Source: CHEM106 Lab 1, NYU Chemistry
Tags: chemical compounds, ionic compounds, covalent compounds, naming compounds, chemical formulas, CHEM106, NYU
Difficulty: Introductory | Prerequisites: Basic periodic table familiarity (metals vs. nonmetals, charges of common ions).
This lab sits right at the start of general chemistry and covers the foundation you will use every time you write a formula or name a compound for the rest of the course. If you cannot classify a compound as ionic, covalent, metallic or organic, and then apply the correct naming convention, every subsequent topic (stoichiometry, reactions, equilibrium) becomes harder than it needs to be.
You should already be comfortable identifying metals and nonmetals on the periodic table and know what an ion is. The three laws covered here (Conservation of Mass, Definite Proportions, Multiple Proportions) also underpin quantitative chemistry, so they will reappear in virtually every unit.
Chemical compounds fall into four main categories: ionic, covalent, metallic and organic. Each type follows its own naming rules, and the order in which you write elements in a formula matters (cation first, anion second for ionic compounds). Three foundational laws govern how compounds form and combine: Conservation of Mass, Definite Proportions and Multiple Proportions.
Ionic compound
A compound formed by the electrostatic attraction between a metal cation and a nonmetal anion. Think of it as: the metal gives electrons to the nonmetal, and the opposite charges hold them together.
Covalent compound (molecular compound)
A compound formed when two or more nonmetals share electrons. In simple terms, neither atom wants to give up electrons, so they share instead.
Metallic compound
A compound created from positive metal ions surrounded by a "sea" of delocalised electrons. Think of it as: metal atoms pooling their electrons into a shared cloud that holds the structure together.
Organic compound
A compound that always contains carbon, usually bonded to hydrogen and often to oxygen, nitrogen or other elements. In simple terms, if carbon is in the formula, you are likely looking at an organic compound.
Cation
A positively charged ion, formed when an atom loses one or more electrons. Think of it as: the atom that gave away electrons and ended up positive.
Anion
A negatively charged ion, formed when an atom gains one or more electrons. In simple terms, the atom that picked up extra electrons and became negative.
Polyatomic ion
An ion made up of two or more atoms covalently bonded together that carries a net charge. Think of it as: a small molecule fragment that acts as a single charged unit (e.g. SO₄²⁻, OH⁻, NO₃⁻).
Oxidation state (oxidation number)
The effective charge an atom would have if all bonds were purely ionic. In simple terms, it tells you how many electrons an atom has lost or gained, and it is indicated by Roman numerals for transition metals, e.g. Iron(II) = Fe²⁺.
Law of Conservation of Mass
Matter is neither created nor destroyed in a chemical reaction; the total mass of reactants equals the total mass of products.
Law of Definite Proportions
A given chemical compound always contains the same elements in exactly the same proportions by mass, regardless of sample size or source.
Law of Multiple Proportions
When two elements combine to form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element are small whole numbers.
Ionic compounds form between a metal and a nonmetal. The metal transfers electrons to the nonmetal, creating cations and anions held together by electrostatic attraction. Examples: NaCl (sodium chloride), MgO (magnesium oxide), KBr (potassium bromide).
Covalent (molecular) compounds form between two nonmetals. Electrons are shared rather than transferred. Examples: CO₂ (carbon dioxide), H₂O (water), SO₃ (sulfur trioxide).
Metallic compounds consist of positive metal ions in a delocalised electron sea. The shared electrons allow electrical conductivity and malleability. Example: bronze (copper-tin alloy).
Organic compounds always contain carbon, typically bonded to hydrogen and often to oxygen, nitrogen or halogens. Examples: CH₄ (methane), C₆H₁₂O₆ (glucose).
Ionic compound naming
Write the metal by its elemental name, then the nonmetal with its root + the suffix "-ide." Example: NaCl = sodium chloride.
If the metal is a transition metal with multiple oxidation states, add a Roman numeral in parentheses after the metal name. Example: Fe²⁺ in a compound = Iron(II); Fe³⁺ = Iron(III). So FeCl₃ = iron(III) chloride.
If the compound contains a polyatomic ion, use the polyatomic ion's standard name as-is. Example: KNO₃ = potassium nitrate; Mg(OH)₂ = magnesium hydroxide.
Covalent compound naming (Greek prefix system)
Use Greek prefixes to indicate the number of each atom: mono- (1, only for the second element), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7).
The second element always gets the "-ide" suffix. Example: SO₃ = sulfur trioxide; CO₂ = carbon dioxide; CF₄ = carbon tetrafluoride.
"Mono-" is dropped from the first element. You say carbon dioxide, not monocarbon dioxide.
Acids
Binary acids (H + nonmetal): "hydro-" + root + "-ic acid." Example: HCl = hydrochloric acid.
Oxyacids (H + polyatomic ion with oxygen): if the ion ends in "-ate," the acid ends in "-ic acid"; if the ion ends in "-ite," the acid ends in "-ous acid." Example: H₂SO₄ (from sulfate) = sulfuric acid; H₃PO₃ (from phosphite) = phosphorous acid.
Order matters. The cation (positive ion, usually the metal) is written first; the anion (negative ion, usually the nonmetal or polyatomic ion) is written second.
Charge balancing. The total positive charge must equal the total negative charge so the formula is electrically neutral. Use subscripts to balance. Example: Mg²⁺ and OH⁻ combine as Mg(OH)₂ because you need two hydroxide ions to balance one magnesium ion.
Parentheses rule. When you need more than one of a polyatomic ion, wrap it in parentheses and add the subscript outside. Example: Co₂(SO₄)₃ for cobalt(III) sulfate.
Transition metals. The Roman numeral in the name tells you the charge of the metal ion. Use that charge to work out subscripts. Example: Mn₂O₃ for manganese(III) oxide, because Mn³⁺ and O²⁻ balance at a 2:3 ratio.
Law of Conservation of Mass. In any chemical reaction, the total mass of the reactants equals the total mass of the products. Nothing is created or destroyed, only rearranged. This is why you balance equations.
Law of Definite Proportions (Proust's Law). A pure compound always contains the same elements in the same mass ratio, no matter where the sample comes from. Water is always 11.2% hydrogen and 88.8% oxygen by mass.
Law of Multiple Proportions (Dalton's Law). When two elements form more than one compound, the ratios of the second element's mass (per fixed mass of the first) are small whole numbers. Example: CO has one oxygen per carbon; CO₂ has two. The ratio of oxygen masses is 1:2.
Naming and formula-writing conventions are not just academic housekeeping. Pharmacists, chemical engineers and lab technicians rely on unambiguous names to avoid dangerous mix-ups: Iron(II) sulfate and Iron(III) sulfate are different compounds with different uses, and writing the wrong Roman numeral could mean the wrong reagent in a reaction vessel.
The Law of Conservation of Mass is the reason stoichiometry works at all. Every industrial process, from fertiliser production to pharmaceutical synthesis, depends on balancing input and output masses to control yield, cost and waste.
Students often think the order of elements in an ionic formula does not matter. It does: the cation is always written first, the anion second. Writing ClNa instead of NaCl is incorrect.
Students confuse when to use Roman numerals. Roman numerals are only needed for metals that can have more than one oxidation state (most transition metals). You do not write Sodium(I) chloride; sodium only forms Na⁺, so the Roman numeral adds nothing.
Students sometimes apply Greek prefixes to ionic compounds. Greek prefixes (di-, tri-, tetra-) are for covalent compounds only. MgCl₂ is magnesium chloride, not magnesium dichloride.
Students assume "organic" means "living" or "natural." In chemistry, organic simply means the compound contains carbon. Plenty of synthetic, lab-made compounds are organic.
⚠️ Be prepared to classify any compound as ionic, covalent, metallic or organic from its formula alone. This is a staple question.
⚠️ Know when to use Roman numerals vs. Greek prefixes. Mixing them up is one of the most common marking errors on naming questions.
⚠️ Charge-balancing for ionic formulas is tested repeatedly. Practise writing formulas from ion pairs until the cross-multiplication method is automatic.
⚠️ Expect at least one question asking you to state or apply one of the three laws (Conservation of Mass, Definite Proportions, Multiple Proportions). Know the names and be able to explain each in one sentence.
True or false: Covalent compounds are formed between a metal and a nonmetal. (False, that describes ionic compounds.)
Fill in the blank: When naming an ionic compound, the cation is written ______ and the anion is written ______. (first; second)
True or false: Greek prefixes (di-, tri-) are used when naming ionic compounds. (False, Greek prefixes are for covalent compounds.)
Fill in the blank: The Roman numeral in Iron(III) chloride tells you the ______ of the iron ion. (oxidation state / charge, which is 3+)
True or false: The Law of Definite Proportions states that a compound can have varying ratios of elements depending on where the sample was collected. (False, the ratio is always the same.)
Q: Write the formula for cobalt(III) sulfate.
A: Co₂(SO₄)₃. Cobalt(III) means Co³⁺; sulfate is SO₄²⁻. Cross-multiply the charges: 2 cobalts and 3 sulfates.
Q: Name the compound TiO₂.
A: Titanium(IV) oxide. Titanium is a transition metal with variable oxidation states, so the Roman numeral is required. Each oxygen is O²⁻, and two of them give a total charge of 4⁻, so Ti must be 4+.
Q: Is SO₃ named using ionic or covalent naming rules, and what is its name?
A: Covalent rules, because both sulfur and oxygen are nonmetals. The name is sulfur trioxide ("tri-" = three oxygens).
Q: A student writes the formula for magnesium hydroxide as MgOH₂. What is wrong?
A: The hydroxide ion OH⁻ is a polyatomic ion and must be enclosed in parentheses when more than one is needed: Mg(OH)₂.
Q: State the Law of Multiple Proportions in one sentence and give an example.
A: When two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. Example: in CO and CO₂, the oxygen masses per fixed mass of carbon are in a 1:2 ratio.
The naming conventions here are a prerequisite for writing balanced chemical equations in the next lab: you cannot balance what you cannot name or formulate. Charge-balancing reappears directly in electrochemistry and redox reactions.
The three laws (Conservation of Mass, Definite Proportions, Multiple Proportions) are the historical foundation of Dalton's atomic theory, which you will encounter in the atomic structure unit. They also underpin stoichiometry, the quantitative backbone of every lab from here on.
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