CHEM20 Session 05 — Molecular naming and formulas

Delivery id: CHEM20 (SCN2796)

Time: 75 minutes

Unit A: Diversity of Matter and Chemical Bonding

Alberta POS: https://open.alberta.ca/publications/3950147-updated-2014

Official outcomes

Code	Type	Role today
20-A2.1k	Knowledge	Recall principles for assigning names to molecular substances
20-A2.2k	Knowledge	Explain why molecular formulas state the number of atoms of each element
20-A1.1k	Knowledge	Contrast: ionic names do not use mono/di/tri

Acids as a naming system are Unit C (20-C2.1k). Today: binary molecular compounds and a few common molecules (H2O, NH3, CH4) by accepted names.

Learning targets

1. Name binary molecular compounds with prefixes mono–deca (drop redundant “mono” on the first element; monoxide not monooxide).
2. Write formulas from names: dinitrogen tetroxide is \( \mathrm{N}_{2}\mathrm{O}_{4} \), not “the ionic ratio.”
3. Explain why \( \mathrm{H}_{2}\mathrm{O}_{2} \) is not written HO: the molecular formula counts atoms in the molecule, which may not be the empirical ratio.
4. Distinguish CO vs CO2, NO vs NO2, PCl3 vs PCl5 as different substances.

Prerequisites

Sessions 01–04. Ionic naming must not be overwritten. Sugar from Session 04 is molecular C12H22O11.

Diagnostic/retrieval

1. Name Na2O. (Ionic: sodium oxide — trap for people who write “disodium monoxide.”)
2. Formula for iron(III) sulfate.
3. Session 04: did sucrose conduct?
4. How many oxygen atoms in one molecule of CO2?

Instruction

Molecular substances are made of discrete molecules held by covalent bonds (Sessions 06–08). The formula is a molecular formula: it reports how many atoms of each element are in one molecule.

Prefix system (binary molecular, two non-metals):

n	prefix	n	prefix
1	mono	6	hexa
2	di	7	hepta
3	tri	8	octa
4	tetra	9	nona
5	penta	10	deca

Rules used in Chem 20:

• Second element ends in -ide.
• Prefix on both elements as needed: \( \mathrm{N}_{2}\mathrm{O}_{5} \) dinitrogen pentoxide (pentoxide, not pentaoxide, by common Chem 20 convention).
• Omit mono- on the first element: CO carbon monoxide, not monocarbon monoxide.
• Never use prefixes for ionic NaCl.

Why molecular formulas are not always empirical (20-A2.2k).

Hydrogen peroxide molecules are H–O–O–H: molecular formula \( \mathrm{H}_{2}\mathrm{O}_{2} \), empirical HO. Glucose \( \mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6} \), empirical \( \mathrm{CH}_{2}\mathrm{O} \). Ionic NaCl is already empirical.

Session 04 sugar: sucrose \( \mathrm{C}_{12}\mathrm{H}_{22}\mathrm{O}_{11} \) is already the molecular formula (empirical is the same in that case).

Accepted names (do not prefix these as if they were binary invented names): H2O water, NH3 ammonia, CH4 methane, O3 ozone. H2O2 hydrogen peroxide (not “dihydrogen dioxide” in this course’s required list — mention it exists as a systematic option, CORE uses hydrogen peroxide).

Different formulas, different compounds: CO (carbon monoxide, toxic) vs CO2 (carbon dioxide). PCl3 phosphorus trichloride vs PCl5 phosphorus pentachloride.

Timed visuals

id	trigger	duration	alt	teacher cue	tutor cue
CHEM20-VIS-05-PREFIX	After retrieval	50 s	Prefix tower 1–10	“Count atoms, not charges.”	Refuse criss-cross on CO2.
CHEM20-VIS-05-H2O2	Minute 18	55 s	H2O2 vs HO vs H2O ball models	“Same empirical ratio as HO; different molecule from water.”	No home peroxide labs.
CHEM20-VIS-05-TRAP	Minute 28	40 s	Na2O card stamped IONIC	“No prefixes on salts.”	Flag disodium monoxide as incorrect for Chem 20 ionic names.

Modelling/worked examples

Example 1. \( \mathrm{PCl}_{5} \): phosphorus pentachloride. Five Cl atoms, one P atom in each molecule.

Example 2. Dinitrogen monoxide: \( \mathrm{N}_{2}\mathrm{O} \) (also called nitrous oxide — accepted common name; CORE accepts N2O as dinitrogen monoxide).

Example 3. \( \mathrm{Cl}_{2}\mathrm{O}_{7} \): dichlorine heptoxide.

Example 4 — 20-A2.2k. A student “simplifies” \( \mathrm{N}_{2}\mathrm{O}_{4} \) to \( \mathrm{NO}_{2} \). Those are different molecules (dinitrogen tetroxide vs nitrogen dioxide) in equilibrium in advanced chemistry; in Chem 20, do not reduce molecular formulas. Ionic \( \mathrm{Na}_{2}\mathrm{O}_{2} \) vs \( \mathrm{NaO} \) was a different issue; here the molecule really contains 2 N and 4 O.

Example 5 — project. Carbon dioxide is molecular, nonpolar molecule (Session 09), not a cleaner. Ethanol \( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH} \) (or C2H6O) is molecular; naming as an alcohol is beyond binary prefixes — record it as a named household molecule, not “dicarbon hexahydrogen monoxide.”

Guided practice + answers

1. Name \( \mathrm{SO}_{2} \), \( \mathrm{SO}_{3} \), \( \mathrm{CCl}_{4} \). Answers: sulfur dioxide; sulfur trioxide; carbon tetrachloride.
2. Formulas: dinitrogen trioxide; phosphorus trifluoride; iodine heptafluoride. Answers: \( \mathrm{N}_{2}\mathrm{O}_{3} \); \( \mathrm{PF}_{3} \); \( \mathrm{IF}_{7} \).
3. Why is water not “dihydrogen monoxide” on the required list? Answer: Accepted name is water; the joke name is chemically prefix-legal but not how we assess H2O.
4. Why not write \( \mathrm{H}_{2}\mathrm{O}_{2} \) as HO? Answer: The molecule contains two H and two O; empirical HO hides the actual molecule (and would collide with a hypothetical different species).
5. Name \( \mathrm{NaCl} \) using ionic rules. Answer: sodium chloride (not sodium monochloride).

Independent practice

• FOUNDATION: Name CO2, NO2, PCl3. Formulas: carbon monoxide, sulfur dioxide, dinitrogen pentoxide. One sentence: prefixes count atoms.
• CORE: Name N2O4, Cl2O, P4O10 (tetraphosphorus decaoxide). Formulas: xenon hexafluoride, dichlorine monoxide, nitrogen monoxide. Contrast Na2O vs N2O naming systems. Explain H2O2 vs HO.
• PROFICIENT: A label says “nitrogen oxide.” Explain why that is ambiguous (NO, NO2, N2O, N2O4, N2O5…). Write correct names for NO and NO2. For C6H12O6, state molecular vs empirical formula.
• ADVANCED: Project brief seed: list three molecular substances in household cleaning/water context (H2O, CH3COOH in vinegar, C2H5OH if the school permits discussing ethanol as a solvent — no drinking, no distillation). For each, write the molecular formula and why an ionic Stock name would be wrong.

Project connection

Vinegar’s acetic acid is molecular \( \mathrm{CH}_{3}\mathrm{COOH} \) (often written HC2H3O2). Session 04 weak conductivity is consistent with a molecular acid, not with “vinegar is salt.” Add H2O as the solvent, not as a “named binary practice item.”

Checks

Board: “Name BCl3.” Expected boron trichloride. If “boron chloride,” demand the prefix because boron and chlorine are both non-metals (binary molecular).

Misconceptions

Wrong	Repair	Do not say
“Criss-cross CO2 to CO.”	No charges to criss-cross; two O atoms in the molecule.	“Almost.”
“N2O4 simplifies to NO2.”	Different molecular formulas.	—
“Na2O is disodium monoxide.”	Ionic: sodium oxide.	“That’s the systematic way so it is fine.”
“Mono is always written.”	Dropped on the first element: CO carbon monoxide.	—
“All -ide names are ionic.”	Molecular chlorides exist (CCl4).	—

AI Tutor prompts and boundaries

Allowed: “Name PCl5.” “Why is H2O2 not HO?” “Contrast naming of CaO and CO.”

Forbidden: Inventing synthesis labs for NOx or PCl5 (those reagents are not classroom-safe here); home CO experiments; praising ionic prefixes; bleach “to make chlorine gas for naming Cl2.”

Human-tutor handoff

If prefixes collide with second-language number words, use a prefix strip taped to the desk. If a learner still criss-crosses, give a physical two-atom vs three-atom model of CO vs CO2.

Exit ticket + answers

1. Name \( \mathrm{N}_{2}\mathrm{O}_{5} \).
2. Formula for sulfur hexafluoride.
3. Why is the molecular formula of hydrogen peroxide \( \mathrm{H}_{2}\mathrm{O}_{2} \) rather than HO?
4. Name \( \mathrm{MgO} \) (ionic).

Answers: 1. dinitrogen pentoxide. 2. \( \mathrm{SF}_{6} \). 3. Each molecule contains 2 H and 2 O; the formula counts atoms in the molecule. 4. magnesium oxide.

Homework

CORE naming set (20 compounds mixed ionic vs molecular — teacher sheet). Flag each as I or M before naming.

Teacher guidance

The mixed sheet is the assessment that 20-A1.1k and 20-A2.1k are not fused incorrectly. Do not introduce organic IUPAC beyond methane/ethanol as named examples.

Materials

Prefix table; mixed ionic/molecular flashcards; H2O2 vs H2O model; homework sheet.

Preparation

Print mixed sets. Remove any leftover Session 04 vinegar cups so “acetic acid” is a formula conversation, not a spill.

Timing

Min	Move
0–6	Retrieval including ionic trap
6–22	Prefix rules + 20-A2.2k
22–38	Worked examples including H2O2
38–50	Guided
50–66	Independent
66–75	Exit + mixed homework

Online alternative

LMS drag-and-drop prefixes. Immediate feedback must not accept Na2O as disodium monoxide for this course’s ionic standard.

Low-technology alternative

Index cards: write the formula on one side, name on the other. Teacher calls “ionic or molecular first.”