Chemical Bonding

adapted from: http://www.chem4kids.com/files/atom_bonds.htmlhttp://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/atomic/differentsubrev1.shtmlhttp://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/General_Principles_of_Chemical_Bonding/Covalent_Bonds

Remember

Atomic number = number of Electrons when in a neutral state

Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells.

Electron shells determine how an atom behaves when it encounters other atoms

Electrons are placed in shells according to rules:The 1st shell can hold up to two electrons, The next 2 shells can hold 8 electrons

Atoms want to have full valence (outer) shells!

Why do elements bond?

-Think about "Happy Atoms." Most atoms want to be happy, just like you.

-Atomic shells like to be full. That's it. If you are an atom and you have a shell, you want your shell to be full.

-Some atoms have too many electrons giving them shells with a tiny numbers of electrons (1 or 2). These atoms like to give up their electrons.

-Some atoms are really close to having a full shell, but don’t have a full shell on their own. Those atoms look for other atoms who can give up an electron.

Let’s look at an example

Sodium and Magnesium both have only one or two electrons in their outer shells! That’s not happy…

-these are “extra”-atoms want to “give these

up”

Oxygen has 6 and Flourine have 7 electrons in their outer shells! That’s not happy!

-these are missing some!-these atoms want to “take

extra”

Atoms want to have full valence (outer) shells!

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons

✓C would like to

✓N would like to

✓O would like to

Gain 4 electrons

Gain 3 electrons

Gain 2 electrons

Check for Understanding1. How many electrons are in the first and second shell?2. Does beryllium want to gain or lose electrons? How many?3. Does sulfur want to gain or lose electrons? How many?

Answers:

1. First Shell: 2 Second Shell: 8

2. Beryllium would give up electrons. Could give up 2.

3. Sulfur wants to gain electrons. Could gain 2

What happens when the valence shells are not full?

If the shells are not full, chemical bonds can occur in an attempt to fill them!

There are two main types of bonds1. Ionic Bonds2. Covalent Bonds

What is an Ionic Bond?So, let’s say we've got a sodium atom that has an extra electron. We've also got a fluorine atom that is looking for one.

When they work together, they can both wind up happy! -Sodium gives up its extra electron to the flourine atom. -The sodium then has a full second shell and the fluorine (F) also has a full second shell. -When an atom gives up an electron, it becomes positive like the sodium ion (Na+). And, when an atom gets an extra electron, it becomes negatively charged like the fluorine ion (F-). This forms an ionic bond.

What creates the Ionic Bond

The attraction of opposite charges is the way they form and maintain the bond.

Any atoms in an ionic/electrovalent bond can get or give up electrons.

The positive and negative charges continue to attract each other like magnets.

IONIC BONDbond formed

between two ions by the transfer of

electrons

Characteristics of Ionic Compounds

• Occur between atoms of metals and nonmetals with very different electronegativity– hint: far apart on the periodic table

• electronegativity increases from left to right on the periodic table– Produce charged ions. Metals form positive ions; non-metals form negative ions.– Bond formed by transfer of electrons (atoms stay together due to opposite

charges)

• The oppositely charged ions are arranged in a regular way to form giant ionic lattices. Ionic compounds often form crystals as a result.

• Are conductors when in a liquid state

• Have high melting point.

• Examples; NaCl, CaCl2, K2O

Check for Understanding1. Define Ionic Bond

2. What type of elements do ionic bonds form between?

3. Explain how a chemical bond would form between sodium and chlorine.

Answers:

1. a bond that forms between two ions by the transfer of electrons

2. form between metals and non-metals

3.

Ionic bond NaCl Example – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged particles or ions.

What if neither atom wants to give up an electron?

Then a….COVALENT BOND

is formed!

In this bond molecules are formed by the sharing of

electrons

Characteristics of Covalent Bonds• Formed between nonmetallic elements of similar electronegativity.

– hint: close together on the periodic table

• Formed by sharing electron pairs

• low melting and boiling points

• Stable non-ionizing particles, they are not conductors at any state

• Types of Covalent Bonds

– Single/double/triple

– Polar and Non-polar

• Examples; O2, CO2, C2H6, H2O, SiC

Single, double, and triple bondsSome bonding creates single covalent bonds, while others make double and triple bonds

They form depending on how many electrons the atom needs to create a full valance shell and how many electrons the atoms can share.

These bonds vary in strength and create molecules/compounds with different properties.

Single bondsA single bond is when two electrons--one

pair of electrons--are shared between two atoms.

depicted by a single line between the two atoms.

Although this form of bond is weaker and has a smaller density than a double bond and a triple bond, it is the most stable

because it has a lower level of reactivity meaning less vulnerability in losing electrons to atoms that want to steal electrons.

Double BondsA Double bond is when two atoms share two

pairs of electrons with each other. depicted by two horizontal lines between

two atoms in a molecule. This type of bond is much stronger than a

single bond, but less stable; this is due to its greater amount of

reactivity compared to a single bond.

Triple BondsA triple bond is when three

pairs of electrons are shared between two atoms in a molecule.

It is the least stable out of the three general types of covalent bonds.

It is very vulnerable to electron thieves!

Polar Covalent Bondswhen electrons are shared but shared unequally

This is caused by one molecule having higher electronegativity than others (further right on table)

Examples: ● HCl and water H2O● water is a polar molecule because oxygen is more

electronegative than hydrogen, and therefore electrons are pulled closer to oxygen.

● HINT: a way to recognize is symmetry. If the molecule is asymmetric, then it will be polar.

Nonpolar Covalent Bondswhen electrons are shared equally

This occurs when they have similar electronegativities

Examples: H2, O2, Cl2

Why does polarity matter?● One reason that polarity matters in chemistry is that polar and non-polar

molecules do not mix to form a solution. ○ That is why oil and water do not mix: oil is non polar, while water is

polar.

● Another reason that chemists are concerned about polarity is that it influences several physical properties of matter, such as

○ solubility○ surface tension○ melting and boiling points.

Check for Understanding1. Explain the difference between single/double/and triple bonds

2. Explain the difference between polar and nonpolar covalent bonds

3. Explain as many differences as you can between covalent and ionic bonds

Answer: single bonds share one electron and are most stable, double bonds share two, and triple bonds share three and are the least stable.

Answer: polar do not share evenly, nonpolar share evenly