Elements in Period 3 and transition elements

This page covers a single Form 4 content standard: elements in Period 3 and the transition elements. It has two parts, the trends you see going across a period, and the special properties of the transition metals, and the exam tests both. Learn the Period 3 pattern (metal to non-metal, basic to acidic oxide) and the four transition-metal characteristics, and you will have this standard covered.

Going across Period 3

Period 3 runs from sodium (Na) to argon (Ar): Na, Mg, Al, Si, P, S, Cl, Ar. As you move from left to right across the period, the proton number increases by one each time, and the number of valence electrons increases from 1 to 8. Several properties change in a regular way:

• Metallic to non-metallic character. The elements change from metals (sodium, magnesium, aluminium), through a metalloid (silicon), to non-metals (phosphorus, sulfur, chlorine), ending with the noble gas argon.
• Atomic size (radius) decreases across the period. Even though electrons are being added, they go into the same number of shells (three), while the proton number increases. The larger nuclear charge pulls the electrons in more strongly, so the atom gets smaller.
• Electronegativity increases across the period, because the smaller atoms with more protons attract bonding electrons more strongly.

The oxides across Period 3

A favourite exam point is how the acid–base nature of the oxides changes across Period 3:

• Metals form basic oxides, sodium oxide (Na₂O) and magnesium oxide (MgO) are basic.
• Aluminium oxide (Al₂O₃) is amphoteric, it can react as both an acid and a base. This is the special case to remember.
• Non-metals form acidic oxides, the oxides of phosphorus, sulfur and chlorine are acidic.

So across the period, the oxides change from basic → amphoteric → acidic, mirroring the change from metal to non-metal.

The transition elements

The transition elements are the block of metals between Group 2 and Group 13, for example the Period 4 metals from scandium to zinc, which includes familiar metals such as iron, copper and zinc. As metals they are hard and strong, with high melting points, high boiling points and high densities. What makes them special are four characteristics you must know:

• They form ions with variable oxidation numbers. For example, iron forms both Fe²⁺ and Fe³⁺, and copper forms Cu⁺ and Cu²⁺.
• They form coloured compounds and ions. For example, Cu²⁺ solutions are blue, Fe²⁺ is green, and Fe³⁺ is brown/yellow.
• They act as catalysts. For example, iron is the catalyst in the Haber process and nickel is used in the hydrogenation of oils.
• They form complex ions, in which the metal ion is surrounded by other molecules or ions.

Worked example

Question. Consider the elements of Period 3. (a) State and explain how the atomic radius changes across the period from sodium to chlorine. (b) State how the acid–base nature of the oxides changes across the period.

Step 1, State the atomic-radius trend. The atomic radius decreases from sodium to chlorine.

Step 2, Explain the trend. Across the period the proton number increases, so the nuclear charge increases, but the electrons are added to the same number of shells (three shells). The greater nuclear charge attracts the outer electrons more strongly and pulls them closer, so the atom becomes smaller.

Step 3, State the oxide trend. The oxides change from basic (Na₂O, MgO), through amphoteric (Al₂O₃), to acidic (the oxides of P, S and Cl).

Answer. (a) The atomic radius decreases because the increasing nuclear charge pulls electrons in the same three shells closer to the nucleus. (b) The oxides change from basic, through amphoteric, to acidic across the period.

Practice question

Question. Iron is a transition element. (a) State two special characteristics of transition elements. (b) Give one example that illustrates each characteristic you named.

Answer. (a) Any two of: they form ions with variable oxidation numbers; they form coloured compounds; they act as catalysts; they form complex ions. (b) For example: iron forms both Fe²⁺ and Fe³⁺ (variable oxidation number); an Fe²⁺ solution is green while an Fe³⁺ solution is brown (coloured compounds); iron is used as a catalyst in the Haber process (catalytic property).

Exam tip

Two traps catch students here. First, atomic radius decreases across a period but increases down a group, do not mix them up, and always explain the across-period trend by the increasing nuclear charge pulling electrons in the same number of shells closer. Second, for the oxides, remember the special case: aluminium oxide is amphoteric, sitting between the basic metal oxides and the acidic non-metal oxides. For the transition elements, memorise the four characteristics as a set, variable oxidation numbers, coloured compounds, catalysts, complex ions, and have one concrete example ready for each, because the exam often asks you to illustrate a characteristic rather than just name it.

Where this fits

This standard completes the The Periodic Table of Elements chapter by pulling together the periodic trends and introducing the transition metals you will meet again in later chapters such as Redox and industrial chemistry. Revise the Period 3 trends and the transition-metal characteristics with the revision notes and test your explanations with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you can explain the atomic-radius trend correctly and can illustrate each transition-metal property with an example, which is where the marks are on this standard across SPM Chemistry.

Quick recap

• Across Period 3: elements change metal → metalloid → non-metal → noble gas, and atomic size decreases.
• Period 3 oxides change basic → amphoteric (Al₂O₃) → acidic.
• Transition elements are hard metals with high melting points and densities.
• Transition characteristics: variable oxidation numbers, coloured compounds, catalysts, complex ions.