Chapter 3: Atoms & the
Periodic Table
These are uranium
atoms. What do they
look like?
What actually are you
seeing?
Section 1: Inside an Atom
• Structure of an Atom
–An atom consists of a
nucleus surrounded by one
or more electrons.
–The nucleus is the tiny,
central core of an atom
–An atom has 3 types of particles:
–(subatomic particles)
• Protons -- have a positive electric
charge & are located in the nucleus
• Neutrons -- have a neutral electric
charge & are located in the nucleus
• Electrons -- have a negative electric
charge & are located outside of the
nucleus
The atom’s nucleus contains protons
and neutrons. The high energy
electrons move in the space around the
nucleus called the electron cloud.
Atomic Number = the number of
protons in a nucleus
• Is a unique property that identifies
the element
–Ex: Every Carbon atom has 6
protons
• In an atom, the # of protons & the #
of electrons are equal, making the
atom neutral.
Atomic Mass = the average mass of
one atom of an element
• Since atoms are so small, they
are measured in atomic mass
units (amu).
–The mass of a proton or a
neutron is about one amu.
–Electrons are much smaller.
• It takes almost 2000 electrons to
equal one amu!
• Therefore, most of an atom’s mass
is in the nucleus of an atom.
• Ex: An atom that has 6 protons, 6
neutrons, and 6 electrons has a
mass of about 12 amu’s
• The number of protons in an
element does not change
• the number of neutrons can
change. (Isotopes of an atom)
–Ex: Carbon atoms always have 6
protons. But they may have 5,6,7,or
8 neutrons. (Isotopes of carbon)
• This means that the amu will vary.
• Since neutrons don’t play a role
in chemical reactions, the
chemical properties of each
element are the same despite
having different masses.
The Role of Electrons
• Electrons move around the nucleus
so fast that it is impossible to
know exactly where any electron is
at a particular time!
–It’s like a spherical cloud of negatively
charged electrons.
Imagine the
blades of a
moving fan.
Little Particles, Big Spaces
video
• The space in which electrons move is
HUGE!!!
• Ex: Imagine standing at the pitcher’s
mound in a baseball stadium. If the
nucleus were the size of a pencil
eraser, the electrons could be in the
outfield or the top row of seats!
So, what composes the
majority of an atom’s
mass?
What composes the
majority of an atom’s
volume?
Valence Electrons
• Electrons in an atom are not
all the same distance away
from the nucleus.
• Valence electrons are those
that are the farthest away.
• A chemical bond forms between 2
atoms when valence electrons
move between them.
• Valence electrons may be
transferred from one atom to
another, or they may be shared
between atoms.
• The number of valence electrons
can vary from 1 to 8.
• Each element has a typical # of
valance electrons.
–Ex: Oxygen has 6, Carbon has 4, &
Hydrogen has 1
Electron Dot Diagrams
• Used to represent valence
electrons.
• A symbol for an element is
surrounded by dots. Each dot
stands for one valence electron.
• When atoms have 8 or 0 valence
electrons, the atom becomes more
stable- or less reactive than they
were before.
Models of Atoms
• 1808- Dalton Model
–each element is made of small
atoms & different elements have
atoms of different masses
–atoms similar to tiny, solid balls
Models of Atoms
• 1897-Thomson Model
–Atom is a positively charged sphere
with electrons embedded in it.
–Similar to a muffin w/ berries
scattered through it
THOMPSON’S
MODEL
Models of Atoms
• 1904- Nagaoka Model
–Atom had a large sphere in the
center with a positive charge.
–Electrons revolved around the
sphere like planets revolve around
the sun.
Models of Atoms
• 1911-Rutherford Model
–Atom is mostly empty space.
–Electrons orbit randomly
around a small, positively
charged nucleus.
RUTHERFORD’S
MODEL
Rutherford’s Experiment - showed that particles were deflected by something in
atoms. He deduced that this was a positively charged nucleus.
Models of Atoms
• 1913-Bohr Model
–Electrons move in specific layers,
or shells
–Atoms absorb or give off energy
when the electrons move from one
shell to another
BOHR’S MODEL
Models of Atoms
• 1932-Chadwick Model
–Discovered the neutron.
–The existence of the neutron
explained why atoms were heavier
than the total mass of their protons
and electrons.
Models of Atoms
• 1920’s to Present- Modern Model
–Electrons form a negatively
charged cloud around the nucleus.
–It’s impossible to determine the
exact location of an electron
Section 2: Organizing the Elements
The Periodic Table
• Mendeleev: patterns appeared
when the elements were arranged
in order of increasing atomic mass.
• Sometimes this method didn’t
work, so he would put the
elements in a “best fit” location.
Modern Periodic Table
• Based on atomic number
(discovered in the 1900’s) rather
than atomic mass
• the properties of the elements
repeat in each period (row) of the
table
This is Henry Mosley,
a British scientist. He
created the modern
periodic table of
elements by placing
the elements in order
of their atomic
number instead of
mass. (He was only 27
years old when he was
killed in WWI.)
Reading the Periodic Table
• Each square of the table
usually includes the
element’s atomic number,
chemical symbol, name, and
atomic mass.
Element’s
Name
Atomic Number
Element’s
Symbol
Atomic Mass
An element’s properties
can be predicted from its
location in the periodic
table!
Organization of the Periodic Table
• Main body of table has 18 vertical
columns & 7 horizontal rows
• The elements in a column are
called a group
• Groups are also known as
families
Group Numbers
• The elements in each group, or
family, have similar
characteristics.
• Ex: elements in group 1A are all
metals that react violently with
water. Elements in group 18 rarely
react at all
• Each horizontal row across the
table is called a period
• Elements in a period have very
different properties
–As you move across a period
from left to right, those
properties change according to a
pattern
Why the Periodic Table Works
• Because it is based on the structure
of atoms, especially the valence
electrons.
–Elements in a family all have the
same number of valence electrons
• This is a reason why the elements in a
particular group have similar
properties
• As you move from left to right
across a period, the atomic number
increases by one
–This means that an element has
one more valence electron than
the element to its left
Section 3: Metals
• Physical properties of metals:
–hardness,
–shininess,
–malleability (can be pounded or
rolled into shapes) ,
–ductility (can be pulled out or
drawn into wires)
• Most metals are good conductors
because they transmit heat and
electricity easily
• Several metals are magnetic
(attracted to magnets)--iron (Fe),
cobalt (Co), and nickel (Ni)
• Most metals are solids at room
temperature ( they have a high
melting point)
Chemical Properties of Metals
• Metals have a wide range of
chemical properties
–Some are very reactive: sodium
(Na), potassium (K)
–Some are unreactive: gold (Au)
and chromium (Cr)
• Some metals are in the middle.
These metals react slowly with
oxygen in the air, forming metal
oxides
–Ex: Iron left unprotected will slowly
rust (turn a reddish brown)
• Process of reaction and wearing
away = Corrosion
Alloys
• Alloy = a mixture of metal
(Remember: a mixture
consists of 2 or more
substances mixed together
but not chemically changed)
• Some common alloys:
–Copper + Tin = Bronze
–Copper + Zinc = Brass
–Iron + Carbon + Chromium
+Vanadium = Stainless Steel
–Gold + Silver + Copper + a little
Zinc = Yellow Gold
–Gold + Nickel + Copper + Zinc =
White Gold
Metals in the Periodic Table
• The metals in a group, or
family, have similar
properties, and these family
properties change gradually
as you move across the table.
Alkali Metals
• Metals in Group 1- from Li to Fr
• These are the most reactive metals
since they only have 1 valence
electron
• In nature, these are never found as
elements--exist only as compounds
• As elements, they are very soft &
shiny
• Most important alkali metals are:
–Sodium (Na)
–Potassium (K)
Alkaline Earth Metals
• Group 2 =alkaline earth metals
• Not as reactive as Group 1 but they
are more reactive than most metals
• Never found uncombined in nature
• They have 2 valence electrons
• Most common alkaline earth
metals:
–magnesium (Mg) -- used to be in
flash bulbs because of the very
bright light
–calcium (Ca) --important in
bones and teeth
Transition Metals
• Elements in group 3 - 12 are the
transition metals
• These metals form a bridge
between the reactive metals on the
left to the less reactive metals on
the right
• Example of transition metals:
–iron, copper, nickel, silver, and
gold
• Most are hard and shiny
• All are good conductors of
electricity
• Are fairly stable, reacting slowly
or not at all with air and water
Metals in Mixed Groups
• Groups 13 - 16 include
metals, nonmetals, and
metalloids
• Not as reactive as the metals
in groups 3 - 12
• Ex: aluminum, tin, and lead
Lanthanides & Actinides
• At the bottom of the periodic table,
– the top row = Lanthanides
–bottom row = Actinides
• These elements are called the rare
earth elements & they fit in
Periods 6 & 7
• Lanthanides: soft, malleable, shiny metals
w/ high conductivity
• Actinides:
–only thorium and uranium exist on
Earth in significant amounts
–All the elements after uranium were
created artificially in the lab
• the nuclei of these elements are very
unstable (many last for only a fraction
of a second after they are made)
Section 4: Nonmetals &
Metalloids
• Nonmetals: elements that lack
most of the properties of metals
• The 17 nonmetals are to the right
of the zig zag on the periodic table
• In general, nonmetal physical
properties are opposites of metal
properties. Most nonmetals are:
–Dull,
–Solid nonmetals are brittle (not
malleable or ductile)
–Poor Conductors of heat &
electricicity
Chemical Properties of Nonmetals
• Most nonmetals form compounds
easily EXCEPT those from Group
18 (Noble Gases)
–Reason: group 18 doesn’t gain,
lose, or share electrons with
other elements. Why?
Compounds of Nonmetals
• When nonmetals and metals react,
valence electrons move from the
metal atoms to the nonmetal
atoms. (See pg 99)
–Table Salt = Na (metal) + Cl
(nonmetal)
• How many valence electrons does
Na (sodium) have?
• How many valence electrons does
Cl (chlorine) have?
• Are valence electrons shared or
transferred to make NaCl?
• Which element transferred the
valence electron in order to make
How many valence electrons does
Hydrogen have? Chlorine?
Are the valence electrons
transferred or shared? & by whom?
• Nonmetals can also form
compounds with other nonmetals
–The atoms share electrons and
become bonded together into
molecules.
–Diatomic molecules=molecules
have only 2 atoms
• Ex: Oxygen (O2), Nitrogen (N2), and
Hydrogen (H2)
Diatomic molecule of H2
(Hydrogen)
Families of Nonmetals
• Carbon Family (Group 14)
–Each element has 4 valence e–Carbon = nonmetal
–Silicon & Germanium =
metalloids
–Tin & Lead = metals
• Nitrogen Family (Group 15):
–Each element has 5 valence e–Nonmetals:
•N: Nitrogen gas is 80% of the
air
•P: always found in compounds
–As & Sb = metalloids
–Bi = metal
• Oxygen Family (Group 16):
–Each element has 6 valence e–Usually gain/share 2 e- when they
react
–Nonmetals:
•Oxygen: very reactive; can
combine with most elements; most
abundant element in Earth’s crust
& 2nd in the atmosphere
•Sulfur: strong, unpleasant odor;
in rubber bands, car tires,
medicine
•Selenium
–Metalloid = Tellurium
–Metal = Plonium
• Halogen Family (Group 17):
–All except Astatine are nonmetals
–All have 7 valence e–Typically gain/share 1 valence e–All are very reactive & most are
dangerous to people
• Fluorine: most reactive nonmetal;
found in nonstick cookware &
toothpaste
•Chlorine: used in table salt
(NaCl), and used to melt snow
(CaCl)
•Bromine: when with Silver
(AgBr) used in photographic
film
• Noble Gases (Group 18):
–Do not usually form compounds
since some don’t gain/lose/share
their valence e–All exist in the atmosphere but in
small amounts
–Can be found in glowing electric
lights (neon lights) but are filled
with Ar, Xe, or other noble gases
• Hydrogen:
–Simplest element (usually 1 proton
& 1 electron)
–Since chemical properties are so
different from other elements, it
can’t really be put into a family
–90+% of the atoms in universe
–1% of the mass of Earth’s crust,
oceans, and atmosphere
–Hydrogen usually found on Earth as
a compound (combined with oxygen
in water)
Metalloids
• 7 of them on the border
between metals and
nonmetals
• Have similar properties as
the metals and nonmetals
• Silicon (Si) is the most common
–when combined with Oxygen, it can
form sand, glass, and cement
• Most useful property is their
varying ability to conduct
electricity
–it would depend on temperature,
exposure to light, or impurities
• The varying ability to conduct
electricity is why these elements
are used to make semiconductors
–Semiconductors=substances that
under some conditions can carry
electricity, while under other
conditions cannot carry electricity
–Semiconductors are used to make
computer chips, transistors, & lasers
Section 5: Elements from Stardust
• Atomic Nuclei Combine
–Sun is made mostly of H which
exists at high temp(15 million
degrees Celsius) & pressure (This
state of matter = plasma)
•Plasma atoms are stripped of their
electrons, & the nuclei are packed
close together
–Usually positively charged nuclei
repel each othen stars (the
plasma state)nuclei are close
enough and moving fast enough
to collide with one another
• In Nuclear Fusion, atomic nuclei
combine to form a larger nucleus,
releasing huge amounts of energy
• Inside stars, nuclear fusion
combines smaller nuclei into larger
nuclei, thus creating heavier
elements
• Think of stars as “element
factories”
• Elements from the Sun
–When 2 hydrogen nuclei with
neutrons join together, they produce
a helium nucleus. Releasing a lot of
energy
•This reaction is a major source of
the sun’s energy
•Estimate that the sun has enough
hydrogen to last another 5 billion
years
• As helium builds up, the volume &
temp of sun’s core changes. This
allows different nuclear fusion
reactions to occur
• Stars the size of the sun do not
contain enough energy to produce
elements heavier than oxygen
• A very massive star can explode (a
supernova) providing enough
energy for nuclear fusion reactions
that create the heaviest elements.
• Theory: Matter in the sun &
planets around it originally came
from a gigantic supernova that
occurred billions of years ago