CHAPTER 20Chemical Bonds

Stability In Bonding

Roughly 110 elements on the periodic table can form a nearly infinite number of compounds

Elements bond with each other, seeking stability

Only one group of elements (Noble Gases) are stable without bonding

All others must bond with either another atom of the same element or a different element to be stable

Combined Elements

Elemental copper is a shiny metal

The Statue of Liberty is copper, why is she a pale green color?

The copper that the statue is made of reacted with oxygen and sulfur to form a compound called copper sulfate, that has its own properties

Combined Elements

Sodium (Na) is a soft, silvery metal that is highly reactive– it actually catches fire in the presence of oxygen

Chlorine (Cl) is a green poisonous gas.

Together, they make a compound that you can put on your food, table salt, NaCl

Table salt’s compound name is Sodium Chloride

Chemical Formulas

A chemical formula tells what elements are in a compound, and in what numbers

Subscripts (a number written below) show how many atoms are in each compound.

If there is no subscript, you can assume it is 1

Example, H2O, water, has two atoms of Hydrogen and one atom of oxygen

Some Familiar Compounds

Atomic Stability

An atom is considered stable if its outer shell electrons are full

This is considered an “Octet”, because 8 electrons fill the outer shell of some atoms

Outer shell electrons are called “valence” electrons and are the electrons that are involved in bonding

Dot Diagrams

Uses dots around the chemical symbol to show number of valence electrons

Dot Diagrams, Cont

Write the symbol of the element Look at the periodic table and determine

the number of outer shell, or valence electrons

Start on the top, put one dot on each side as you move clockwise Exceptions: He (helium) has a full shell at two

electrons, so put those dots on top Group 1: Alkali metals (including H) put the

one dot to the right

Stability is Reached

Atoms form chemical bonds with each other to reach chemical stability– to fill up their outer shells

They will either give away, take, or share electrons to do this

Types of Bonds

When atoms gain or lose an electron, they become an ion

A positive ion, such as Ca +2, gives away 2 electrons

A negative ion, such as Chlorine -1, gains one electron

A bond where electrons are transferred, (given or taken) is called an IONIC bond

Ionic Bonds

Ionic bond is the force of attraction between opposite charges in an ionic compound

For the most part, ionic bonds are formed by elements that are far apart on the periodic table

A metal and a non-metal Metals tend to lose electrons Non-metals tend to gain electrons Once ionic bonds are formed, the

resulting compound has a net charge of zero

Sharing Electons

Non metals, when bonding with other non metals, are unlikely to gain or lose electrons

The attraction that forms between atoms when they share electrons is called a covalent bond

A neutral particle that forms due to a covalent bond is called a molecule

Types of Covalent Bonds

Single covalent bonds share one pair of electrons, example is water H2O this is called a single bond

A multiple covalent bond shares two or three pairs of electrons, called a double or a triple bond, example N2

Covalent bonds form between non-metals Many covalent compounds are liquids or

gases at room temperature

Bond Types

Polar vs Non-Polar

Polar covalent compounds are molecules that have ends that have partial opposite charges

This means that the electrons are shared unevenly

One atom has a greater “pull” on the shared electrons than another

A non-polar compound mean that the electrons are shared evenly with no partial charges

Polar vs. Non Polar

Water is PolarCarbon Tetrachloride is non-polar

Diatomic Atoms

Some atoms are so reactant that they can’t exist as only one atom

They must bond with themselves in order to reach stability

They are called “diatomic” which literally means “two atoms”

The diatomic atoms are: Nitrogen, Oxygen, Fluorine, Chlorine, Iodine, Hydrogen, Bromine

Magic 7- they make a 7 and then add hydrogen

They are all written with a subscript: Cl2

Diatomic Atoms

Binary Ionic Compound

A binary ionic compound is one that consists of two elements bonded together where electrons are transferred

You need to know what elements are involved and how many electrons are gained or lost

The element’s OXIDATION NUMBER tells how many electrons are transferred when ions are formed

Oxidation numbers are often referred to as “charge”

Oxidation Numbers

Special Ions- have more than one oxidation number

Compounds are Neutral

When compounds are formed, their net charge must be zero

Therefore, when you are putting compounds together and writing their formulas, their oxidation numbers must equal zero

Sodium (Na +1) and Chlorine (Cl -1) together are NaCl

Magnesium (Mg +2) and Chlorine (Cl -1) together are MgCl2

It takes two chlorine atoms to cancel out the +2 charge of Mg to zero

Criss-Cross method

Naming Ionic Compounds

Step 1: Write the name of the positive ion

Step 2: Determine if the positive ion has a special oxidation number (refer to table 2.) If it does, you’ll have to determine its oxidation number and write roman numerals after the name

Step 3: Write the root of the negative ion (chlor-, ox-, phosph-)

Add –ide to the end of the root of the negative ion

Naming Ionic Compounds

NaCl Step one: Sodium Step two: not needed, it only has one

possible oxidation number Step three: Sodium Chlor- Step four: Sodium Chloride Now, you do MgI

Naming Compounds with polyatomic ions

A polyatomic ion is a special ion that is made of many atoms

Look at the chart to the right

To write the names of a special compound with polyatomic ion, you write the name of the positive ion and then the name of the polyatomic ion

Ex K2SO4 is potassium sulfate

Naming Covalent Compounds

Naming covalent compounds uses prefixes that tell how many of the atoms are present in the molecule

Often the prefix for one, mono is omitted but is used for emphasis in some cases

Naming Covalent Compounds

CO is Carbon Monoxide CCl4 is Carbon Tetrachloride H2O is Dihydrogen Monoxide

Your Turn

NO2

N2O5