元素周期表：化学周期性

Periodicity 周期性 means that properties repeat in a regular pattern as you go across each period 周期 of the Periodic Table. Period 3 (Na to Ar) is the standard example.

The melting point and conductivity follow from the structure and bonding:

• Na, Mg, Al are giant metallic 金属晶体. Melting points rise (Na → Al) because each atom gives more delocalised electrons and the ions get smaller, so the bonding is stronger. They conduct well.
• Si is giant molecular 原子晶体 (giant covalent). It has the highest melting point, because strong covalent bonds must be broken. It barely conducts.
• P, S, Cl, Ar are simple molecular 分子晶体 (or single atoms). Their melting points are low, because the only forces that break are the weak van der Waals' forces 范德华力 between the molecules - the strong covalent bonds inside each molecule are never broken at all. They do not conduct.

Name van der Waals' forces when you explain that drop: it is the marking point, and "the covalent bonds are weak" is the error that loses it. Melting $\text{S}_8$ pulls whole molecules apart from each other; it does not touch a single $\text{S–S}$ bond. Their sizes still follow the molecule: $\text{S}_8$ melts higher than $\text{P}_4$ because it is bigger, with more electrons and so stronger van der Waals' forces, while $\text{Ar}$ (a single atom) is lowest of all.

Reactions with oxygen, chlorine and water

With oxygen:

With chlorine:

With water (only Na and Mg react):

Sodium reacts fast; magnesium reacts only very slowly with cold water.

Oxidation number of the oxides and chlorides

The oxidation number 氧化数 of the Period 3 element in its oxide or chloride rises across the period, because it equals the number of outer-shell (valence shell 价层) electrons 电子 the atom uses in bonding:

The chlorides $\text{NaCl}$, $\text{MgCl}_2$, $\text{AlCl}_3$, $\text{SiCl}_4$, $\text{PCl}_5$ show oxidation numbers $+1$ to $+5$ in the same way.

Oxides with water, and acid–base behaviour

Across the period the oxides 氧化物 change from basic to acidic:

Metal oxides (left) are basic; non-metal oxides (right) are acidic. $\text{Al}_2\text{O}_3$ and its hydroxide 氢氧化物 $\text{Al(OH)}_3$ are amphoteric — they react with both acids and bases:

Chlorides with water

• $\text{NaCl}$ and $\text{MgCl}_2$ are ionic. They simply dissolve, giving a near-neutral solution.
• $\text{SiCl}_4$ and $\text{PCl}_5$ are covalent. They undergo hydrolysis 水解 (react with water) to make acidic solutions and fumes of $\text{HCl}$:

Explaining the trends

These trends follow from the change in bonding and electronegativity 电负性. On the left, the elements are metals with low electronegativity, so their oxides and chlorides are ionic and basic (or neutral). On the right, the elements are non-metals with high electronegativity, so their oxides and chlorides are covalent and acidic. You can use a chloride's or oxide's properties (melting point, conductivity, effect on water) to suggest whether its bonding is ionic or covalent.

Worked example. Predict the pH when $\text{Na}_2\text{O}$, $\text{Al}_2\text{O}_3$ and $\text{SO}_3$ are each added to water. Acid-base character follows the metal to non-metal change across Period 3. $\text{Na}_2\text{O}$ is ionic and basic: it dissolves to give $\text{NaOH}$, so the pH is about 13. $\text{SO}_3$ is covalent and acidic: it reacts to give $\text{H}_2\text{SO}_4$, so the pH is about 1. $\text{Al}_2\text{O}_3$ sits at the changeover - it is amphoteric and essentially insoluble in water, so the pH stays about 7. The trap is that last one: amphoteric does not mean neutral, it means the oxide reacts with acids and with alkalis - it simply does not react with water.

The same idea works for any group. If you know the pattern down a group and across a period, you can:

• predict the properties of an element from its position (for example, a Group 1 element will be a reactive metal forming a $+1$ ion).
• deduce the likely position and identity of an unknown element from its physical and chemical properties.

Common mistakes.

• Explaining the atomic-radius trend by "more electrons". Across the period the electrons go into the same shell with similar shielding, while the nuclear charge rises, so the outer electrons are pulled closer.
• Explaining silicon's high melting point with intermolecular forces. Silicon is a giant covalent lattice: melting means breaking many strong covalent bonds.
• Saying $\text{Al}_2\text{O}_3$ gives a neutral solution because it is amphoteric. It is insoluble, so the pH stays 7; "amphoteric" means it reacts with both acids and bases.
• Saying $\text{SiO}_2$ is basic because it does not dissolve. $\text{SiO}_2$ is weakly acidic: it reacts with hot, concentrated $\text{NaOH}$ but not with acids.
• Saying $\text{SiCl}_4$ dissolves in water like $\text{NaCl}$. Covalent chlorides are hydrolysed, giving steamy fumes of HCl and an acidic solution.
• Giving the same oxidation number to sulfur in both of its oxides. Sulfur is $+4$ in $\text{SO}_2$ and $+6$ in $\text{SO}_3$.

• Always pair the trend with its reason: atomic radius falls across Period 3 (rising nuclear charge, similar shielding).
• Explain the melting-point pattern by structure: giant metallic (Na→Al), giant covalent (Si, highest), then simple molecular ($\text{P}_4$, $\text{S}_8$, $\text{Cl}_2$) and monatomic (Ar).
• Oxides go basic → amphoteric ($\text{Al}_2\text{O}_3$) → acidic across the period; link to ionic-to-covalent bonding.
• For reactions of oxides/chlorides with water, give the products and the resulting pH.