Chemistry
PHYSICAL SCIENCE HONORS
Chemical Bonds and Electrons
● A chemical bond forms
when atoms transfer or
share electrons.
● A covalent bond is
formed when atoms
share electrons.
Chemical Formula
● A molecule’s chemical formula tells you the ratio of
atoms of each element in the compound.
Ionic Bonds
● Not all compounds are made of molecules.
● Ionic bonds are bonds in which electrons are transferred
from one atom to another
● Sodium and chlorine form an ionic bond
because the positive sodium ion is attracted
to the negative chloride ion.
Why Chemical Bonds Form
● It takes energy to separate
atoms that are bonded
together.
● The same energy is released
when chemical bonds form.
● Atoms form bonds to reach
a lower energy state.
Reactivity
● In chemistry, reactive means an element readily
forms chemical bonds, often releasing energy.
● Some elements are more reactive than others.
● The closer an element is to having the same number
of electrons as a noble gas, the more reactive the
element is.
Valence Electrons
● Chemical bonds are formed only between the electrons in
the highest unfilled energy level.
● These electrons are called valence electrons.
Valence Electrons and Bonding
Valence electrons and the periodic table
● Going from left to right across a period each new element
has one more valence electron than the one before it.
● How many valence electrons does nitrogen
have?
Valence electrons and the periodic table
● Oxygen combines with one beryllium atom because
beryllium can supply two valence electrons to give
oxygen its preferred number of 8.
Valence electrons and the periodic table
● Carbon has four valence electrons.
● Two oxygen atoms can bond with a single carbon atom,
each oxygen sharing two of carbon’s four valence electrons.
● The bonds in carbon dioxide (CO2) are double bonds
because each bond involves 2 electrons.
Lewis dot diagrams
● A clever way to keep track
of valence electrons is to
draw Lewis dot diagrams.
● A dot diagram shows the
element symbol surrounded
by one to eight dots
representing the valence
electrons
● What is the dot
structure for nitrogen?
Chemical Formulas and Oxidation Numbers
● All compounds have an electrical charge of zero (they
are neutral).
● An oxidation number indicates the charge on the
atom (or ion) when electrons are lost, gained, or
shared in chemical bonds.
Oxidation Numbers
● A sodium atom always
ionizes to become Na+ (a
charge of +1) when it
combines with other atoms
to make a compound.
● Therefore, we say that
sodium has an oxidation
number of 1+.
● What is chlorine’s
oxidation number?
Oxidation
Ionic Bonds
● On the periodic table, strong electron donors are the
left side (alkali metals).
● Strong electron acceptors are on the right side
(halogens).
● The further apart two elements are on the periodic
table, the more likely they are to form an ionic
compound.
Covalent Bonds
● Covalent compounds form when elements have
roughly equal tendency to accept electrons.
● Elements that are both nonmetals and therefore
close together on the periodic table tend to form
covalent compounds
Oxidation numbers and chemical formulas
● Remember, the oxidation numbers for all the atoms
in a compound must add up to zero.
Oxidation numbers
● Some periodic tables list
multiple oxidation
numbers for most
elements.
● This is because more
complex bonding is
possible.
Solving Problems
● Iron and oxygen combine to form a compound. Iron
(Fe) has an oxidation number of 3+. Oxygen (O) has
an oxidation number of 2–.
● Predict the chemical formula of this compound.
Solution
Looking for:
1.
○
…formula for a binary compound
Given
2.
○
… Fe3+ and O2–
Relationships:
3.
○
Write the subscripts so that the sum of the oxidation
numbers equals zero.
Solution
4.
○
○
Two iron atoms = 2 × (3+) = 6+
Three oxygen atoms = 3 × (2–) = 6–
Polyatomic Ions
● Compounds can contain more than two elements.
● Some of these types of compounds contain
polyatomic ions.
● A polyatomic ion has more than one type of atom.
● The prefix poly means “many.”
Some Polyatomic Ions
● Al3+ combines with sulfate (SO4)2– to make
aluminum sulfate.
● Write the chemical formula for aluminum sulfate.
Looking for:
1.
○
…formula for a ternary compound
Given
2.
○
… Al3+ and SO42–
Relationships:
3.
○
Write the subscripts so that the sum of the oxidation
numbers equals zero.
Solution
4.
○
○
Two aluminum ions = 2 × (3+) = 6+
Three sulfate ions = 3 × (2–) = 6–