Chemical Bonding

Compounds

• Compounds are substances made up of two or more elements in fixed proportions.– Electrically neutral (equal numbers of positive and

negative charges)– Atoms combine by gaining, losing, or sharing electrons

to form chemical bonds– Atoms achieve greater stability in bonding with other

atoms

3 Types of bonding

• Metallic– (metal + metal)

• Ionic– (metal + nonmetal)

• Covalent/molecular– (nonmetal + nonmetal)

Metallic Bonding and Compounds

Metallic Bonds

• Electrons are shared• Electrostatic (positive-

negative) attraction between cations and a “sea” of free floating valence electrons

Characteristics of Compounds

• Metal + metal• Conduct electricity

and heat• Usually solid at room

temp (range of m.p./b.p.)

• Not soluble in water• Luster is shiny• Malleable and ductile

Metallic Bonding and Compounds

• Alloy: a mixture (usually a solid) that contains two or more elements and has the characteristics of a metal

• Compositions of Selected Alloys– stainless steel 74% Fe, 18% Cr, 8%

Ni, .18% C– coinage silver 90% Ag, 10% Cu– plumber’s solder 67% Pb, 33% Sn– brass 67% Cu, 33% Zn– 18 carat gold 75% Au, 10-20% Ag, 5-15%

Cu– nichrome 60% Ni, 40% Cr

*note that the elements in an alloy are not present in specific ratios (the percentages may be adjusted)

Ionic Bonding and Compounds

Ionic Bonding• electrons are transferred• electrostatic attraction

between cation and an anion (ions)

• atoms are often less stable than ions

• metals lose electrons to form positive ions to achieve stability cations– example: sodium 

• non-metals gain electrons to form negative ions to achieve stability anions– example: chlorine

Characteristics of Compounds

• metal + nonmetal• called “salts”• solids at room

temperatures (high m.p. and b.p.)

• may dissolve in water to form strong electrolytes

• conduct electricity when molten (liquid at high temperature)

• brittle• crystalline, NOT molecules

– they form 3D crystal arrays of alternating anions and cations

Covalent Bonding and Compounds

Covalent Bonds• electrons are shared• electrostatic attraction

between electrons and nuclei

Characteristics of Compounds

• nonmetal + nonmetal• solids/liquid/gas at room

temperatures (variable m.p. and b.p.)

• may dissolve in water but doesn’t form an electrolyte

• doesn’t conduct electricity when solid or molten (liquid at high temperature)

• forms molecules

Covalent Bonding and Compounds

• unshared pair - valence electrons that are not shared in bonds

• single bond - only a single pair of electrons are shared between two atoms (see examples above)

• double bond - two pairs of electrons are shared between two atoms– examples: formaldehyde- H2CO

oxygen- O2

• triple bond - three pairs of electrons are shared between two atoms– examples: nitrogen N2

ethyne, or acetylene, C2H2

Classify each compound as:

• KCl• Brass (Cu + Zn + Sn)• CO2

• NO2

• Sterling silver (Ag + Cu)• SnF2

• CH4

• MgCl2• NH3

• LiF

M- MetallicI- IonicC- Covalent

Chemical Formulas

• Chemical formula- what elements it contains and the ratio of the atoms of those elements– Example: NaCl (sodium chloride)

Contains 1 sodium atom and 1 chloride atom

– Example: H2O (water)The formula is a combination of the symbolsH and O and the subscript number 2

Contains 2 Hydrogen atoms and 1 Oxygen atom

Chemical Formulas

• Subscript means “written below” and is written after the symbol. It tells how many atoms of that element are in one unit of the compound. If the symbol has no subscript, the unit contains only one atom of that element.

Familiar Name Chemical Name Formula

Lye Sodium Hydroxide NaOH

Ammonia Ammonia NH3

Sand Silicon Dioxide SiO2

Battery Acid Sulfuric Acid H2SO4

Octet Rule• Octet Rule: atoms tend to gain, lose, or share

electrons in order to acquire a full set of valence electrons

• Think of ionic bond formation as a process: – electrons are lost/gained to achieve a stable octet of electrons– ions form – ions brought together by electrostatic attractions.

• Lewis Dot Diagrams:• Recall that a way to show and emphasize an atom’s

valence electrons is to draw the element’s dot diagram

• Li Be N O Cl Si Ar

Empirical vs. Molecular Formulas

• empirical formula – shows the lowest whole number ratio of atoms in a

compound– always used for ionic compounds – can be useful for partially describing covalent compounds

• example: Ca2+ and F- combine to form CaF2

• molecular formula – shows the actual number of atoms in a single molecule – cannot be used for ionic compounds

• examples: sucrose- C12H22O11 and glucose- C6H12O6

• Questions: – What is the empirical formula for sucrose? – What is the empirical formula for glucose? ________