IB Chemistry SL — Classification of Matter

Complete Study Guide

Topics Covered

1. The Periodic Table — Trends and Periodicity (S3.1)
2. Group Properties — Alkali Metals, Halogens, Noble Gases (S3.1)
3. Classification of Organic Compounds by Functional Group (S3.2)

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Prerequisites: This guide builds directly on atomic structure and electron configurations. Revisit Atomic Structure & Periodicity (Structure 1) if you need a refresher on ionisation energy anomalies or electronegativity definitions. For the functional group section, familiarity with basic organic nomenclature from Organic Chemistry (Reactivity 3) is helpful but not required.

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1. The Periodic Table — Trends and Periodicity (S3.1)

The periodic table is the single most important tool in chemistry. Elements are arranged in order of increasing atomic number ($Z$), and this arrangement reveals repeating patterns — called periodicity — in physical and chemical properties. Structure 3.1 formalises the trends you need to be able to explain, predict, and apply.

What is Periodicity?

Periodicity refers to the regular, repeating variation in element properties as atomic number increases. This repetition arises because elements in the same group have the same number of valence electrons and therefore similar chemical behaviour.

The table is divided into:

• Periods (horizontal rows): numbered 1–7. Moving across a period increases the nuclear charge by one proton per element while electrons fill the same principal energy level.
• Groups (vertical columns): numbered 1–18 (IUPAC). Elements in the same group share the same number of valence electrons → similar chemical properties.
• Blocks ($s$, $p$, $d$, $f$): defined by the sublevel being filled. Groups 1–2 are the $s$-block; Groups 13–18 the $p$-block; Groups 3–12 the $d$-block (transition metals).

Periodic Trend 1 — Atomic Radius

The atomic radius (covalent radius: half the distance between nuclei of two bonded identical atoms) changes predictably:

Direction	Trend	Reason
Across a period (→)	Decreases	Nuclear charge $Z$ increases; electrons added to the same energy level are pulled closer to nucleus
Down a group (↓)	Increases	Each new period adds an energy level further from nucleus; shielding by inner shells also increases

Periodic Trend 2 — First Ionisation Energy (IE$_1$)

The first ionisation energy is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in the ground state:

$\text{X}(g) \rightarrow \text{X}^+(g) + e^- \quad \Delta H = \text{IE}_1 \; (\text{always positive})$

Direction	General Trend	Reason
Across a period (→)	Increases (with exceptions)	Higher $Z$ → stronger nuclear attraction on outer electrons
Down a group (↓)	Decreases	Outer electron further from nucleus + more shielding by inner shells

Two key anomalies across a period (you must know these):

Anomaly 1 — Group 2 to Group 13 (e.g. Mg → Al): IE$_1$(Al) < IE$_1$(Mg)

• Magnesium’s outer electron is in $3s$ ($[\text{Ne}]\, 3s^2$).
• Aluminium’s outer electron is in $3p$ ($[\text{Ne}]\, 3s^2\, 3p^1$).
• The $3p$ sublevel is higher in energy and further from the nucleus than $3s$, and is partially shielded by the filled $3s$ pair. The $3p$ electron is therefore easier to remove despite Al having higher nuclear charge.

Anomaly 2 — Group 15 to Group 16 (e.g. N → O or P → S): IE$_1$(O) < IE$_1$(N)

• Nitrogen: $2p^3$ — three $2p$ orbitals each hold one electron (Hund’s rule, no pairing).
• Oxygen: $2p^4$ — one $2p$ orbital holds two paired electrons.
• The electron–electron repulsion in the paired orbital makes one electron easier to remove from O than from N.

Periodic Trend 3 — Electronegativity

Electronegativity is the ability of a bonded atom to attract the shared electron pair towards itself. It is measured on the Pauling scale (F = 4.0, most electronegative element).

Direction	Trend	Reason
Across a period (→)	Increases	Higher $Z$ + smaller atomic radius → stronger attraction on bonding electrons
Down a group (↓)	Decreases	Larger radius + greater shielding → weaker attraction on bonding electrons

Periodic Trend 4 — Electron Affinity

Electron affinity (EA) is the energy change when one mole of electrons is added to one mole of gaseous atoms in the ground state:

$\text{X}(g) + e^- \rightarrow \text{X}^-(g) \quad \Delta H = \text{EA}$

• EA is usually negative (energy is released) for non-metals — they attract electrons.
• EA generally becomes more negative (more energy released) across a period and less negative down a group, following the same logic as electronegativity.
• Exception: EA of fluorine is less negative than that of chlorine. The small size of F means adding an electron causes significant electron–electron repulsion in the already-crowded 2$p$ shell.

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2. Group Properties — Alkali Metals, Halogens, Noble Gases (S3.1)

Three groups illustrate periodicity most clearly in IB Chemistry SL. You need to know the physical and chemical trends within each group and be able to explain them using atomic structure.

Group 1 — Alkali Metals (Li, Na, K, Rb, Cs, Fr)

All alkali metals have one valence electron ($ns^1$). They react by losing this electron to form a $1+$ cation.

Physical trends down Group 1:

Property	Trend	Reason
Atomic radius	Increases	Each element has an additional electron shell
Melting point	Decreases	Larger atoms → weaker metallic bonding (delocalised electrons further from nuclei)
Density	Generally increases (Li, Na, K are less dense than water; Rb, Cs are denser)	Increase in mass outpaces increase in atomic volume

Chemical trend — reactivity with water increases down the group:

$2\text{M}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{MOH}(aq) + \text{H}_2(g)$

• Li reacts steadily. Na reacts vigorously. K reacts very vigorously (flame). Rb and Cs react explosively.
• Reactivity increases because the outer $ns^1$ electron is increasingly far from the nucleus (more shielding, larger radius) → easier to remove → lower ionisation energy → metal more readily loses its electron.

Group 17 — Halogens (F, Cl, Br, I, At)

All halogens have seven valence electrons ($ns^2\, np^5$) and react by gaining one electron (forming $\text{X}^-$ halide ions) or by sharing electrons in covalent bonds.

Physical trends down Group 17:

Property	Trend	Reason
Atomic radius	Increases	Additional electron shell each period
Electronegativity	Decreases	More shielding; nucleus less able to attract bonding electrons
State at room temperature	$\text{F}_2$ (pale yellow gas) → $\text{Cl}_2$ (yellow-green gas) → $\text{Br}_2$ (red-brown liquid) → $\text{I}_2$ (grey-black solid)	Larger molecules have stronger London dispersion forces → higher boiling points
Colour	Becomes darker (yellow → brown → purple/black) as size increases

Chemical trend — reactivity decreases down the group:

Halogens react by gaining an electron. The ability to attract an electron (electron affinity) decreases down the group as the atomic radius increases and shielding increases. Therefore:

• $\text{F}_2$ is the most reactive non-metal (oxidising agent).
• Reactivity decreases: $\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2$.

Displacement reactions demonstrate this reactivity order:

A more reactive halogen will displace a less reactive halide from solution:

$\text{Cl}_2(aq) + 2\text{KBr}(aq) \rightarrow 2\text{KCl}(aq) + \text{Br}_2(aq)$

$\text{Cl}_2(aq) + 2\text{KI}(aq) \rightarrow 2\text{KCl}(aq) + \text{I}_2(aq)$

$\text{Br}_2$ will displace $\text{I}^-$ but not $\text{Cl}^-$. $\text{I}_2$ cannot displace either $\text{Br}^-$ or $\text{Cl}^-$.

Group 18 — Noble Gases (He, Ne, Ar, Kr, Xe, Rn)

Noble gases have full outer shells (He: $1s^2$; others: $ns^2\, np^6$). This makes them exceptionally stable and almost chemically inert.

Key properties:

• Exist as monoatomic gases (lowest boiling points of any element at a given period).
• Very low reactivity — do not form ions or covalent bonds under normal conditions.
• Boiling point increases down the group as atoms become larger and London dispersion forces between atoms increase.
• Used industrially: He (balloons, MRI cooling), Ne (neon lights), Ar (inert atmosphere for welding), Kr/Xe (specialist lighting).

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3. Classification of Organic Compounds by Functional Group (S3.2)

Structure 3.2 requires you to be able to identify and classify organic molecules based on their functional groups. This overlaps with the organic chemistry in Reactivity 3, but the focus here is on classification and the connection between functional group structure and physical properties — not on mechanisms.

What is a Functional Group?

A functional group is an atom or group of atoms in an organic molecule that is responsible for the characteristic chemical reactions of that molecule. Molecules with the same functional group belong to the same homologous series and share similar chemical properties.

The Eight Functional Groups for IB SL

You must be able to recognise these from structural formulas, name the class, and give an example.

Class	Functional Group	Key Feature	Example
Alcohol	$\text{-OH}$	Hydroxyl group on C chain	$\text{CH}_3\text{CH}_2\text{OH}$ (ethanol)
Aldehyde	$\text{-CHO}$	Carbonyl (C=O) at chain end	$\text{CH}_3\text{CHO}$ (ethanal)
Ketone	$\text{C=O}$ (internal)	Carbonyl in middle of chain	$\text{CH}_3\text{COCH}_3$ (propanone)
Carboxylic acid	$\text{-COOH}$	Carbonyl + hydroxyl at chain end	$\text{CH}_3\text{COOH}$ (ethanoic acid)
Ester	$\text{-COO-}$	Formed from acid + alcohol	$\text{CH}_3\text{COOCH}_2\text{CH}_3$ (ethyl ethanoate)
Amine	$\text{-NH}_2$	Amino group on C chain	$\text{CH}_3\text{NH}_2$ (methylamine)
Amide	$\text{-CONH}_2$	Carbonyl bonded to N	$\text{CH}_3\text{CONH}_2$ (ethanamide)
Halogenoalkane	$\text{C-X}$ (X = F, Cl, Br, I)	Halogen bonded to C	$\text{CH}_3\text{CH}_2\text{Br}$ (bromoethane)

Distinguishing Aldehydes from Ketones

Both aldehydes and ketones contain a carbonyl group ($\text{C=O}$), so they can be confused. The distinction:

• Aldehyde — the carbonyl carbon is bonded to at least one hydrogen atom: $\text{-CHO}$ (always at the end of the carbon chain).
• Ketone — the carbonyl carbon is bonded to two other carbon atoms: the C=O is always in the middle of the chain.

Practical test: Tollens’ reagent (ammoniacal silver nitrate) oxidises aldehydes to carboxylic acids, forming a silver mirror. Ketones do not react (they cannot be further oxidised at the carbonyl).

Distinguishing Amines from Amides

• Amine — the nitrogen atom ($\text{-NH}_2$) is bonded directly to a carbon chain but there is no carbonyl adjacent to it.
• Amide — the nitrogen is bonded to a carbonyl group: $\text{-CO-NH}_2$ or $\text{-CO-NH-}$.

Amides are formed when a carboxylic acid reacts with an amine (condensation reaction). They are present in proteins (peptide bonds are amide bonds).

Physical Properties and Functional Groups

The functional group determines not just reactivity but also physical properties, especially boiling point and solubility.

Class	Intermolecular Force	Boiling Point (relative)	Water Solubility
Alkane	London dispersion only	Low	Insoluble
Halogenoalkane	London dispersion + dipole–dipole	Higher than alkane	Low (non-polar C-X bond)
Amine	H-bonding (N-H···N), dipole–dipole	Moderate	Soluble (short chain)
Alcohol	H-bonding (O-H···O)	High	Soluble (short chain)
Carboxylic acid	H-bonding (dimerisation)	Very high	Soluble (short chain)
Amide	H-bonding (N-H···O=C)	Very high	Varies
Ester	Dipole–dipole (no H-bond donor)	Moderate — lower than the alcohol of same mass	Low
Aldehyde/Ketone	Dipole–dipole (C=O polar; no O-H)	Moderate	Moderate (short chain)

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Practice Questions

Test your understanding with these IB-style questions.

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Question 1

Which of the following correctly describes the trend in first ionisation energy across Period 3 (Na to Ar)?

A. It increases continuously with no exceptions.

B. It generally increases, with a dip from Mg to Al and a dip from P to S.

C. It decreases across the period because atomic radius increases.

D. It is constant across Period 3 because all elements are in the same period.

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Question 2

Which statement correctly explains why the first ionisation energy of aluminium (Al) is lower than that of magnesium (Mg)?

A. Aluminium has a smaller atomic radius than magnesium.

B. Aluminium has fewer electrons than magnesium.

C. The outer electron of aluminium is in a $3p$ sublevel, which is higher in energy and further from the nucleus than the $3s$ sublevel of magnesium.

D. Aluminium has a higher nuclear charge than magnesium, making electrons harder to remove.

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Question 3

A student tests an unknown organic compound with Tollens’ reagent. A silver mirror forms in the test tube. Which functional group is present?

A. Ketone

B. Carboxylic acid

C. Ester

D. Aldehyde

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Question 4

Which of the following correctly explains why iodine ($\text{I}_2$) is a solid at room temperature while chlorine ($\text{Cl}_2$) is a gas?

A. $\text{I}_2$ has ionic bonds and $\text{Cl}_2$ does not.

B. $\text{I}_2$ molecules are larger and have stronger London dispersion forces than $\text{Cl}_2$ molecules.

C. $\text{Cl}_2$ is more reactive than $\text{I}_2$, so it does not form stable solid structures.

D. $\text{I}_2$ has permanent dipole interactions while $\text{Cl}_2$ does not.

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Question 5

Which pair of functional groups both contain a carbonyl group (C=O)?

A. Alcohol and amine

B. Ester and halogenoalkane

C. Aldehyde and ketone

D. Carboxylic acid and alcohol

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Show Answers

1. B — IE$_1$ generally increases across Period 3 because nuclear charge increases while electrons are added to the same shell. There are two exceptions: IE$_1$(Al) < IE$_1$(Mg) because Al’s outer electron is in the higher-energy $3p$ sublevel; and IE$_1$(S) < IE$_1$(P) because S has a paired electron in one $3p$ orbital, creating extra repulsion that makes one electron easier to remove. A is wrong because of these two exceptions. C and D are incorrect descriptions of the trend.

2. C — Magnesium’s outer electron ($3s^2$) is in the $3s$ sublevel. Aluminium’s outer electron is in $3p^1$, which is higher in energy and further from the nucleus than $3s$, and is additionally shielded by the filled $3s$ electrons. Despite Al having a higher nuclear charge (one more proton), the $3p$ electron is easier to remove than the $3s$ electron. A is wrong — Al is smaller than Mg. B is wrong — Al has more electrons than Mg (13 vs. 12). D correctly states the nuclear charge trend but fails to explain the anomaly.

3. D — Tollens’ reagent (ammoniacal silver nitrate) is a mild oxidising agent that reacts specifically with aldehydes, oxidising them to carboxylic acids. The silver ions in the reagent are reduced to silver metal, forming a silver mirror on the glass. Ketones (A) cannot be oxidised by Tollens’ reagent. Carboxylic acids (B) and esters (C) do not react with Tollens’ reagent under these conditions.

4. B — Both $\text{Cl}_2$ and $\text{I}_2$ are non-polar diatomic molecules; the only intermolecular forces between their molecules are London dispersion (temporary induced dipole) forces. $\text{I}_2$ has more electrons than $\text{Cl}_2$ (54 vs. 34) and a larger, more polarisable electron cloud, giving rise to much stronger London dispersion forces. These stronger forces require more energy to overcome, resulting in higher boiling and melting points — hence $\text{I}_2$ is solid at room temperature. A is wrong — both are covalent. C and D are incorrect.

5. C — Aldehydes contain $\text{-CHO}$ (carbonyl at chain end) and ketones contain C=O (carbonyl in chain middle). Both contain the carbonyl group. Alcohols contain $\text{-OH}$ (not C=O). Amines contain $\text{-NH}_2$. Esters contain $\text{-COO-}$ (this does contain C=O, but paired with an ether linkage; the direct pair asked about is aldehyde and ketone). Halogenoalkanes contain C-X bonds. Carboxylic acids contain $\text{-COOH}$ (which includes C=O), and alcohol contains $\text{-OH}$, so D is wrong as alcohol has no carbonyl.

Next Topics

This guide connects to two further areas:

• Stoichiometry (Reactivity 2.1 & Structure 2.4) — quantitative chemistry using the mole concept builds on the element classifications here.
• Organic Chemistry (Reactivity 3) — reactions of functional groups (oxidation of alcohols, addition across alkenes, nucleophilic substitution of halogenoalkanes) apply the classifications from S3.2 in a mechanistic context.