Looking for Patterns in
Chemical Reactivity
Elements and Compounds
An element is a pure substance that
cannon be broken down into simpler
substances by ordinary means such as a
chemical reaction. (Iron, Oxygen, and
Carbon are all elements.)
A compound is a pure substance made
up of two or more elements that are
chemically combined (Water, or H2O, is a
compound.)
The Periodic Table of Elements
Atoms and Subatomic Particles
All atoms are composed of protons,
neutrons, and electrons.
Protons are positively charged particles
found in the nucleus of an atom.
Neutrons are uncharged particles that are
also found in the nucleus.
Electrons are negatively charged particles
that orbit the nucleus of an atom.
Atoms and Subatomic Particles
Protons and Neutrons are about the same
size and mass.
Electrons are much smaller than Protons
and Neutrons.
Protons and Electrons have charges that
are equal in size but opposite in sign.
Some Notation
Atomic Number and Atomic Mass
Atomic number is the number of protons
and electrons in each atom of that element
The atomic mass is the number of protons
and neutrons in an atom (these make up
the nucleus of an atom)
By subtracting the atomic number from the
atomic mass you can find the number of
neutrons
Isotopes
An isotope of an element has the same
atomic number but different mass
numbers because the number of neutrons
has changed.
For example: we can have Carbon-12 with
6 protons and 6 neutrons but we can also
have Carbon-14 with 6 protons and 8
neutrons
Metals, Non-metals, and Metalloids
Properties of Metals, Non-metals, and
Metalloids
Malleable – able to be pounded or formed into a shape
Ductile – able to be drawn into a wire
Organization of the Periodic Table
The periodic table arranged elements into
periods (horizontal rows) and groups
(vertical columns) according to their
atomic numbers.
Elements in the same group have similar
chemical properties.
Electron Shells
Electron Shell Patterns
Period 1 elements have a single electron
shell, while Period 2 elements have two
electron shells and Period 3 elements
have 3 electron shells.
The first electron shell can hold two
electrons while the second and third shells
can each hold eight electrons.
Elements in the same group have the
same number of electrons in their
outermost shell.
Predicting Chemical Reactivity
Electron shell diagrams are useful
because they show the number of
electrons in the outer shells.
Knowing this is the key to understanding
the organization of the periodic table and
the chemical properties of elements.
Elements in the same group have similar
chemical properties because they have
the same number of electrons in their
outer shell.
Families Within the Periodic Table
Valence Electrons
Knowing the number of outer shell
electrons will help us predict the formation
of compounds, name the compounds, and
write their chemical formulas.
The outer shell is called the valence shell
and the electrons that occupy it are called
valence electrons.
A chemical bond forms between two
atoms when their valence electrons form a
stable arrangement together.
Valence Electrons
 The chemical properties of elements are related
to the energy changes that take place when their
atoms lose, gain, or share electrons to obtain a
filled valence shell.
 Metals are elements that tend to lose their
valence electrons relatively easily and this
accounts for many of their physical and chemical
properties.
 One important property of valence shells is that
they generally like to be full.
Ions
Any atom or group of atoms that carries an
electrical charge is called an ion.
When a neutral atom gives up one or more
electrons, the positively charged ion that
results is called a cation.
HINT: cation – think t or + for positive
When a neutral atom gains one or more
electrons, the negatively charged ion that
results is called an anion.
HINT: anion – think n for negative
Cations
 Adding energy to a neutral sodium atom removes its one
valence electron and leaves a positively charged sodium
ion (cation) and a free electron.
Anions
 Adding an electron to a neutral chlorine atom results in a
negatively charged chlorine ion (anion) and the release
of energy.
Electron Dot Diagrams
Electron dot diagrams depict valence
electrons as dots.
The inner electrons and the atomic nuclei
are represented by the symbol for the
element and dots are placed one at a time
moving around the symbol.