Electrons are the key to Electrons are the key to chemical bonding chemical bonding

– – specifically outer shell electrons.specifically outer shell electrons. Only outer shell electrons are Only outer shell electrons are

involved in bonding.involved in bonding.

Valence Electrons

– The outer shell is called the valence shell.– Electrons in the outer shell are called valence

electrons

Valence shell

Valence electrons

All atoms want to be “happy”

– a happy atom has a full valence shell.

It has all of the electrons it can hold!

• The first period of elements - hydrogen and helium - can hold up to 2 electrons in the outer shell.

– So to be happy, hydrogen wants to gain an electron.

I want another electron to fill my shell…

Hooray! My shell is full!

For the other periods… …the atoms want to have 8 electrons in

their outer shell.1

2

3

4 5

6

7

8

Chlorine wants to gain one electron

Potassium wants to lose one! (its easier to lose one electron than to gain seven!)

How many does Carbon want?

• Carbon can either gain or lose 4 electrons!

Why is this atom sad?

It’s outer shell is not full!

How do atoms bond?

• Atoms bond by gaining, losing, or sharing electrons!

• ….But if you gain an electron, there will be more electrons than protons!

• That’s right! The atom is no longer neutral – it now has a charge!

-An atom that has a charge is called an ion.

Because I have an extra

electron, I have a -1 charge!

• Ion – Atom with a charge formed when one or more valence electrons is gained or lost

• Ionic bond – force of attraction between oppositely charged atoms

• When an atom gains or loses electrons, it gains a charge – it becomes positive or negative.

• Positive ions are attracted to negative ions

+ -

The Chlorine atom from before…

…gained one electron to become happy. This electron gave the chlorine a negative charge

Cl-

- The potassium atom would want to lose an electron. – becoming positive!

K+

• So if a potassium and a chlorine meet, the chlorine will take the electron from the potassium, making both atoms into ions!

ChlorinePotassium

Cl-K+

• The Positive potassium is attracted to the negative chlorine – and an ionic bond is formed

Cl-K+KCl

Ionic compounds

• Ionic compounds are usually hard, brittle, and dissolve in water.

• When metals bond with nonmetals, it forms ionic compounds

• Sodium chloride (table salt) and hydrochloric acid are ionic compounds

Sodium chloride (table salt) and hydrochloric acid (stomach acid)

are ionic compounds

Na+ Cl-

NaCl

H+ Cl+

HCl

• Covalent bond – when atoms are held together by sharing electrons

• Many atoms hang onto their electrons much too tightly for them to be removed – This means they cannot form ions!

• These atoms still want to fill their outer shell, but they can’t gain or lose electrons – So what can they do?

They Share!

• Oxygen cannot pull the electrons off of hydrogen, but it still wants a full shell.

• With a little energy, hydrogen atoms will share their electrons with the oxygen atom to make it happy, and the oxygen will share one electron with each hydrogen atom!

The electrons are shared

• The hydrogen atoms now have 2 electrons each and the oxygen atoms have 8.

Water is a covalent Water is a covalent compound.compound.

Covalent compounds Covalent compounds are much more common are much more common than ionic compounds. than ionic compounds. Everything from sugar Everything from sugar to gasoline to ammonia to gasoline to ammonia is a covalent compound! is a covalent compound!

Covalent Compounds

Metallic bondingMetallic bonding

• In metallic bonding, atoms are In metallic bonding, atoms are surrounded by a “sea” of free-flowing surrounded by a “sea” of free-flowing electrons. electrons.

• They are held together by the forces They are held together by the forces within that “sea”, where a positively within that “sea”, where a positively charged metal ion is attracted to the charged metal ion is attracted to the electrons in a metal.electrons in a metal.