CHEMICAL BONDING

Chapter 7

THE OCTET RULE

• Atoms tend to gain, lose, or share electrons in order to get a full set of valence electrons.
• “octet” – most atoms need 8 valence electrons for a full set
• Gaining or losing → ions = ionic bonding
• Sharing = covalent bonding
• “Dogs Teaching Chemistry”
• https://www.youtube.com/watch?v=_M9khs87xQ8

IONIC BONDING

PROPERTIES OF IONIC COMPOUNDS

• High melting points
• Brittle
• Usually salts
• Many dissolve in water
• Can conduct electricity because ions separate and are charged in the solution

IONIC BONDS

• Electrons are transferred from one atom to another creating ions
• Cations are attracted to anions (positives and negatives attract)
• Metal + nonmetal
• Metals form cations
• Nonmetals form anions

TYPES OF IONS

• Monatomic = “one-atom”
• H⁺, Ca²⁺, Br^-, N^3-
• Polyatomic = “many-atoms”
• NH₄⁺, OH^-, SO₄^2-,

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LEWIS DOT STRUCTURES

• Developed by American chemist Gilbert Lewis (1875-1946)
• Valence electrons represented by dots around the element symbol
• No more than two dots per side
• Can be used to show rearrangement of electrons during chemical reactions

BINARY IONIC COMPOUNDS

• Contain ions of only two elements
• Formula: Cation written first, then anion
• Charges of ions written as superscripts, # of atoms in a compound written as subscripts
• Ratio written in lowest terms = empirical formula
• (REMEMBER THIS!)

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BINARY IONIC COMPOUNDS

• Draw the Lewis Dot Structures for sodium and chlorine
• Using an arrow, identify how the transfer of 1 electron can create 2 new ions

• Sodium transfers an electron to chlorine.
• Sodium becomes a positive ion with a +1 charge.
• Chlorine becomes a negative ion with a -1 charge.

BINARY IONIC COMPOUNDS

Na⁺ + Cl^- → NaCl

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• The total (net) charge on the compound should be zero.
• You must determine how many of each ion will need to be in the compound to balance out the charges.

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COMPOUND FORMULA PRACTICE

magnesium ion + oxide ion

Mg²⁺ + O^2- →

Mg²⁺ + O^2- → MgO

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calcium ion and bromide ion

Ca²⁺ + Br^- →

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strontium ion and nitride ion

Sr²⁺ + N^3- →

Mg₂O₂

CaBr₂

Sr₃N₂

THE CRISSCROSS METHOD FOR WRITING COMPOUND FORMULAS

• Write the ion symbols (with their charges as superscripts) for the cation and anion
• Criss-cross the two charges, moving them diagonally from one ion’s superscript to the other ion’s subscript
• Drop the sign!

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CRISSCROSS METHOD PRACTICE

magnesium ion and chloride ion

Mg²⁺ Cl^-1

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Mg with Cl =

MgCl₂

NAMING IONIC COMPOUNDS

• Name the cation using its element name.
• Name the anion by dropping the ending of the element name and adding –ide.

Ca₃P₂

calcium phosphide

• If the anion is polyatomic, simply name it using the ion’s name

Mg₃(PO₄)₂

magnesium phosphate

NAMING IONIC COMPOUNDS

• If the cation has more than one valence (it can have different charges), indicate the charge using roman numerals in parenthesis after the cation name.

FeO = iron (II) oxide

Fe₂O₃ = iron (III) oxide

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COVALENT BONDING

COVALENT BONDS

• Formed by a shared pair of electrons between two atoms
• Make up molecules (which make up molecular substances)
• Between nonmetals

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FORMULAS

• Empirical formula gives the lowest ratio of types of atoms in a compound
• Molecular formula gives the exact number of atoms of each element in a single molecule of a compound
• Structural formula shows how atoms are bonded together

FORMULA EXAMPLE: GLUCOSE

molecular formula

C₆H₁₂O₆

empirical formula

CH₂O

structural formula

LEWIS DOT STRUCTURES

• For molecules:
• Show pairs of electrons that are shared between atoms using 2 dots or 1 dash.
• Leave electrons not involved in bonds as dots.

LEWIS DOT STRUCTURES

Draw the Lewis dot structures for:

F₂

NH₃

H₂O

H₂CO

C₂H₂

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EXCEPTIONS TO �THE OCTET RULE

• Less than an octet
• BF₃
• More than an octet
• SF₄
• Odd number of electrons
• NO

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PROPERTIES OF COVALENT BONDS

• Polar covalent bonds = Unequal sharing
• Due to electronegativity difference
• More electronegative atom gets slightly negative charge (higher electron density)
• Less electronegative atom gets slightly positive charge (lower electron density)
• Nonpolar covalent bonds = equal sharing
• No electronegativity difference

PROPERTIES OF COVALENT BONDS

• Low melting points
• Soft, flexible
• Many won’t dissolve in water
• Cannot conduct electricity even if they do dissolve (due to no charges being present)

NAMING COVALENT COMPOUNDS (MOLECULES)

• Similar to naming ionic compounds, but prefixes must be added to tell the ratio of atoms in the compound.

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NAMING COVALENT COMPOUNDS (MOLECULES)

• Most electronegative element written last in formula and name.
• Drop ending of this element’s name and add

–ide.

Si₂Br₆

disilicon hexabromide

• Don’t include mono- prefix for 1^st element listed.

CF₄

carbon tetrafluoride

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NAMING COVALENT COMPOUNDS (MOLECULES)

• Shorten prefixes to make names easier to say.

H₂O

dihydrogen monoxide

not dihydrogen monooxide

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• Sometimes common names are used.

O₂ = oxygen

NH₃ = ammonia

HYDRATES

NAMING HYDRATES

• Hydrates are ionic compounds that absorb water into their solid structures.
• Anhydrous substances are water-free
• Example:

MgSO₄ • 7 H₂O