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Chapter: Atomic Structure
and Chemical Bonds
Table of Contents
Section 1: Why do atoms combine?
Section 2: How Elements Bond
Atomic Structures
Why do atoms combine?
1
• The nucleus, containing protons and
neutrons, is at the center of an atom and is
surrounded by the electron cloud, an
area of space around the nucleus where
electrons travel.
Atomic Structures
Why do atoms combine?
1
– Electrons have a negative charge and
do not travel in well-defined orbits.
• Each element has a different atomic structure consisting of a particular number of protons, neutrons, and electrons.
Electrons
Why do atoms combine?
1
Element Structure
Why do atoms combine?
1
• The number and arrangement of electrons
in the electron cloud determines the
physical and chemical properties of the
element.
Electron Arrangement—Electron Energy
Why do atoms combine?
1
• Electrons are
arranged in different
energy levels at
different distances
from the nucleus.
Number of Electrons
Why do atoms combine?
1
• The farther an energy level is from the
nucleus, the more electrons it can hold
• 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.
• The closer a negatively charged electron is to the positively charged nucleus, the more strongly it is attracted to the nucleus. Therefore, removing electrons that are close to the nucleus takes more energy than removing those that are farther away from the nucleus.
Energy Steps
Why do atoms combine?
1
• Data from the periodic table can be used to understand energy levels.
Periodic Table and Energy Levels
Why do atoms combine?
1
Periodic Table and Energy Levels
Why do atoms combine?
1
• The atomic number is the same as the number of protons (or electrons) in an electrically neutral atom.
Electron Configurations
Why do atoms combine?
1
• The number of electrons in an element’s outermost energy level increases from left to right across a period.
Fig. 5, p. 467
• The first energy level is complete with two electrons, and the first period has two elements.
Electron Configurations
Why do atoms combine?
1
Electron Configurations
Why do atoms combine?
1
• Therefore, helium’s outer energy level is complete.
Electron Configurations
Why do atoms combine?
1
• Atoms with a complete outer energy level are stable. Therefore, helium is stable.
Electron Configurations
Why do atoms combine?
1
• The last element in each of the other
periods has eight electrons in its outer
energy level.
Element Families
Why do atoms combine?
1
• Each column in the periodic table contains
one element family or group with similar
chemical properties.
Noble Gases
Why do atoms combine?
1
• The noble gases in
Group 18 do not
combine easily
with other
elements because
their energy levels
are stable.
Halogens
Why do atoms combine?
1
• The halogens have
7 electrons in their
outer energy
levels; reactivity of
the halogens
decreases down
the group.
• Fluoine is the most
reactive.
Halogens
Why do atoms combine?
1
• The reactivity of the halogens decreases down the group as the outer energy levels of each element’s atoms get farther from the nucleus.
Alkali Metals
Why do atoms combine?
1
• The alkali metals in
Group 1 have one
electron in their
outer energy level;
reactivity of the
alkali metals
increases down the
group.
Electron Dot Diagrams
Why do atoms combine?
1
• An electron dot diagram is the symbol for
the element surrounded by as many dots
as there are electron in its outer energy
level.
• Electrons in the outer energy level are
called valence electrons.
• Dots are written on four sides of the
element symbol.
• One dot represents a single electron.
• Paired electrons are represented by two
dots.
• The dots are written in pairs on four sides of the element symbol.
How to Write Them
Why do atoms combine?
1
• Start by writing one dot on the top of the element symbol, then work your way around adding dots to the right, bottom, and left.
How to Write Them
Why do atoms combine?
1
• Add a fifth dot to the top to make a pair. Continue in this manner until you reach eight dots to complete the level.
Using Dot Diagrams
Why do atoms combine?
1
• A chemical bond is the force that holds
two atoms together.
– Electron dot diagrams can be used to
show how atoms bond with each other.
– Atoms bond with other atoms so that
each atom has a stable energy level.
1
Section Check
Question 1
Electrons are now known to swarm around the
nucleus of an atom in a configuration known as
the _______.
A. electron circle
B. electron cloud
C. electron configuration
D. electron swarm
1
Section Check
Answer
The answer is B. The “cloud” includes all the
regions where an electron might be found.
1
Section Check
Answer
This image is an example of an electron dot
diagram. It tells you that nitrogen contains five
electrons in its outer energy level.
1
Section Check
Question 3
The _______ an energy level is from the
nucleus, the _______ electrons it can hold.
A. closer, more
B. closer, less
C. farther, less
D. farther, more
1
Section Check
Answer
The answer is D. The farthest shells contain the
greatest number of electrons.
Ionic Bonds—Loss and Gain
How Elements Bond
2
• Atoms form bonds by losing electrons, by
gaining electrons, by pooling electrons, or
by sharing electrons.
Ionic Bonds—Loss and Gain
How Elements Bond
2
• An atom that is no longer neutral because
it has lost or gained an electron is called
an ion.
– Cations are positive
– Anions are negative
• An ionic bond forms when positive and
negative ions attract each other.
Ionic Bonds—Loss and Gain
How Elements Bond
2
• Sodium chloride is formed from
sodium ions and chloride ions.
• Sodium has one valence electron
and wants to lose one electron to
become stable.
• Chlorine has seven valence
electrons and wants to gain one
electron to become stable.
Bond Formation
How Elements Bond
2
• Two or more elements that are chemically
bonded form a compound.
More Gains and Losses
• Some atoms can gain or lose more than one electron when they form ions.
How Elements Bond
2
• Magnesium can lose these two electrons and achieve a completed energy level.
More Gains and Losses
• Some atoms, such as oxygen, need to gain two electrons to achieve stability.
How Elements Bond
2
• The two electrons released by one magnesium atom could be gained by a single atom of oxygen.
• When this happens, magnesium oxide (MgO) is formed.
Metallic Bonding—Pooling• In a metal, the electrons in the outer energy
levels of the atoms are not held tightly to individual atoms.
How Elements Bond
2
• Instead, they move freely among all the ions in the metal, forming a shared pool of electrons.
Metallic Bonding—Pooling
How Elements Bond
2
• Metallic bonds form when metal atoms
share their pooled electrons.
The Convalent Bond
How Elements Bond
2
• A covalent bond forms between atoms that
share electrons.
The Convalent Bond
How Elements Bond
2
• Atoms sharing electrons form a neutral
particle called a molecule.
The Convalent Bond
How Elements Bond
2
• Covalently bonded compounds are called molecular compounds.
• No electrons are gained or lost.
Double and Triple Bonds
How Elements Bond
2
• Two pairs of electrons are involved in a
double bond; three pairs are in a triple
bond.
Polar and Nonpolar Molecules
How Elements Bond
2
• A polar bond occurs when electrons are
unevenly shared.
The Polar Water Molecule
How Elements Bond
2
• Polar molecules, such as water, have two
opposite ends, or poles, like a magnet.
The Polar Water Molecule
How Elements Bond
2
• Nonpolar bonds form between atoms of
the same element.
Chemical Shorthand• Alchemists used elaborate symbols to
describe elements and processes. Modern chemical symbols are letters that can be understood all over the world.
How Elements Bond
2
Symbols for Atoms
How Elements Bond
2
• Symbols are used to represent atoms and
compounds.
– Elements are represented by a one,
two, or three letter symbol
Symbols for Compounds
How Elements Bond
2
• Compounds are described using element
symbols and numbers.
• In the formula H2, the small 2 after the H is
called a subscript, and indicates the
number of atoms of hydrogen that are in
the molecule.
Chemical Formulas
How Elements Bond
2
• A chemical formula is a combination of
chemical symbols and numbers.
– It tells which elements are present and
how many atoms of each element are
present.
– No subscript means that one atom of
that element is present.
Ex: Sugar (C12H22O11)
• C12 – 12 atoms of carbon
• H22 – 22 atoms of hydrogen
• O11 – 11 atoms of oxygen
Ex: Water (H2O)
• H2 – 2 atoms of hydrogen
• 1 atom of oxygen
2
Section Check
Question 1
What is the term for an atom which has lost or
gained an electron and therefore has a net
charge?
A. compound
B. ion
C. molecule
D. polar compound
2
Section Check
Answer
The answer is B. When this happens to an
atom, we say it has become “ionized.”
2
Section Check
Question 2
When two or more elements are chemically
bonded to form a substance, that substance is
called a _______.
2
Section Check
Answer
A compound is a pure substance containing
two or more elements that are chemically
bonded. An example of a compound is salt, or
sodium chloride.
2
Section Check
Question 3
A _______ bond occurs when atoms of
nonmetals share electrons.
A. covalent bond
B. ionic bond
C. metallic bond
D. polar bond
2
Section Check
Answer
The answer is A. When this type of bonding
happens between metal atoms it is called
pooling.
