Atoms and Elements
Fundamental Particles
Dalton’s Atomic Theory
In Dalton’s Atomic Theory, atoms
• are tiny particles of matter.
• of an element are similar and
•
•
different from other elements.
of two or more different elements
combine to form compounds.
are rearranged to form new
combinations in a chemical reaction.
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Law of Multiple Proportions
MultiProportionsMov800x600.MOV
Subatomic Particles
Atoms contains subatomic particles,
• protons have a positive (+) charge.
• electrons have a negative (-) charge.
• like charges repel and unlike charges
•
attract.
neutrons are neutral.
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Rutherford’s Gold-Foil Experiment
In Rutherford’s gold-foil experiment, positively
charged particles
•
were aimed at atoms of gold.
•
mostly went straight through the atoms.
•
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged
nucleus in the atom that deflects positive particles
that come close.
Rutherford’s Gold-Foil Experiment
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The Nuclear Atom: Atoms Have Parts
An English physicist named J. J. Thomson (1856–1940)
discovered a smaller and more fundamental particle
called the electron.
Thomson discovered:
Electrons are negatively charged.
Electrons are much smaller and lighter than atoms.
Electrons are uniformly present in many different kinds
of substances.
He proposed that atoms must contain positive charge
that balanced the negative charge of electrons.
Plum pudding model of the atom: In the model
suggested by J. J. Thomson, negatively charged
electrons (yellow) were held in a sphere of positive
charge (red).
Structure of the Atom
An atom consists
• of a nucleus that
contains protons
and neutrons.
• of electrons in a
large empty space
around the nucleus.
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Atomic Masses
An atomic mass unit (amu) is defined as
exactly one-twelfth the mass of a carbon-12
atom
1 u = 1.66054 × 10–24 g
The atomic mass of an element is the
weighted average of the masses of the
naturally occurring isotopes of that element
EOS
Atomic Mass Scale
On the atomic mass scale for subatomic particles,
• 1 atomic mass unit (amu) has a mass equal to 1/12 of the
mass of the carbon-12 atom.
• a proton has a mass of about 1 (1.007) amu.
• a neutron has a mass of about 1 (1.008) amu.
• an electron has a very small mass, 0.000549 amu.
Particles in the Atom
The periodic table of the elements lists all known
elements according to their atomic numbers.
Atomic Number
The atomic number
• is specific for each element.
• is the same for all atoms of an element.
• is equal to the number of protons in an atom.
• appears above the symbol of an element.
Atomic Number
Symbol
11
Na
Atomic Number and Protons
Examples of atomic number and number of protons:
• Hydrogen has atomic number 1; every H atom has one
proton.
• Carbon has atomic number 6; every C atom has six
protons.
• Copper has atomic number 29; every Cu atom has 29
protons.
• Gold has atomic number 79; every Au atom has 79
protons.
Comprehension Check
State the number of protons in each.
A. A nitrogen atom
1) 5 protons
2) 7 protons
3) 14 protons
B. A sulfur atom
1) 32 protons
2) 16 protons
3) 6 protons
C. A barium atom
1) 137 protons
2) 81 protons
3) 56 protons
Solution
State the number of protons in each.
A. A nitrogen atom
2) atomic number 7; 7 protons
B. A sulfur atom
2) atomic number 16; 16 protons
C. A barium atom,
3) atomic number 56; 56 protons
Electrons in An Atom
An atom of
• an element is electrically neutral; the net charge of an
atom is zero.
• has an equal number of protons and electrons.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net charge
is zero.
13 protons (13+) + 13 electrons (13 -) = 0
Mass Number
The mass number
• represents the number of particles in the nucleus.
• is equal to the number of protons + the number of
neutrons.
Atomic Models
Comprehension Check
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B. How many neutrons are in the zinc atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom that has
37 neutrons?
1) 37
2) 65
3) 67
Solution
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
(atomic number 30)
B. How many neutrons are in the zinc atom?
2) 35
(65 – 30 = 35)
C. What is the mass number of a zinc atom that has 37
neutrons?
3) 67
(30 + 37 = 67)
Comprehension Check
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14.
2) 16.
3) 34.
2) 16.
3) 34.
B. Its mass number is
1) 14.
C. The element is
1) Si.
2) Ca. 3) Se.
Solution
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
(14 + 20 = 34)
C. The element is
1) Si
(Atomic number 14)
Isotopes
Isotopes
• are atoms of the same element that have different mass
numbers.
• have the same number of protons, but different numbers
of neutrons.
Isotopes:
When the Number of Neutrons Varies
All atoms of a given element have the same
number of protons.
They do not necessarily have the same
number of neutrons.
Atoms with the same number of protons but
different numbers of neutrons are called
isotopes.
All elements have their own unique percent
natural abundance of isotopes.
Isotopes: Isotope Symbols
A second notation for isotopes is the chemical symbol (or chemical
name) followed by a hyphen and the mass number of the isotope.
In this notation, the neon isotopes are:
Ne-20
neon-20
Ne-21
neon-21
Ne-22
neon-22
Atomic Mass: The Average Mass of an
Element’s Atoms
The atomic mass of each element listed in the
periodic table represents the average mass of
the atoms that compose that element.
Naturally occurring chlorine consists of
75.77% chlorine-35 (mass 34.97 amu) and
24.23% chlorine-37 (mass 36.97 amu).
Its atomic mass is:
Atomic Mass: The Average Mass of an
Element’s Atoms
In general, atomic mass is calculated according to
the following equation:
Atomic mass =
(Fraction of isotope 1 × Mass of isotope 1) +
(Fraction of isotope 2 × Mass of isotope 2) +
(Fraction of isotope 3 × Mass of isotope 3) + …
where the fractions of each isotope are the percent
natural abundances converted to their decimal
values.
EXAMPLE - Calculating Atomic Mass
Gallium has two naturally occurring
isotopes: Ga-69, with mass 68.9256 amu
and a natural abundance of 60.11%, and Ga71, with mass 70.9247 amu and a natural
abundance of 39.89%. Calculate the atomic
mass of gallium.
EXAMPLE - Calculating Atomic Mass
Convert the percent natural abundances into decimal
form by dividing by 100.
Solution:
Fraction Ga-69 = 60.11 = 0.6011
100
Fraction Ga-71 = 39.89 = 0.3989
100
EXAMPLE- Calculating Atomic Mass
Use the fractional abundances and the atomic masses of
the isotopes to compute the atomic mass according to
the atomic mass definition given earlier.
Atomic mass = (0.6011 × 68.9256 amu) + (0.3989 × 70.9247 amu)
= 41.4321 amu + 28.2919 amu
= 69.7231 = 69.72 amu
Isotopes in the Environment
The nuclei of some isotopes of a given
element are not stable.
These atoms emit a few energetic subatomic
particles from their nuclei and change into
different isotopes of different elements.
The emitted subatomic particles are called
nuclear radiation.
The isotopes that emit them are termed
radioactive.
Isotopes in the Environment
Nuclear radiation can be harmful to humans and
other living organisms because the energetic
particles interact with and damage biological
molecules.
Some isotopes, such as Pb-185, emit significant
amounts of radiation only for a very short time.
Other isotopes, such as Pu-239, remain radioactive
for a long time—thousands, millions, or even
billions of years.
Isotopes in the Environment
Radioactive isotopes are not always harmful.
Many have beneficial uses.
For example, technetium-99 (Tc-99) is often
given to patients to diagnose disease.
The radiation emitted by Tc-99 helps doctors
image internal organs or detect infection.
Nuclear Symbol
A nuclear symbol
• represents a particular atom of an element.
• gives the mass number in the upper left corner and the
atomic number in the lower left corner.
Example: An atom of sodium with atomic number 11
and a mass number 23 has the following atomic symbol:
mass number
23
Na
atomic number
11
Information from Nuclear Symbols
From the nuclear symbol, we can determine the number of
protons (p+), neutrons, (n), and electrons (e-) in a particular
atom.
16
8
31
O
8 p+
8n
8 e-
P
15
15 p+
16 n
15 e-
65
Zn
30
30 p+
35 n
30 e-
Comprehension Check
Naturally occurring carbon consists of three isotopes, 12C,
13C, and 14C. State the number of protons, neutrons, and
electrons in each of the following.
protons
12C
13C
14C
6
6
6
______
______
______
neutrons ______
______
______
electrons
______
______
______
Solution
12C
13C
14C
6
6
6
6 p+
6 p+
6 p+
neutrons 6 n
7n
8n
6 e-
6 e-
6 e-
protons
electrons
Comprehension Check
Write the nuclear symbols for atoms with the following
subatomic particles.
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e-
___________
Solution
A. 8 p+, 8 n, 8 e-
16O
8
B. 17p+, 20 n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Isotopes of Magnesium
In naturally occurring magnesium,
there are three
isotopes.
Isotopes
of Mg
24Mg
12
25Mg
12
26Mg
12
Atomic Mass
The atomic mass of an element
• is listed below the symbol of each element on
the periodic table.
• gives the mass of an “average” atom of each
element compared to 12C.
• is not the same as the mass number.
• 1 u = 1.66054 × 10–24 g
Na
22.99
Isotopes of Some Elements and
Their
Atomic
Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
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Atomic Mass for Cl
The atomic mass of chlorine is
• due to all the Cl isotopes.
• not a whole number.
• the average of two isotopes:
35Cl and 37Cl.
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Comprehension Check
Using the periodic table, specify the atomic mass of
each element.
A. calcium
__________
B. aluminum
__________
C. lead
__________
D. barium
__________
E. iron
__________
Solution
Using the periodic table, specify the atomic mass of
each element:
A. calcium
40.08 amu
B. aluminum
26.98 amu
C. lead
207.2 amu
D. barium
137.3 amu
E. iron
55.85 amu
Groups and Periods
On the periodic table,
• elements are arranged according to similar properties.
• groups contain elements with similar properties in vertical
columns.
• periods are horizontal rows of elements.
Groups and Periods
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Periodic Table
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Group Numbers
Group Numbers
• use the letter A for the representative elements (1A to 8A)
and the letter B for the transition elements.
• also use numbers 1-18 to the columns from left to right.
Names of Some Representative
Elements
Several groups of representative elements are known by
common names.
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Alkali Metals
Group 1A(1), the alkali metals, includes lithium,
sodium, and potassium.
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Halogens
Group 7A(17) the
halogens, includes
chlorine, bromine,
and iodine.
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Comprehension Check
Identify the element described by the following.
A. Group 7A(17), Period 4
1) Br
2) Cl
3) Mn
B. Group 2A(2), Period 3
1) beryllium
2) boron
3) magnesium
C. Group 5A(15), Period 2
1) phosphorus
2) arsenic
3) nitrogen
Solution
A. Group 7A (17), Period 4
1) Br
B. Group 2A (2), Period 3
3) magnesium
C. Group 5A(15), Period 2
3) nitrogen
Metals, Nonmetals, and Metalloids
The heavy zigzag line
separates metals and
nonmetals.
• Metals are located to the
left.
• Nonmetals are located to
the right.
• Metalloids are located
along the heavy zigzag
line between the metals
and nonmetals.
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Properties of Metals, Nonmetals,
and Metalloids
Metals
• are shiny and ductile.
• are good conductors of heat and electricity.
Nonmetals
• are dull, brittle, and poor conductors.
• are good insulators.
Metalloids
• are better conductors than nonmetals, but not as
good as metals.
• are used as semiconductors and insulators.
Comprehension Check
Identify each of the following elements as
1) metal
2) nonmetal
3) metalloid
A. sodium
B. chlorine
C. silicon
D. iron
E. carbon
____
____
____
____
____
Solution
Identify each of the following elements as
1) metal
2) nonmetal
3) metalloid
A. sodium
B. chlorine
C. silicon
D. iron
E. carbon
1 metal
2 nonmetal
3 metalloid
1 metal
2 nonmetal
Comprehension Check
Match the elements to the description.
A. Metals in Group 4A(14)
1) Sn, Pb
2) C, Si
3) C, Si, Ge, Sn
B. Nonmetals in Group 5A(15)
1) As, Sb, Bi
2) N, P 3) N, P, As, Sb
C. Metalloids in Group 4A(14)
1) C, Si, Ge,
2) Si, Ge
3) Si, Ge, Sn, Pb
Solution
Match the elements to the description.
A. Metals in Group 4A (14)
1) Sn, Pb
B. Nonmetals in Group 5A(15)
2) N, P
C. Metalloids in Group 4A(14)
2) Si, Ge