5.2 Chemical Bonding and Formulas

Why Bonding Appears on Praxis 5442

Domain II asks teachers to connect electron behavior to ionic, covalent, and metallic bonding, read simple chemical formulas, interpret electron-dot and structural models, name common compounds, and predict ions released when ionic solids dissolve. Expect items that name a formula (MgO, SO₂, O₂) and ask for bond type, or that show a methane model and ask what the dots or lines represent.

Three Bond Types at a Glance

Atoms bond to achieve more stable electron arrangements—often described in middle school as completing a valence shell (octet idea for many main-group elements; hydrogen “duet”).

Teaching language that earns credit: ionic = transfer + electrostatic attraction; covalent = sharing within molecules; metallic = delocalized electrons explaining conductivity and malleability.

Bond-type checks the exam loves

• MgO (magnesium oxide): magnesium (metal) + oxygen (nonmetal) → ionic. Mg tends to lose two electrons (Mg²⁺); O tends to gain two (O²⁻). The solid is a lattice of ions, not discrete Mg–O molecules in the salt sense.
• SO₂ (sulfur dioxide): sulfur + oxygen, both nonmetals → covalent (molecular compound with shared electron pairs).
• O₂ (oxygen gas): two identical nonmetal atoms share electrons → covalent (specifically a diatomic molecular element with a double bond in the usual Lewis picture).

If the stem lists only nonmetals, lean covalent. If it pairs a metal from the left/center of the periodic table with a nonmetal, lean ionic. Pure metals or alloys → metallic.

Formulas and What They Communicate

A chemical formula states the types of atoms and their ratio in a compound or molecule:

• MgO → 1 Mg : 1 O (empirical formula of the ionic compound)
• CaCl₂ → 1 Ca : 2 Cl
• CH₄ → 1 C : 4 H in each methane molecule
• Na₂SO₄ → 2 Na : 1 SO₄ group (sodium sulfate)

Subscripts multiply only the atom or group immediately before them. Parentheses (more advanced) group polyatomic ions, e.g., Ca(OH)₂ has two hydroxide units. Praxis middle-school items stay mostly with simple binary formulas and a few common polyatomic ions (sulfate, hydroxide, nitrate, carbonate).

Structural formulas and Lewis (electron-dot) diagrams show connectivity and valence electrons, not just counts.

Electron-dot and structural models for CH₄

Methane, CH₄, is a covalent molecule. Carbon has 4 valence electrons; each hydrogen has 1. Carbon shares with four hydrogens so that carbon is surrounded by 8 shared electrons (4 bonds) and each hydrogen has 2.

Electron-dot (Lewis) idea in words:

• C in the center with four H atoms around it
• Each C–H connection is a shared pair (often drawn as two dots between atoms, or as a line)

Structural formula commonly drawn as:

      H
      |
  H — C — H
      |
      H

Each line = one shared pair (single covalent bond). Praxis items may ask what the dots represent (valence electrons / shared pairs) or why carbon forms four bonds (four valence electrons available for sharing).

Naming Common Compounds

Middle-grades naming mixes ionic stock names and molecular prefixes. Memorize patterns rather than endless lists.

Support rules:

• Ionic binary (metal + nonmetal): metal name + nonmetal root + -ide (sodium chloride, magnesium oxide, calcium chloride).
• Ionic with polyatomic: metal name + polyatomic name (sodium sulfate, calcium carbonate)—do not change the polyatomic ending to -ide.
• Molecular binary (two nonmetals): use prefixes mono-, di-, tri-, tetra-… on the second element (and on the first if more than one): CO₂ carbon dioxide, SCl₂ sulfur dichloride. Mono- is often dropped on the first element (CO is carbon monoxide, not monocarbon monoxide).

Ions from CaCl₂ in Water

Calcium chloride, CaCl₂, is ionic. In the solid, Ca²⁺ and Cl⁻ occupy a lattice. When CaCl₂ dissolves in water, water molecules separate the ions and surround them. The dissolved species are Ca²⁺(aq) and Cl⁻(aq) in a 1:2 ratio—not neutral CaCl₂ molecules floating intact.

Dissociation equation (teaching form):

CaCl₂(s) → Ca²⁺(aq) + 2 Cl⁻(aq) (in water)

Why this matters on Praxis:

• Explains electrical conductivity of salt water (mobile charged particles)
• Contrasts with sugar water (covalent molecules dissolve as neutral molecules, poor conductivity)
• Connects formula subscripts to ion counts: one formula unit yields one calcium ion and two chloride ions

Classroom Models and Common Traps

1. Ball-and-stick vs space-filling: both can represent CH₄; sticks emphasize bonds, space-filling emphasizes occupied space.
2. "Molecules of NaCl": avoid saying ionic solids are made of molecules; say formula units or ion lattice.
3. O₂ vs ionic oxides: O₂ gas is covalent; MgO is ionic—same element oxygen, different bonding context.
4. Metallic bonding ≠ magnets: metallic bonding explains metal properties; magnetism is a related but separate topic.

When a teaching scenario asks students to justify MgO vs SO₂ bonding, require them to cite metal/nonmetal vs nonmetal/nonmetal and transfer vs sharing—not merely “because the teacher said so.”