Chemical Bonding

Basic Atom Information

Atoms are neutralAtoms are the smallest particles of an elementElements are arranged on the table by

increasing atomic numberThe atomic number tells you the proton

numberThe mass number tells you the number of

protons and neutrons

Basic Information (continued)

The group number tells you the number of valence electrons

The period number tells you the number of energy levels

Group 1 is the most reactive group on the periodic table

Group 17 is the most reactive non-metal on the periodic table

Ionic Bonding

Occurs when electrons are transferred from one atom to another, forming two ions

The ions stay together because of electrostatic attractions (opposites attract)

Ionic bonds NEVER form moleculesThe metal becomes a cation because it loses

valence electrons. The non-metal becomes an anion because it gains valence electrons.

The Octet Rule

Atoms tend to gain, share, or lose valence electrons in order to obtain a full set of valence electrons (in most cases this equals 8)

Exceptions: Those elements where energy level 1 is the outermost energy level

Example

Na + Cl Na + Cl+ -

Properties of Ionic Compounds

Ionic compounds do not form molecules; they form a crystal lattice, which is an alternating arrangement of cations and anions

The green spheres are Na+ and the red spheres are Cl -

This is a crystal of CaCl2. Each ion is held rigidly in place by strong electrostatic forces that bond it to several oppositely charged ions. The crystal lattice is what gives ionic compounds most of their properties.

Other Ionic Properties

Normally form between metals and nonmetals, usually forms a salt

Ionic compounds have ions that form very strong bonds, which makes them hard and brittle

They have high melting points and high boiling points

When dissolved in water, the solution will conduct electricity

Names of Charged Particles

When electrons are lost, the ion has an overall positive charge and is called a cation Examples: Na+, Ca2+

When electrons are gained, the ion has an overall negative charge and is called an anion Examples: F-, S2-

The anion will attract the cation and form a bond. The anion takes on the ending –ideexample: Sodium Fluoride NaF

All ionic compounds are electrically neutral

Ionic Compounds

Writing formulas for binary ionic compoundsCompounds composed of two elements are

called binary compoundsWhen the formula is written, the charge of the cation

must be balanced by the charge of the anionThe overall charge of the ion combination must be

zeroThe cation is always written first in the formula

How to Determine the Charge on an IonAn atom is neutral because the proton

number equals the electron numberIons are not neutral. If an ion has more electrons than protons,

it is negative (anion)If an ion has more protons than electrons

it is positive (cation)

Example of How to Determine the Charge on an IonMg has 12 protons (+) and 12 total

electrons (-). Mg has 2 valence electrons. If it gives them away it now has 12 protons (+) and 10 electrons (-). There are 2 more positive charges than negative charges so the charge on the Mg ion is +2.

Example of How to Determine the Charge on an IonN has 7 protons (+) and 7 total electrons (-). N has 5 valence electrons. It is a non-

metal. It will not give these valence electrons away because the energy of ionization is too high. N will share with another non-metal or take from a metal. If it takes 3 from a metal it will now have 7 protons (+) and 10 electrons (-). There are now 3 more negative charges so the charge on the N ion is -3.

Energy of Ionization

For a metal to lose an electron and become a cation energy is required. The more electrons it needs to lose the greater the amount of energy required.

Whenever an electron is accepted by its new atom, forming an anion, energy is released.

If the energy released is greater than the energy needed to take the electron, the ionic bond easily forms.

Covalent Bonding

Covalent bonds form between non-metals that are very reluctant to give away their valence electrons

Most reactive non-metals have 5, 6, or 7 valence electrons (there are some with 4)

Too much ionization energy is required to remove these valence elctrons from the non-metals

Molecules

Covalently bonded substances contain molecules.

Molecules are particles that contain 2 or more covalently bonded atoms. These atoms may be alike or different.

Properties of Covalently Bonded SubstancesLow boiling/melting pointSoluble, but not in waterSince they do not form ions in water,

covalent substances do NOT conduct electricity

Examples: carbon dioxide, carbon monoxide

Diatomic Elements

There are 7 elements that when found pure in nature will always be covalently bonded to another one of themselves.

This covalent bond results in the formation of a molecule.

Diatomic elements: H, O, N, F, Cl, Br, IExamples of diatomic molecules: H2 Cl2

Electron Dot Diagrams

The dots around each element symbol represent the valence electrons for that element.

Electron Dot Diagrams Showing Covalent Bonds

Electron Dot Diagram Showing an Ionic Bond