S4 Chemistry: Periodic Table and Trends prep guide | Atomic radius, ionisation energy and metallic character

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students say metallic character falls across a period, then stall when asked why magnesium has a higher first ionisation energy than aluminium. Every trend in this lesson hangs on one cause: nuclear charge against shielding.

Objectives — Students can: (1) use proton number and shells to explain the rows and columns of the table; (2) explain the radius and ionisation-energy trends across a period, including the aluminium and sulfur exceptions; (3) infer oxide character, metallic character and bond type from an element’s position.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Atomic structure	Electron arrangement of sodium	2, 8, 1
Ions	Where the charges on Na+ and Cl- come from	Gain or loss of electrons
Symbols	The eight elements of period 3	Na Mg Al Si P S Cl Ar

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-12 min	How the table is built	Rows count shells, columns count outer electrons	Same group, similar chemistry
12-26 min	Atomic radius trend	Na 186 pm down to Cl 99 pm	Narrows by 1.88 times
26-40 min	First ionisation energy	Na 496 up to Ar 1521 kJ/mol	Cl / Na = 2.52
40-50 min	The two exceptions	Al below Mg by 160, S below P by 12 kJ/mol	Electron arrangement stability
50-56 min	Oxides and metallic character	Si melts at 1414 C, P at only 44 C	Ionic gives way to covalent
56-60 min	Exit check	Three questions collected	Done independently

03 Teaching script (read it out)

Scene 1: The table is an electron-arrangement map, not a list

A row (period) shares the number of shells; a column (group) shares the number of outer electrons. That is why a group behaves alike, since chemistry is decided mainly by the outer shell.
Across period 3 sodium (2, 8, 1) becomes argon (2, 8, 8). The outer count climbs from one to eight, so the character shifts from reactive metal through the metalloid silicon to a noble gas.
The transition metals form their own block because the 3d subshell fills after 4s, which is why they show variable oxidation states, coloured ions and catalytic behaviour.
One line to remember: position fixes the arrangement, the arrangement fixes the properties. No trend needs to be memorised once that chain is in place.

Task: write the electron arrangements of potassium (period 4, group 1) and bromine (period 4, group 7) and give one property of each.

Scene 2: Radius and ionisation energy: two faces of one cause

Across a period the nuclear charge rises from +11 to +17 while the added electrons enter the same shell, so shielding barely changes. The effective pull grows, the radius shrinks (Na 186 pm to Cl 99 pm, a factor of 1.88).
Down a group an extra shell appears, shielding jumps, the radius grows and the first ionisation energy falls.
First ionisation energies in period 3: Na 496, Mg 738, Al 578, Si 786, P 1012, S 1000, Cl 1251 and Ar 1521 kJ/mol. The trend is upwards overall, with chlorine 2.52 times sodium.
Two exceptions must be taught explicitly: aluminium sits 160 kJ/mol below magnesium because its electron comes from a singly occupied 3p orbital, easier to remove than a full 3s pair; sulfur sits 12 kJ/mol below phosphorus because the 3p3 half-filled set is extra stable and sulfur must also overcome a paired electron repulsion.

Task: explain the radius trend with “charge up, shielding unchanged”, then explain the aluminium exception using the 3p electron.

Scene 3: Reading properties off the position: oxides, melting points and bonding

Metal oxides (Na2O, MgO, Al2O3) give alkaline solutions; non-metal oxides (SO3, Cl2O7) give acidic ones; SiO2 is insoluble but reacts with hot alkali, the two-faced behaviour of a metalloid.
Melting points show the bonding change best: Na 98 C, Mg 650 C and Al 660 C rise with metallic bonding strength, Si peaks at 1414 C as a giant covalent network, then P collapses to 44 C, S 115 C, Cl2 -101 C as simple molecular solids held only by van der Waals forces, with Ar at -189 C as single atoms. The 1,370 C gap between silicon and phosphorus is the boundary between bonding types.
Electronegativity: Na 0.93 and Cl 3.16 differ by 2.23, and a gap above roughly 1.8 points to ionic bonding, which is why NaCl is ionic while CCl4 (about 0.5) is covalent.
Metallic character means the tendency to lose electrons, falling across a period and rising down a group, exactly mirroring the ionisation-energy trend.

Task: order MgO, SiO2 and SO3 by oxide character and write one equation each with water or alkali.

04 Question types and worked steps

Type	Example	Steps	Answer
Trend	Radius and ionisation energy	Locate period and group, then argue charge and shielding	186 to 99 pm
Exception	Irregular ionisation energy	Write the electron arrangement first	Al 160 kJ/mol lower
Inference	An unknown element	Outer electrons from the position	Same group, similar chemistry
Bonding	Oxides and melting points	Sort into metal, non-metal and metalloid	Si 1414 C against P 44 C

Sanity check (teach this): Check the units: pm for radius, kJ/mol for ionisation energy, and always compare within one period or one group, since cross-period radius comparisons mislead.
The three trends must agree: radius falls, ionisation energy rises and metallic character falls from left to right. A student claiming radius and ionisation energy both rise has contradicted themselves.
Magnitude check: first ionisation energies lie between a few hundred and about two thousand kJ/mol, so 4.96 or 49,600 signals a unit slip, since 496 kJ/mol is 496,000 J/mol.
An exception does not overturn the trend: the 160 and 12 kJ/mol dips are far smaller than the overall rise across the period, so “trending upwards” stays correct.

05 Independent practice (3 questions)

Question	Answer	What to watch
Largest and smallest radius in period 3	Na at 186 pm, Cl at 99 pm	Shrinks left to right
Ratio of Cl to Na first ionisation energy	2.52	1251 / 496
Why aluminium is below magnesium	160 kJ/mol lower	A single 3p electron leaves more easily
Melting point gap between Si and P	1,370 C	Giant covalent against simple molecular

06 Common mistakes and what to say

Mistake	Student writes	Say this
Swapping group and period	Calling group 3 a row	Rows are periods, columns are groups. Swap them and most of the paper collapses.
Answering “because that is the trend”	No mention of charge or shielding	The trend is the result. I want the cause: charge up, shielding unchanged.
Ignoring the exceptions	Claiming ionisation energy always rises left to right	It rises overall, but aluminium and sulfur are exceptions and a good answer says so.
Explaining melting point with radius	Saying silicon melts high because its atoms are small	Melting point follows bonding type. Silicon is a giant covalent network, so radius is not the argument.

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index — All chapters
Chemistry practice paper — Periodic table questions 5 to 10

08 Exit check (last 5 minutes)

Question	Answer	Decision
Radius trend across period 3	Falls left to right	Correct means bonding comes next
Why Al is below Mg	A single 3p electron	Wrong means redo the exceptions
Why NaCl is ionic	Electronegativity gap 2.23, above 1.8	Two errors means restart the first act

09 Tutor FAQ

Why do noble gases have the highest ionisation energies?

The outer shell is already full, so removing an electron breaks a stable octet and costs a great deal of energy.

Textbooks give different radius values, so which is right?

Sources use different definitions (metallic, covalent, van der Waals). The trend is what matters, and in an exam use the data given.

How large must an electronegativity gap be for ionic bonding?

There is no sharp cut-off; about 1.8 is a common guide, and lattice energy and physical state decide the rest.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.