Chapter 6
Atomic Structure and
Chemical Bonds
I. Atomic Structure
• At the center of every atom is a nucleus
containing protons and neutrons
• Electrons travel in an area of space around the
nucleus called the electron cloud
• The electron cloud is mostly empty space and the
exact location of any one electron cannot be
determined
A. Electrons
• Electrons have negative charges
• Electrons travel in predictable areas, but it
is impossible to know their exact location
B. Element structure
• Each element has a specific number of
protons, neutrons, and electrons
• The number of protons and electrons is
always the same for a neutral atom of a
given element
II. Electron Arrangement
A. Electron energy
• The different areas for an electron in an
atom are called energy levels
• Each energy level represents a different
amount of energy
B. Number of electrons
• Each energy level has a specific number of
electrons it can hold
• The farther an energy level is from the
nucleus, the more electrons it can hold
• To determine the maximum number of
electrons that can occupy an energy level, use
the formula, 2n2, where n equals the number
of the energy level
Level 1 = 2 electrons
= 2(12)
Level 2 = 8 electrons
= 2(22)
Level 3 = 18 electrons
= 2(32)
Level 4 = 32 electrons
= 2(42)
= 32 e= 18 e= 8 e= 2 e-
C. Energy steps
• Electrons in the level closest to the nucleus
have the lowest amount of energy
• Electrons farthest from the nucleus have
the highest amount of energy and are the
easiest to remove
- the closer a negatively charged electron is
to the positively charge nucleus, the more
strongly it is attracted to the nucleus
= 32 e= 18 e= 8 e= 2 e-
III. Periodic Table and Energy Levels
• The atomic number for each element is the
same as the number of protons in that
element
• The number of protons equals the number
of electrons in a neutral atom
• Therefore, the atomic number also tells you
how many electrons are in a neutral
element
IV. Electron Configuration
• Atoms with a complete outer energy level
are stable
• Atoms with exactly eight electrons in an
outer energy level are also stable
• Each period in the periodic table ends with
a stable element
V. Element Families
• Each column of the periodic table contains
one element family
• Members of element families have similar
chemical properties because they have the
same number of electrons in their outer
energy levels
A. Noble Gases
• Elements in Group 8
have eight electrons in
their outer energy level
• Because their energy
levels are stable, they
do not combine easily
with other elements
• Helium is also stable
because its single
energy level is
complete with only
two electrons
B. Halogens
• Elements in Group 7 need one
more electron to obtain a stable
outer energy level
• The easier it is for a halogen to
gain this electron, the more
reactive it is
• The reactivity of the halogens
decreases down the group as the
outer energy levels get farther
from the nucleus
- the protons cannot pull electrons
in as easily when they are
farthest from the nucleus
C. Alkali metals
• Elements in Group 1 each have one electron in
their outer most energy level
• The easier it is to remove an electron, the more
reactive the atom is
• The reactivity of the alkali metals increases down
the group as the outer energy levels get farther
from the nucleus
- less energy is needed to remove an electron
from an energy level that is farther from the
nucleus
VI. Electron Dot Diagrams
• Because it is the outermost electrons that
determine an atoms chemical properties, it
is helpful to make models that show only
those electrons
• An electron dot diagram is the symbol for
the element surrounded by as many dots as
there are electrons in its outer energy level
A. How to write them
• Dots are written in pairs on four sides of
the element symbol
• Group numbers 1 and 2 and 3-8 tell you
how many electrons are in the outer shell
for those elements
B. Using dot diagrams
• Dot diagrams help show how many
chemical bonds an atom can make
• A chemical bond is the force that holds two
atoms together
• Atoms bond with other atoms so that each
atom has a stable outer shell
- each atom likes eight electrons in its outer
shell except for hydrogen and helium (they
like two)
VII. Ionic Bonds-Loss and Gain
• Atoms form bonds with other atoms in one
of four ways:
1. losing electrons
2. gaining electrons
3. pooling electrons
4. sharing electrons with another atom
• An ionic bond forms between two atoms
when one atom losses an electron and
another atom gains an electron
A. Ions – A question of balance
• Sodium has only one electron in its outer
level
• The sodium atom loses an electron and
becomes more stable with eight electrons
in its outer level
• An ion is an atom that has gained or lost an
electron
• Sodium lost an electron, so it now has one
more proton than electron
• Sodium is now a positive ion, Na+, because
it lost an electron
• Chlorine has seven electrons in its outer level
• The chlorine atom gains one electron and
becomes more stable with eight electrons in
its outer level
• Chlorine gained an electron, so it now has one
more electron than proton
• Chlorine is now a negative ion, Cl-, because it
gained an electron
B. Bond formation
• The positive sodium ion and the negative
chloride ion are strongly attracted to each
other
• This attraction which holds the ions close
together creates an ionic bond
• A compound is a pure substance containing
two or more elements that are chemically
bonded
• The compound made by sodium chloride is
table salt (NaCl)
VIII. Metallic Bonding – Pooling
• Metals form bonds with other metals by
forming a shared pool of electrons
• Metallic bonding allows metals to conduct
electricity because the electrons are
allowed to move from one atom to the next
to transmit current
IX. Covalent Bonds – Sharing
• The chemical bond that forms between nonmetal
atoms when they share electrons
• Shared electrons are attracted to both nuclei so each
atom has a stable outer energy level some of the
time
• A molecule is the neutral particle formed when two
atoms share electrons and have an equal number of
protons and neutrons
A. Double bonds
• When two pairs of electrons are involved in
a covalent bond
ex. In carbon dioxide, CO2, the carbon atom
shares two of its electrons with each
oxygen atom
B. Triple bonds
• Three pairs of electrons are shared
between two atoms
ex. N2
X. Polar and Nonpolar Molecules
• Some electrons are not shared equally
between two atoms
• Some atoms have a greater attraction for
electrons than others do
• An unequal sharing of electrons makes one
side of the bond more negative than the
other
• A polar bond is a bond in which electrons
are shared unevenly
A. The polar water molecule
• In a water molecule, H2O, the oxygen atom
has a greater share of the electrons in each
bond
• The oxygen end of a water molecule has a
slight negative charge and the hydrogen
end has a slight positive charge
• Water is said to be polar; it has two oppositely
charged ends like poles on a magnet
• Molecules that do not have these uneven
charges are called nonpolar molecules