Chapter 1: Atomic Structure and the
Periodic Table
1.1 Atoms are the smallest form of elements.
• All matter is made of atoms. About 100
different atoms, or elements, make up
everything on Earth.
Hydrogen is the most abundant element
in the universe, it makes up 90% of all
elements in the universe.
Oxygen is the most abundant element in
the human body and in Earth's crust.
• The structure of an atom
• Nucleus: is at the center of the atom, and is a
combination of protons and neutrons.
Protons: are positively charged particles.
Neutrons: are particles that have NO
electrical charge.
Protons and neutrons have approximately
the same mass.
Electrons: are tiny particles with a
negative charge.
o They form a cloud, because at the same time
that they repel each other, they are attracted to
the positively charged protons in the nucleus.
(2000 times smaller than protons and
neutrons)
What part of the atom has a positive
charge, negative charge and no charge?
What is the smallest sub-atomic particle?
• Atomic Number: is the number of protons in
an atom’s nucleus.
• Atomic mass number: is the combined
number of protons and neutrons in a nucleus.
• Isotopes: are atoms of an element that have a
different number of neutrons.
For example, most carbon atoms contain 6
protons and 6 neutrons. Some carbon
atoms, however, have 6 protons and 8
neutrons.
Chemists will often put the atomic mass
number after the element's name, such as
carbon-12 or carbon-14, to signify
different isotopes.
How would we write these isotopes? What is
the difference between each isotope?
Atomic Mass: of an element is the average
mass of all the element’s isotopes.
• Ions are atoms that have lost or gained one or
more electrons.
They no longer contain equal numbers of
protons and electrons, so they have an
electric charge.
Atoms that have lost electrons are
positive ions; atoms that have gained
electrons are negative ions.
1.2 Elements make up the periodic table.
• Mendeleev’s Periodic Table
A Russian chemist, Dmitri Mendeleev,
was the first to produce a periodic table
of the elements.
His periodic table was organized by
increasing atomic mass and similarities in
chemical properties.
The Na atom is now Na+ because it has one more positive proton than negative electrons.
Sodium ion (Na+) 11 protons & 10 electrons
Sodium atom (Na) 11 protons & 11 electrons
An electron is lost
An electron is gained
Chlorine atom (Cl) 17 protons & 17 electrons
Chlorine atom (Cl-) 17 protons & 18 electrons
The Cl atom is now Cl-
because it has one more negative electron than positive proton.
• The periodic table has a system of
organization.
The number at the top is the atomic
number.
The big bold letters are the chemical
symbol
The name of the element is below the
chemical symbol.
Under the name is the atomic mass
number.
• The periodic table is organized into groups
and periods.
In a group (vertical column) of the
periodic table, elements show similarities
in their physical and chemical properties.
In a period (horizontal row) of the
periodic table, the physical and chemical
properties change in a predictable way
from one to the next, and the atomic
number increases by one across a period.
How is the modern periodic table different
then Mendeleev’s?
• An element's position in the periodic table
dictates what type of ion it will form.
Atoms at the left of the table tend to lose
an electron to form positive ions
Atoms at the right of the table tend to
gain electrons to form negative ions.
Atoms in group 18 rarely form ions at all.
Atoms in groups 3-12 all form positive
ions.
• Atomic size and density show patterns in
the periodic table.
Density generally increases from the top
of a group to the bottom of a group.
Elements to the left and right sides of the
periodic table are less dense than the
elements in the middle of the table.
Atomic size increases, or gets larger, from
top to bottom.
Atomic size decreases, or gets smaller,
from left to right.
1.3 The periodic table is a map of elements.
• The periodic table has distinct regions
Metals (Yellow)
Nonmetals (Green)
Metalloids (Purple)
• The element's position on the periodic table
determines how reactive it is
Reactivity is a measure of how likely an
element is to undergo a chemical
change.
• Most elements are metals.
Metals are elements that conduct
electricity and heat well and have a
shiny appearance.
• The most reactive metals in the periodic
table.
Alkali metals in Group 1 (most reactive)
Alkaline earth metals in Group 2
(more reactive than most other metal
elements, but not as reactive as those in
group 1)
• Transition metals in the middle of the
periodic table, like iron and copper, are less
reactive and were among the first known
elements.
• Rare earth elements are the elements in
period 6, the portion taken out of the body of
the periodic table.
• Nonmetals
Most nonmetals are gases at room
temperature.
In the solid state, nonmetals have dull
surfaces and can be hammered into
shape or drawn into a wire.
Halogens in Group 17 are very reactive
nonmetals.
The noble gases in Group 18 almost
never react with other elements.
• Metalloids
Have properties of both metals and
nonmetals.
Metalloids make up the semiconductors
in electronic devices.
The metalloid silicon is the second most
abundant element in the Earth's crust.
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