atomic number (Z).
• Its value determines the identity of the atom.
• For example, any atom that contains eleven protons is the
element sodium and has the atomic number 11, regardless of
how many neutrons or electrons it may have.
NEUTRAL ATOMS
• A neutral atom must contain the same number of positive
and negative charges. Meaning the number of protons equals
the number of electrons.
• Therefore, the atomic number also indicates the number of
electrons in a neutral atom.
MASS NUMBER (A)
• The total number of protons and neutrons in an atom is
called its mass number (A).
• The number of neutrons is therefore the difference between
the mass number and the atomic number.
• Atomic number (Z) = number of protons
• Mass number (a) = number of protons + number of neutrons
• A – z = number of neutrons
Cobalt-59 (also written Co-59) has:
• 27 protons
• 59-27 = 32 neutrons
• 27 electrons
IONS
• When the number of protons and electrons are NOT equal,
the atom is electrically charged and called an ion.
• Charge of an atom = number of protons – number of
electrons
• Atoms (and molecules) acquire charge by losing or gaining
electrons.
CATIONS AND ANIONS
• An atom that gains one or more electrons will exhibit a
negative charge and is called an anion.
• Example: a neutral oxygen atom (z = 8) has eight
electrons, and if it gains two electrons it will become an
anion with a 2− charge (8 − 10 = 2−).
• An atom that loses one or more electrons will exhibit a
positive charge and is called a cation.
• Example: a neutral sodium atom (z = 11) has 11
electrons. If this atom loses one electron, it will become a
cation with a 1+ charge (11 − 10 = 1+).
CHEMICAL SYMBOLS
• A chemical symbol is an abbreviation that we use to indicate
an element or an atom of an element.
• Some symbols are derived from the common name of the
element; others are abbreviations of the name in another
language, typically latin.
• Most symbols have one or two letters.
• Only the first letter of a chemical symbol is capitalized.
SOME COMMON ELEMENTS
Element
Symbol
Element
Symbol
aluminum
Al
iron
Fe (from ferrum)
bromine
Br
lead
Pb (from
plumbum)
calcium
Ca
magnesium
Mg
carbon
C
mercury
Hg (from
hydrargyrum)
chlorine
Cl
nitrogen
N
chromium
Cr
oxygen
O
cobalt
Co
potassium
K (from kalium)
copper
Cu (from cuprum)
silicon
Si
fluorine
F
silver
Ag (from
argentum)
gold
Au (from aurum)
sodium
Na (from natrium)
helium
He
sulfur
S
hydrogen
H
tin
Sn (from stannum)
iodine
I
zinc
Zn
ELEMENTS TO MEMORIZE
Elements to Know Symbol
Name
Elements to Know Symbol
Name
Elements to
Know
Name
Symbol
Hydrogen
H
Helium
He
Lithium
Li
Beryllium
Be
Boron
B
Carbon
C
Nitrogen
N
Oxygen
O
Fluorine
F
Neon
Ne
Sodium
Na
Magnesium
Mg
Aluminum
Al
Silicon
Si
Phosphorus
P
Sulfur
S
Chlorine
Cl
Argon
Ar
Potassium
K
Calcium
Ca
Scandium
Sc
Titanium
Ti
Vanadium
V
Chromium
Cr
Manganese
Mn
Iron
Fe
Cobalt
Co
Nickel
Ni
Copper
Cu
Zinc
Zn
Gallium
Ga
Germanium
Ge
Arsenic
As
Selenium
Se
Bromine
Br
Krypton
Kr
Silver
Ag
Iodine
I
Barium
Ba
Gold
Au
Mercury
Hg
Lead
Pb
13
ISOTOPES
• Isotopes are elements that have the same number of protons
but have a different number of neutrons.
• The symbol for a specific isotope of any element is written
by placing the mass number as a superscript to the left of the
element symbol.
• For example, magnesium exists as a mixture of three
isotopes.
•
24Mg, 25Mg,
and 26Mg
• All isotopes have 12 protons, but the number of neutrons
are different.
NUCLEAR COMPOSITIONS OF HYDROGEN
• Hydrogen exists as a mixture of three isotopes
Element
Symbol
hydrogen
1
1
Atomic
Number
Number
of
Protons
Number
of
Neutrons
Mass
(amu)
%
Natural
Abundan
ce
1
1
0
1.0078
99.989
1
1
1
2.0141
0.0115
1
1
2
3.01605
trace
H
(protium)
2
1
H
(deuteriu
m)
3
1
H
(tritium)
ATOMIC MASS
• Each proton and each neutron has a mass of ~ 1 amu.
• Each electron weighs far less.
• Therefore the atomic mass of a single atom in amu is
approximately equal to its mass number.
• However, most elements exist naturally as a mixture of two
or more isotopes.
• The periodic table lists the weighted, average mass of all the
isotopes present in a naturally occurring sample of that
element.
ATOMIC WEIGHT
average mass = å( fractional abundance ´ isotopic mass)
i
i
Isotope
Isotopic mass (amu)
Natural abundance (%)
12C
12.00000
98.93
13C
13.003355
1.07
Average mass (C) = (0.9893)(12.00000 amu) + (0.0107)(13.003355 amu)
= 11.8716 + 0.1391359
= 12.01 amu
MASS SPECTROMETRY (MS)
• The occurrence and natural abundances of isotopes can be
experimentally determined using an instrument called a mass
spectrometer.
WORKED EXAMPLE 2.3
Oxygen has three stable isotopes, 168O (99.757 percent), 178O (0.038
percent), 188O (0.205 percent), are 15.9949, 16.9991, and 17.9992
amu, respectively. Calculate the average atomic mass of oxygen.
CHEMICAL FORMULAS
• Molecular formula: A representation of a molecule or
compound which consists of the following:
•
Chemical symbols to indicate the types of atoms.
•
Subscripts after the symbol to indicate the number of
each type of atom in the molecule.
• Subscripts are used only when more than one atom of a given
type is present.
• A structural formula shows the same information as a
molecular formula but also shows how the atoms are
connected.
FOR EXAMPLE: METHANE
A methane molecule can be represented as:
• (a) a molecular formula,
• (b) a structural formula,
• (c) a ball-and-stick model, and
• (d) a space-filling model.
ELEMENTS THAT HAVE MORE THAN 1
ATOM
• Many elements consist of discrete, individual atoms.
• Some elements only exist as multiple atoms
• Diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2
• The most common form of elemental sulfur exists as S8.
CAREFUL HOW YOU WRITE HYDROGEN
The symbols H, 2H, H2, and 2H2 represent very different
entities.
THE MOLE AND MOLAR MASS
The mole is defined as the amount of a substance that contains as
many elementary entities as there are atoms in exactly 12 g of
carbon-12.
This experimentally determined number is called Avogadro’s
number (NA).
NA = 6.0221415 x 1023
We normally round this to 6.022×1023.
1 mole = 6.022×1023, just like 1 dozen = 12 or 1 gross = 144.
THE MOLE (2)
One mole each of some substances:
CONVERSION BETWEEN
MOLES AND ATOMS
Small particles such as atoms are counted using the mole.
Can interconvert moles and atoms of elements using NA
1 mole of an element = 6.022x1023 atoms of that element
# 𝒎𝒐𝒍𝒆𝒔 𝒐𝒇 𝑬𝒍𝒆𝒎𝒆𝒏𝒕 x
6.022 𝑥 1023 𝑎𝑡𝑜𝑚𝑠 𝑁𝐴
1 𝑚𝑜𝑙𝑒 𝐸𝑙𝑒𝑚𝑒𝑛𝑡
#
𝒂𝒕𝒐𝒎𝒔 𝒐𝒇 𝑬𝒍𝒆𝒎𝒆𝒏𝒕
1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡
6.022 𝑥 1023 𝑎𝑡𝑜𝑚𝑠 𝑁𝐴
x
=
=
𝑎𝑡𝑜𝑚𝑠
𝑜𝑓
𝐸𝑙𝑒𝑚𝑒𝑛𝑡
moles
𝑜𝑓
𝐸𝑙𝑒𝑚𝑒𝑛𝑡
WORKED EXAMPLE 2.4A
A typical human body contains roughly 30 moles of calcium.
Determine (a) the number of Ca atoms in 30.00 moles of calcium
and (b) the number of moles of calcium in a sample containing
1.00×1020 Ca atoms.
MOLAR MASS
The molar mass of a substance is the mass in grams of one mole of
the substance.
By definition, the mass of a mole of carbon-12 is exactly 12 g.
 Mass of 1 carbon-12 atom: exactly 12 amu
 Mass of 1 mole of carbon-12: exactly 12 g
Although molar mass specifies the mass of one mole, making the
units (g), we usually express molar masses in units of grams per
mole (g/mol) to facilitate cancellation of units in calculations.
INTERCONVERTING MASS, MOLES, AND
NUMBER OF ATOMS
Molar mass is the conversion factor from mass to moles, and vice
versa.
Avogadro’s constant converts from moles to atoms.
WORKED EXAMPLE 2.4B
Determine (a) the number of moles of C in 25.00 g of carbon, (b) the number of
moles of He in 10.50 g of helium, and (c) the number of moles of Na in 15.75 g
of sodium.
WORKED EXAMPLE 2.4C
Determine (a) the number of C atoms in 0.515 g of carbon, and (b) the mass of
helium that contains 6.89×1018 He atoms.
EXAMPLE 2.4D
• A packet of an artificial sweetener contains
40.0 mg of saccharin (C7H5NO3S)
• Given that saccharin has a molar mass of
183.18 g/mol, how many saccharin
molecules are in a 40.0-mg sample of
saccharin? How many carbon atoms are in
the same sample?