Atoms and the Periodic Table
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A pure substance that cannot be broken
down into simpler substances by a chemical
reaction
Identified by a one- or two-letter symbol
Arranged in the periodic table
Its location on the periodic table indicates a
lot about its chemical properties
Can be a metal, nonmetal, or a metalloid
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Some of the
element symbols
will be familiar to
you already
It will take time
and practice to
learn others
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Metals- located to the left of the stair step
line (top located near Boron)
◦ Shiny
◦ Good conductors of heat and electricity
◦ Solid at room temperature except for mercury
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Nonmetals – except for hydrogen (H) these
are located to the right of the stair step line
(top located near Boron)
◦ Do not have a shiny appearance
◦ Generally poor conductors of heat and electricity
◦ Usually a solid or gas at room temperature
(exception: Br2 is a liquid at room temperature)
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Metalloids – 7 elements located along the
stair step line (top located near Boron)
◦ Properties in between metals and nonmetals
◦ Metallic shine but brittle
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Boron (B)
Silicon (Si)
Germanium (Ge)
Arsenic (As)
Antimony (Sb)
Tellurium (Te)
astatine (At)
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A pure substance formed by chemically
combining two or more elements
A chemical formula for a compound consists
of:
◦ Element symbols to show the identity of the
elements that form a compound
◦ Subscripts to show the ratio of atoms in the
compound
H2O
2 H atoms 1 O atom
C3H8
3 C atoms
8 H atoms
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Can be drawn many ways but we are always
making a simplified representation of reality
O
H
H
Line
Lewis Dot Diagram
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All mater is composed of building blocks
called atoms
Atoms are composed of three subatomic
particles
◦ Proton
◦ Neutron
◦ Electron
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The mass of an atom is very small since each
proton and neutron has a mass of 1.6726 X
10-24
Chemists use the atomic mass unit (amu) to
measure the mass of an element
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The nucleus is the dense core that contains the
protons and neutrons
Most of the mass of an atom is in the nucleus
Electron cloud is composed of electrons that are
in almost empty space around nucleus
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Opposite charges attract each other
Like charges repel each other
Protons and electrons attract each other but
electrons repel each other
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The number of protons in the nucleus of an
atom is its atomic number
◦ Every atom of a given element has the same
number of protons in the nucleus
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We can symbolize the number of protons with
the generic letter Z
A neutral atom has no overall chare so:
◦ Z= number of protons = electrons
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Example: Lithium
3
Li
Atomic number (Z) is
the number of protons
in the nucleus.
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Atoms of the same element that have a
different number of neutrons
◦ The number of protons remains constant
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Different forms of the same element
The mass number (A) represents the number
of protons plus the number of neutrons
◦ A = Z + number of neutrons
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Example – two forms of carbon:
◦ C-12 has 6 neutrons
◦ C-14 has 8 neutrons
◦ Z = 6 for both forms of carbon
Mass number (A)
35
Atomic number (Z)
17
# of protons
Cl
=
17
# of electrons =
17
# of neutrons
A – Z = 35 – 17 = 18
=
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The weighted average of the masses of the
naturally occurring isotopes of a particular
element reported in atomic mass units (amu)
◦ This information can be found on the periodic table
6
C
12.01
atomic number (Z)
element symbol
atomic weight (amu)
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The weighted average of the ass of the
naturally occurring isotopes
Example
What is the atomic weight of chlorine?
Step [1]
List each isotope, it’s mass in atomic
mass units, and it’s abundance in nature.
Isotope
Mass (amu)
Isotopic Abundance
Cl-35
34.97
75.78% = 0.7578
Cl-37
36.97
24.22% = 0.2422
Step [2]
Multiply the isotopic abundance by the mass
of each isotope, and add up the products.
The sum is the atomic weight of the element.
34.97 x 0.7578 =
26.5003 amu
36.97 x 0.2422 =
8.9541 amu
35.4544 amu =
4 sig. figs.
35.45 amu
Answer
4 sig. figs.
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Periodic table was put together over time as
the elements were discovered, isolated, and
studied.
Many versions exist but the most common is
based on the one developed by Dmitri
Mendeleev in 1869
A row in the periodic table is called a period
◦ Elements in the same row are similar in size
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A column in the periodic table is called a
group
◦ Elements in the same group have similar electronic
and chemical properties
1A
2A
B
B
7A
8A
Alkali Metals
Alkaline
Earth Metals
Transition
Metals
Lanthanide
& Actinide
Halogens
Nobel Gases
Very
reactive
Reactive
Metals
except for
H
+1 ions
React with
Oxygen to
form
compounds
that
dissolve
into
alkaline
solutions in
water
+2 ions
Metals
Form ions
with several
Oxygen
different
compounds charges
are strongly (oxidation
alkaline
states)
Many are
not water
soluble
Tend to
Reactive
form +2
and +3 ions Form
diatomic
Lanthanides molecules
58 – 71
in
elemental
Actinides
state
90 – 103
-1 ions
Actinides
are
Salts with
radioactive alkali
metals
Inert
Heavier
elements
have limited
reactivity
Do not
form ions
Monoatomi
c gases
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Carbon’s ability to join with itself and other
elements gives it a versatility not seen with
any other element in the periodic table
Elemental forms of carbon include the
following carbon-only structures:
diamond
graphite
buckminsterfullerene
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“the party animal of the atomic world,
latching onto many other atoms (including
itself) and holding tight, forming molecular
conga lines of hearty robustness-the very
trick of nature necessary to build proteins
and DNA”
◦ Bill Bryson from A Short History of Nearly Everything
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The chemical properties of an element are
determined by the number of electrons in an
atom
◦ Electrons do not move freely in space – restricted to
a region with a particular energy
◦ Electrons occupy discrete energy levels that are
restricted to specific values – the energy is
“quantized”
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Electrons are confined to regions called the
principal energy levels or shells
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The shells are numbers, n=1, 2, 3, 4…
Moving out from the nucleus
Electrons closer to the nucleus are held more
tightly are lower in energy
Electrons farther from the nucleus are held
less tightly and are higher in energy
The farther a shell is from the nucleus, the
larger its volume, and the more electrons it
can hold
Nucleus
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Each shell is divided into subshells made up
of orbitals
◦ These are identified as s, p, d, f
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Each orbital can hold two electrons
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The maximum number of electrons that can
occupy a shell is determined by the number
of orbitals in a shell
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The electron configuration shows how the
electrons are arranged in an atom’s orbitals
The ground state is the lowest energy
arrangement
The outtermost shell is the valance shell
The electrons in the valence shell are called
valence electrons
The chemical properties of an element
depend on the number of electrons in the
valence shell
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Rule 1:
◦ Electrons are placed in the lowest energy orbital
beginning with the 1s orbital.
◦ Orbitals are then filled in order of increasing
energy.
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Rule 2:
◦ Each orbital holds a maximum of 2 electrons
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Rule 3:
◦ 1 electron is added to each orbital until all of
orbitals are half-filled
◦ Then, the orbitals can be completely filled
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What would the electron configuration for
fluorine (F) be?
◦ Atomic Number = 9 so it has nine electrons
◦ Shells fill: 1s22s22p5
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How many valence electrons does it have?
◦ There are 7 electrons in the 2nd shell so there are 7
valence electrons
Be
Cl
Atomic number: 4
Atomic number: 17
1s22s2
1s22s22p63s23p5
valence shell: n = 2
valence shell: n = 3
# of
valence electrons = 2
# of
valence electrons = 7
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Elements in the same group have similar
electron configurations.
Elements in the same group have the same
number of valence electrons.
The group number, 1A–8A, equals the
number of valence electrons for the main
group elements.
The exception is He, which has only 2 valence
electrons.
The chemical properties of a group are
therefore
very similar.
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Dots representing valence electrons are placed
on the four sides of an element symbol
Each dot represents one valence electron
For 1 to 4 valence electrons, single dots are used
With more than 4 valence electrons the dots are
paired
H
C
O
Cl
# of Valence electrons:
1
4
6
7
Electron-dot symbol:
H
C
O
Cl
Element:
Increases
The size of atoms
increases down a
column, as the
valence e− are
farther from the
nucleus.
Decreases
•The size of atoms decreases across a row, as
the number of protons in the nucleus increases,
pulling the valence electrons in closer.
The ionization energy is the energy needed to remove
an electron from a neutral atom.
Na + energy
Na+ + e–
Decreases
Increases
•Ionization energies
decrease down a
column as the
valence e− get
farther away from
the positively
charged nucleus.
•Ionization energies increase across a row as the
number of protons in the nucleus increases.