Bonding

Why do Atoms Bond?• Atoms bond when their valence electrons interact

• Generally, atoms join to form bonds so that each atom has a full outermost energy level.

• Examples – Draw the Bohr model for:

Ne F O Li

2 2 228 7 6 1

Full outer level

Needs 1 e Needs 2 e Give away 1 e

Ionic Bonds

• Formed when atoms TRANSFER electrons

• This causes two oppositely charged ions to form

• What is an ion? It is an atom with a charge

• Both ions that form have a filled outermost energy level

• The result is a POSITIVE ion and a NEGATIVE ion.

Calculating Charges of Ions• Sodium normally has 11 electrons, 1 valence

electron. It gives it away. It still has 11 protons, but now has 10 electrons.

+11

-10

+1 charge

• If an atom loses an electron, its charge is +1

• If an atom loses two electrons, its charge is +2

• If an atom gains an electron, its charge is –1

• If an atom gains two electrons, its charge is -2

Examples: Lithium & FluorineLi F

First, draw the Bohr Model

Lithium has an extra electron, fluorine needs one moreLithium gives its valence electron to FluorineLi now has a +1 charge. F now has a -1 charge. Oppositely charged ions attract each other and form an

ionic bond.

+1 -1

+3

-2

+9

-10

Ionic Bonding Animation (NaCl)

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The Dog Bone AnalogyTaken from http://ithacasciencezone.com/chemzone/lessons/03bonding/dogbonds.htm

• Ionic bonding can be best imagined as one big greedy dog steeling the other dog's bone.

More Facts About Ionic Bonds

• When melted or dissolved in water, ionic compounds conduct electricity

• Elements in ionic bonds form COMPOUNDS from a METAL and a NONMETAL

• How do you determine metals vs nonmetals?– Look on your periodic table…there is a little stair

step that divides metals from nonmetals– Elements to the right of this line (including

Hydrogen!) are nonmetals– Elements to the left of this line are metals

Reactivity of Metals• Reactivity lab – think back to the reactivity lab…which

was more reactive: Al or Mg?• Mg was…it had fewer valence electrons.• In the case of a METAL, fewer valence electrons = more

reactive…but WHY?– Metals are trying to get rid of their electrons. – Electrons are attracted to protons. Farther to the left on the

periodic table, elements have fewer protons.– Fewer protons, less attraction, easier to get rid of electrons

• Predict which is more reactive, Na or K.• K…it has more energy levels.

– The farther an electrons is from the nucleus, the easier it is to give away.

• Nonmetals are OPPOSITE from metals

Covalent Bonds

• Atoms joined by covalent bonds SHARE electrons

• Covalent bonds form MOLECULES

• Made from 2 or more NONMETALS

• In the case of covalent bonds, atoms don’t transfer electrons because each needs to gain electrons

Examples of Covalent Bonds

F2

Both need 1 electron so they share one electron each

Covalent Bonding Animation (Cl2)

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Non-Polar Covalent Bonds

• When both atoms are the same, the electron is shared EQUALLY

• Atoms may share more than one pair of electrons

• Examples: O2 or Cl2

The Dog Bone Analogy

• Covalent bonds can be thought of as two or more dogs with equal attraction to the bones.  Since the dogs (atoms) are identical, then the dogs share the pairs of available bones evenly. 

Polar Covalent Bonds• When two UNLIKE atoms share

electrons, the electrons are NOT shared equally

• Electrons are attracted to elements that are located farther to the right and closer to the top of the periodic table

• The occurs because:– Farther to the right has more protons– Closer to the top has fewer energy

level

• Example: CO2

The Dog Bone Analogy

• These bonds can be thought of as two or more dogs that have different desire for bones.  The bigger dog has more strength to possess a larger portion of the bones.  Sharing still takes place but is an uneven sharing. 

Polyatomic Bonds• Two or more covalently bonded atoms form an ion

because they have either gained or lost electrons

• Examples (you do not have to memorize these). • Hydroxide OH-

• Carbonate CO32-

• Sulfate SO42-

• Nitrate NO3-

• Nitrite NO2-

• Phosphate PO43-

• Hydrogen Sulfate HSO4-

• Cyanide CN-

Review Questions• In what type of bond are electrons shared equally?

– Nonpolar covalent

• In what type of bond are electrons transferred?– ionic

• Determine if the following compounds/molecules are likely to have ionic or covalent bonds:– MgO

• Ionic (a metal and a nonmetal)

– SrCl2

• Ionic (a metal and a nonmetal)

– O3

• Covalent (nonpolar – all the same nonmetal)

– CH4O• Covalent (polar – different nonmetals)

• Explain why electrons are shared equally in ozone, O3, and unequally in carbon dioxide, CO2.