CH 2.4: How do Elements Form Compounds?

Chemical Bonding

CH 2.4 - How Do Elements Come Together To Form Compounds?

Key Points

• The formula of a compound shows the proportions of the elements in the compound.

• Compounds account for the huge variety of matter on Earth.

Key Points

• Ionic compounds are made of ions.

• Covalent compounds are made of molecules.

• Covalent bonding also occurs in elements and network solids.

Multiple Choice

Select the correct option that describes the composition of the compound Li₂Cr₂O₇

• 7 atoms of Chromium, 2 atoms of Lithium, and 7 atoms of Oxygen

• 2 atoms of Lithium, 2 atoms of Chromium, and 7 atoms of Oxygen

• 2 molecules of Lithium, 2 atoms of Chromium, and 7 molecules of Oxygen

Multiple Select

Select the correct proportions of each element in this compound C₁₂H₂₂O₁₁

• 22 atoms of Carbon

• 22 atoms of Hydrogen

• 12 atoms of Carbon

• 12 atoms of Oxygen

• 11 atoms of Oxygen

Multiple Choice

What is the composition of this compound: (NH₄)₃PO₄

• 1 N, 4 H, 3 P, 4 O

• 3 N, 7 H, 1 P, 4 O

• 3 N, 12 H, 1 P, 4 O

Concept 1: Compounds account for the huge variety of matter on Earth.

All the compounds that exist on Earth are built from elements

• 118 elements are on the periodic table; only 80 commonly form compounds

• 10 million known compounds; billions of possible compounds

Concept 2: Ionic compounds are made of ions

Ionic Compounds

• Ionic compound: a compound made of oppositely charged ions

• Ions are held together with ionic bonds (a strong attraction between oppositely charged ions)

• Ionic bonds are very strong

Binary ionic compounds

• Contain two elements (metal and a non-metal)

• Form when atoms of the metal element each lose one or more electrons to atoms of the non-metal element

Binary ionic compounds

• Results in the formation of ions that have full valence shells

• Stability of a full valence shell drives the formation of compounds

Example: Sodium chloride (salt)

• Sodium (metal) reacts with chlorine (gas)

• Forms when sodium atoms each transfer one electron to chlorine atoms

• Each sodium atom becomes positive ion (Na+)

• Each chlorine atom becomes a negative ion (Cl−)

Example: Sodium chloride (salt)

• Valence shells of both the sodium ion (Na+) and chlorine ion (Cl−) are full

• Recall: The stability of a full valence shell drives the formation of compounds

Structure of Ionic Compounds

• Ionic compounds consist of positive and negative ions arranged in regular repeating patterns called lattices

• Example: Sodium chloride

  ◦Sodium chloride crystals consist of sodium and chloride ions arranged in a lattice

Properties of Ionic Compounds

Ionic compounds have high melting points

• Melting requires breaking ionic bonds: the strong forces holding the ions together in the lattice structure

• A large amount of energy is required to break ionic bonds

• Example: Melting point of sodium chloride is 801°C

Ionic compounds are hard and brittle

• Ionic solids are hard because ionic bonds are very strong

• When enough force is applied, ions will shift

• Causes ions with the same charge to be close together

• Results in repulsive forces that break the solid apart

Ionic compounds conduct electric current when liquid or dissolved

• Electric current: the flow of charged particles

• Solid form: do not conduct electric current since ions are held rigidly in place 

• Dissolved or liquid form: ions are free to move, and can conduct electric current

Concept 3: Covalent compounds are made of molecules.

Covalent Bonds

• Covalent bond: a strong attraction between atoms that forms when atoms share valence electrons

Covalent compound

• Made of molecules

• Molecule: a particle made up of two or more neutral atoms bonded together by covalent bonds

Covalent Bonds: Tug of War

• Each team (atom) tries to pull the rope (shared electrons) toward itself

• Neither side wins, and the bond is the rope that connects them

Achieving Stability by Sharing Electrons

• Formation of a covalent compound is based on achieving stability with a full valence shell

• Non-metals in covalent compound share electrons to get a full valence shell

Example: Water

• Covalent bond is formed from a single pair of shared electrons

• Each hydrogen atom contributes a single electron to the shared pair of electrons

• Each oxygen atom contributes a single electron to the shared pair of electrons

Example: Water

• Hydrogen atoms achieve a full valence shell of 2 electrons

• Oxygen atom achieves a full valence shell of 8 electrons

Three Ways That Atoms Become Stable (Achieve a Full Valence Shell)

• Metal atoms can lose electrons to achieve a full valence shell

• Non-metal atoms can gain electrons to achieve a full valence shell

• Non-metal atoms can share electrons with other non-metal atoms to achieve a full valence shell

1. Metals can lose electrons to achieve a full valence shell

• Form positive ions because they lose electrons

• Retain the same number of protons in the nucleus

1. Metals can lose electrons to achieve a full valence shell

• Example:

• Group 1 metal ions have a 1+ charge because they have lost one electron

• Group 2 metal ions charge: 2+

• Group 3 metal ions charge: 3+

2. Non-metal atoms can gain electrons to achieve a full valence shell

• Form negative ions because they gain electrons

• Non-metal ions end in –ide

• Ex. Fluorine becomes Fluoride, Chlorine becomes Chloride, Sulfur becomes Sulfide

2. Non-metal atoms can gain electrons to achieve a full valence shell

• Example:

• Group 17 non-metals ion charge is 1- because they have gained one electron

• Group 16 non-metal ions charge: 2-

• Group 15 non- metal ion charge: 3-

3. Non-metal atoms can share electrons with other non-metal atoms to achieve a full valence shell

Properties of Covalent Compounds

Covalent compounds have low melting points:

• Forces holding atoms together in a molecule are strong

• Bond that attract molecules to one another are relatively weak; therefore, not as much energy is needed to “break” the weak bond melt at low temperatures

Covalent Properties

• Relatively soft: Weak forces between molecules mean that it’s easier for molecules to move and shift

• Poor conductors: covalent compounds do not have free electrons, and they are poor conductors of electric current and heat

Concept 4: Covalent bonding also occurs in elements and network solids.

Seven diatomic elements are made up of molecules held together with covalent bonds under normal conditions: 

Diatomic Elements

• H2, F2, Cl2, Br2, I2: Two atoms share one electron in a covalent bond

• O2: Two atoms share two pairs of electrons to form two covalent bonds (double bond)

• N2: Two nitrogen atoms share three pairs of electrons to form three covalent bonds (triple bond)

Network solid:

• Consists of non-metal elements containing covalent bonds that connect their atoms in one large network; essentially consist of one giant molecule 

Network Solid

• Example: Silicon dioxide (SiO2)

• Example: Diamond (C) 

• Atoms in Diamond are bonded in a regular, repeating structure by covalent bonds (B) In real Diamonds, billions of carbon atoms are bonded together in the same repeating structure, forming one giant molecule.

Carbon (diamond form – network solid):

• Each carbon atom is bonded to four other carbon atoms by covalent bonds

  Forms strong 3-D structure

Carbon (graphite form – not a network solid):

• Each carbon atom forms covalent bonds with three other carbon atoms, forming sheets

• Sheets are weakly attracted to each other and can slide around

• As you write on paper, layers of graphite slide off the pencil tip and onto the page

Polyatomic Ions

• Covalent and ionic bonds can occur together

• A molecule can gain or lose electrons to become charged, forming a polyatomic ion.

Polyatomic Ions

• Polyatomic ions form compounds like other ions.

• Example: Ammonium ion (NH4+)

• There are many types of

  polyatomic ions, but they occur

  in a few basic shapes.