Trends in Chemical Properties of Period 3 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Period 3 Elements and Their Compounds

4 lessons

Lesson 1 of 4

Physical Properties of the Period 3 Elements

25 min

Eight elements, one shell

Walk along the third row of the periodic table and you pass a soft metal you can cut with a knife (sodium), the light metal of window frames and cooking pans (aluminium), the grey semiconductor at the heart of every phone chip (silicon), the yellow solid that crusts volcanic vents (sulfur), a green poisonous gas (chlorine) and finally an unreactive gas (argon). Yet all eight atoms fill the same outer shell, the third, adding one proton and one electron at each step. This lesson explains how such small steps produce such different elements, and the regular trends hidden underneath.

Definition

Period: a horizontal row of the periodic table. All the elements in Period 3 have their outer electrons in the third shell ( $n = 3$ ), from $[\mathrm{Ne}]\,3s^1$ (Na) to $[\mathrm{Ne}]\,3s^2\,3p^6$ (Ar).

Effective nuclear charge: the net positive charge felt by an outer electron, roughly the nuclear charge minus the shielding by inner electrons. Across Period 3 the nuclear charge rises from +11 to +18, but the inner shielding (the 10 electrons of $[\mathrm{Ne}]$ ) stays the same, so the outer electrons are pulled in more strongly.

Periodicity: the regular, repeating pattern of properties across each period of the table.

Structure and bonding change across the period

Table 14.1: Physical properties of the Period 3 elements

Element	Outer electrons	Structure	m.p. / K	b.p. / °C	Covalent radius / pm	First IE / kJ mol⁻¹	Electronegativity (Pauling)	Electrical conductivity
Na	$3 s^{1}$	giant metallic	370.9	883	155	495.8	0.93	good ( $2.10 \times 10^{7}\ \mathrm{S\,m^{-1}}$ )
Mg	$3 s^{2}$	giant metallic	923	1090	139	737.7	1.31	good ( $2.28 \times 10^{7}\ \mathrm{S\,m^{-1}}$ )
Al	$3s^2\,3p^1$	giant metallic	933.5	2519	126	577.5	1.61	very good ( $3.77 \times 10^{7}\ \mathrm{S\,m^{-1}}$ )
Si	$3s^2\,3p^2$	giant covalent	1687	3265	116	786.5	1.90	semiconductor
P (white)	$3s^2\,3p^3$	simple molecular, $\mathrm{P_4}$	317.3	280.5	111	1011.8	2.19	none
S	$3s^2\,3p^4$	simple molecular, $\mathrm{S_8}$	388.4	444.7	103	999.6	2.58	none
Cl	$3s^2\,3p^5$	simple molecular, $\mathrm{Cl_2}$	171.6	−34.0	99	1251.2	3.16	none
Ar	$3s^2\,3p^6$	single atoms	83.8	−185.8	(96)	1520.6	none	none

Conductivities are at 20 °C, from measured resistivities. The m.p. of sulfur is for the monoclinic form.

Figure 14.1: Melting points across Period 3, with the type of structure shaded above. The peak at silicon is a giant covalent lattice; the collapse at phosphorus marks the change to small molecules.

Na, Mg, Al: giant metallic lattices. Positive ions sit in a sea of delocalised electrons. From Na to Al each atom gives 1, 2, then 3 electrons to the sea, and the ions become smaller and more highly charged, so metallic bonding gets stronger: melting point, boiling point and conductivity all rise.
Si: a giant covalent lattice. Every silicon atom is bonded to four others in a diamond-like network. Melting means breaking many strong Si–Si covalent bonds, so silicon has the highest melting point in the period. Its electrons are held in bonds, so it is a semiconductor, not a metal.
P, S, Cl: simple molecules ( $\mathrm{P_4}$ , $\mathrm{S_8}$ , $\mathrm{Cl_2}$ ). Melting only overcomes weak London forces between molecules, so melting points are low and depend on the size of the molecule.
Ar: separate atoms with the weakest London forces of all.

Figure 14.2: Covalent (single-bond) radius from sodium to chlorine. Argon is left out: it forms almost no bonds, so its radius is not measured the same way.

Atomic radius decreases across the period: each step adds one proton, while the extra electron enters the same shell and shields very little. The effective nuclear charge rises, so the outer shell is pulled closer. For the same reason electronegativity increases from 0.93 (Na) to 3.16 (Cl), and first ionisation energy increases overall (Figure 14.3), with two small dips.

Figure 14.3: First ionisation energies across Period 3. The overall rise follows the increasing effective nuclear charge; the dips at Al and S are explained in Worked example 2.

Worked example 1: why sulfur melts above phosphorus and chlorine

Use the formulas of the molecules to explain the order of melting points: S (388.4 K) > P (317.3 K) > Cl (171.6 K) > Ar (83.8 K).

Solution. All four are held together only by London forces, which grow with the number of electrons in each particle. $\mathrm{S_8}: 8 \times 16 = 128\ e^- \qquad \mathrm{P_4}: 4 \times 15 = 60\ e^- \qquad \mathrm{Cl_2}: 2 \times 17 = 34\ e^- \qquad \mathrm{Ar}: 18\ e^-$ The order of electrons per particle, $\mathrm{S_8} > \mathrm{P_4} > \mathrm{Cl_2} > \mathrm{Ar}$ , is exactly the order of melting points. The melting point depends on the size of the molecule, not on the atomic number of the element.

Worked example 2: the two dips in ionisation energy

Explain why the first ionisation energy of aluminium (577.5 kJ mol⁻¹) is lower than that of magnesium (737.7), and that of sulfur (999.6) is lower than that of phosphorus (1011.8).

Solution. Mg → Al. Magnesium loses a $3 s$ electron; aluminium loses its single $3 p$ electron ( $3s^2\,3p^1$ ). A $3 p$ electron is higher in energy than a $3 s$ electron and is partly shielded by the $3 s^{2}$ pair, so it is easier to remove despite the extra proton.

P → S. Phosphorus has three unpaired $3 p$ electrons, one in each orbital ( $\uparrow\ \uparrow\ \uparrow$ ). In sulfur ( $\uparrow\downarrow\ \uparrow\ \uparrow$ ) two electrons share one orbital and repel each other; removing one of this pair needs less energy.

Warning

Common mistakes. (1) Explaining the low melting point of phosphorus by "weak covalent bonds": the P–P bonds inside $\mathrm{P_4}$ do not break on melting; only the London forces between molecules are overcome. (2) Saying atoms get bigger across a period because they have more electrons: they get smaller, because the extra electrons go into the same shell while the nuclear charge rises. (3) Calling silicon a metal because it is shiny and grey: it is a metalloid and a semiconductor. (4) Forgetting that sulfur and phosphorus exist as $\mathrm{S_8}$ and $\mathrm{P_4}$ molecules.

Safety. Sodium is stored under oil and cut with a dry knife on a tile; it reacts violently with water, including sweat. White phosphorus catches fire in air at about 30 °C and is kept under water; schools use red phosphorus instead. Chlorine is toxic and is used only in a fume cupboard. Wear goggles throughout.

Tip

The melting-point mountain: it climbs with metallic bonding (Na → Al), peaks at the giant covalent lattice (Si), then falls off a cliff to simple molecules (P, S, Cl) and single atoms (Ar). Across the period, everything about the nucleus's grip goes up (IE, electronegativity) and the radius goes down.

chemistNATE: Melting Point Trend Across Period 3 (4 min). Watch for how the type of structure (metallic, covalent network, molecular) decides how each element melts.

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Practice

Write the electron configurations of Mg and of S in the form $[\mathrm{Ne}]\ldots$ .
State and explain the trend in atomic radius from Na to Cl.
Explain why aluminium has a higher melting point and better electrical conductivity than sodium.
Explain why silicon has the highest melting point in Period 3.
Explain why white phosphorus melts at a lower temperature than sulfur.
Explain why the first ionisation energy of argon is the highest in the period.
Calculate the electrical resistivity of aluminium from its conductivity, $3.77 \times 10^{7}\ \mathrm{S\,m^{-1}}$ , and compare it with sodium's resistivity of $4.77 \times 10^{-8}\ \Omega\,\mathrm{m}$ .
An element of Period 3 melts at 1687 K, does not conduct when pure and cold but conducts a little when heated, and has an electronegativity of 1.90. Identify it and describe its structure.

Show answersHide answers

Mg: $[\mathrm{Ne}]\,3s^2$ ; S: $[\mathrm{Ne}]\,3s^2\,3p^4$ .
It decreases. The nuclear charge rises by one each step, but the added electron enters the same third shell, so shielding hardly changes; the greater effective nuclear charge pulls the outer shell in.
Al gives 3 electrons per atom to the delocalised sea (Na gives 1), and $\mathrm{Al^{3+}}$ is smaller and more highly charged than $\mathrm{Na^+}$ . The attraction between ions and electrons is stronger (higher m.p.) and there are more mobile electrons (better conduction).
It is a giant covalent lattice: melting requires breaking a very large number of strong Si–Si covalent bonds.
Both are simple molecular. $\mathrm{S_8}$ has 128 electrons, $\mathrm{P_4}$ only 60, so London forces between $\mathrm{S_8}$ molecules are stronger.
Argon has the greatest nuclear charge in the period (+18) with the same inner shielding, so its outer electrons feel the largest effective nuclear charge and are held most tightly; its $3 p$ sub-shell is full.
$\rho = \frac{1}{\sigma} = \frac{1}{3.77 \times 10^{7}} = 2.65 \times 10^{-8}\ \Omega\,\mathrm{m}$ , about 0.56 times sodium's: aluminium is the better conductor.
Silicon: a giant covalent (diamond-like) lattice in which each atom is bonded to four others; a semiconductor.

Quick check

Which element of Period 3 has the highest melting point, and why?

Key points

Across Period 3 the structure changes: giant metallic (Na, Mg, Al) → giant covalent (Si) → simple molecular ( $\mathrm{P_4}$ , $\mathrm{S_8}$ , $\mathrm{Cl_2}$ ) → single atoms (Ar).
Melting point rises from Na to a peak at Si, then drops sharply; among the molecular elements it follows the size of the molecule ( $\mathrm{S_8} > \mathrm{P_4} > \mathrm{Cl_2} > \mathrm{Ar}$ ).
Atomic radius decreases and electronegativity increases across the period, because effective nuclear charge rises while shielding stays the same.
First ionisation energy rises overall, with dips at Al ( $3 p^{1}$ ) and S (paired $3 p$ electrons). Conductivity: Na < Mg < Al (metals), Si semiconductor, the rest insulators.

Sources and credits

Image: Sodium sample.jpg by Nefronus, via Wikimedia Commons, licensed under CC0.
Data: Melting points, Boiling points, Atomic radii, Electronegativities, Electrical resistivities and Molar ionization energies of the elements (data pages), Wikipedia contributors, licensed under CC BY-SA 4.0, adapted by Ganzaa.
Video: Melting Point Trend Across Period 3, chemistNATE, embedded from YouTube.
The text, table layout, charts, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Eight elements, one shell

2Structure and bonding change across the period

Eight elements, one shell

Structure and bonding change across the period