G3 Chemistry study guide

The Periodic Table: groups, trends and questions

Learn how atomic number, periods and groups fit together, then test the patterns with eight worked questions. This guide uses the group numbers in Singapore's 2027 SEC G3 Chemistry syllabus.

Content checked against the 2027 SEC G3 Chemistry syllabus (K324).

Start with the pattern

How to read the Periodic Table

The Periodic Table arranges elements in order of increasing proton (atomic) number. Its position clues connect this topic to atomic structure and chemical bonding.

Proton number

The number of protons in an atom's nucleus. It identifies the element.

Relative atomic mass, Ar

The average mass of the atoms of an element, weighted by the abundance of its isotopes, relative to one-twelfth of the mass of a carbon-12 atom.

Period

A horizontal row. For the elements studied here, the period number tells you how many electron shells an atom has. Sodium and phosphorus are in Period 3.

Group

A vertical column. Main-group elements in the same group have the same number of outer-shell electrons, so they often show similar chemical properties.

Do not read the modern group number as the valence-electron count. Groups 1 and 2 have one and two outer electrons; Groups 13 to 18 usually have three to eight. Helium is the exception in Group 18, with two. This simple rule does not apply to transition elements.

Simplified main-group position map
Period Group 1 Group 2 Group 13 Group 14 Group 15 Group 16 Group 17 Group 18
1 Hhydrogen   Hehelium
2 Lilithium Beberyllium Bboron Ccarbon Nnitrogen Ooxygen Ffluorine Neneon
3 Nasodium Mgmagnesium Alaluminium Sisilicon Pphosphorus Ssulfur Clchlorine Arargon

On a small screen, swipe sideways within the table to see every group. This is a teaching map, not the complete Periodic Table. Hydrogen is placed in Group 1 but is not an alkali metal.

For example, sodium has proton number 11 and electron arrangement 2,8,1, placing it in Period 3, Group 1. Chlorine has proton number 17 and arrangement 2,8,7, placing it in Period 3, Group 17.

Across Period 3, elements generally change from metallic on the left to non-metallic on the right. The proton number increases, while the number of occupied shells stays at three.

Question 1 of 8

Which property decreases across Period 3 from sodium to chlorine?

  1. Number of occupied electron shells
  2. Metallic character
  3. Proton number
  4. Number of outer-shell electrons
Show answer and explanation

Answer: B, metallic character. Across Period 3, the proton number and outer-shell electron count increase. The atoms still have three occupied shells, while metallic character generally decreases.

The alkali metals

Group 1: lithium, sodium and potassium

These are relatively soft, low-density metals with one outer-shell electron. They react with water to produce a metal hydroxide solution and hydrogen gas. The hydroxide makes the solution alkaline.

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Melting point decreases down Group 1
The metal ions become larger, so the attraction between positive ions and delocalised electrons weakens.
Reaction with water becomes more vigorous
The outer electron is farther from the nucleus and more shielded, making it easier to lose.
Low density and softness
Lithium, sodium and potassium float on water and can be cut. Density does not rise perfectly down the group: potassium is less dense than sodium.
Predicting ions
A Group 1 atom loses one outer electron to form a 1+ ion, such as Na+ or K+.

Question 2 of 8

Which listed properties does sodium have?

1. Conducts electricity   2. Is a hard solid   3. Has high density

  1. 1 only
  2. 1 and 2 only
  3. 1 and 3 only
  4. 1, 2 and 3
Show answer and explanation

Answer: A, 1 only. Sodium conducts electricity, but it is soft and has low density.

Question 3 of 8 · extension example

Rubidium is in Group 1. Which statement is likely to be correct?

  1. It forms a sulfate with formula Rb2SO4.
  2. It forms an insoluble hydroxide.
  3. It can be extracted from its ore by reduction with carbon.
  4. It reacts slowly with water at 20 °C.
Show answer and explanation

Answer: A. Rubidium forms Rb+; sulfate is SO42−, so two rubidium ions are needed. Rubidium hydroxide is soluble, rubidium is too reactive for extraction by carbon reduction at this level, and its reaction with water is vigorous. Rubidium extends the Li/Na/K examples named in the SEC syllabus.

The halogens

Group 17: chlorine, bromine and iodine

Halogens are non-metals with seven outer-shell electrons. They occur as diatomic molecules. The 2027 G3 syllabus names chlorine, bromine and iodine for their colours, states and displacement reactions.

Cl2 · chlorineYellow-green gas at room temperature
Br2 · bromineReddish-brown liquid at room temperature
I2 · iodineGrey-black solid at room temperature

Down Group 17, the halogens become darker and change from gas to liquid to solid at room temperature.

Melting and boiling points increase down the group
Larger molecules have stronger London dispersion forces between them, so more energy is needed to separate the molecules.
Reactivity decreases down the group
Chlorine can displace bromide and iodide ions from solution; bromine can displace iodide ions. Iodine cannot displace chloride or bromide ions.

A displacement reaction

A more reactive halogen replaces a less reactive halogen in a solution of its halide. For example, chlorine displaces bromine from potassium bromide:

Cl2 + 2KBr → 2KCl + Br2

Question 4 of 8

Chlorine water is added to potassium bromide solution. Which new substances form?

  1. Potassium chloride and bromine
  2. Potassium bromide and chlorine
  3. Potassium fluoride and bromine
  4. Potassium chloride and iodine
Show answer and explanation

Answer: A. Chlorine is more reactive than bromine, so it displaces bromine from bromide ions. The products are potassium chloride and bromine.

Question 5 of 8

Which statement about iodine is not correct?

  1. It exists as I2 molecules.
  2. It can form iodide ions, I−.
  3. It is more reactive than chlorine.
  4. It is a grey-black solid at room temperature.
Show answer and explanation

Answer: C. Reactivity decreases down Group 17. Iodine lies below chlorine and is less reactive.

The noble gases

Group 18: full outer electron shells

Helium has two electrons in its first and only shell. Neon, argon and the other familiar noble gases have eight outer-shell electrons. These arrangements make the gases very unreactive under ordinary conditions.

Noble gases exist as single atoms rather than molecules at room temperature. Their boiling points are low, but generally increase down the group because dispersion forces between larger atoms are stronger. Their low reactivity makes them useful for inert environments, such as argon in light bulbs or steelmaking.

Remember helium: a full outer shell does not always mean eight electrons. Helium's first shell is full with two.

Question 6 of 8

Which statements about noble gases are correct?

1. Argon reacts readily with oxygen.   2. Helium and neon both have eight outer-shell electrons.   3. Noble gases exist as single-atom gases at room temperature.

  1. 2 only
  2. 3 only
  3. 2 and 3 only
  4. 1, 2 and 3
Show answer and explanation

Answer: B, 3 only. Argon is very unreactive under ordinary conditions. Helium has two electrons in its full outer shell, whereas neon has eight. Noble gases are monatomic at room temperature.

Beyond the main groups

Typical transition elements

The transition elements sit between Groups 2 and 13. Typical transition elements have high melting points and densities, can form ions with different oxidation states, often form coloured compounds, and can act as catalysts or form catalytic compounds. These are typical patterns, not rules for every individual element.

Why the valence shortcut stops here: the simple “group number tells outer electrons” rule used for the main groups does not describe the transition elements.

Put the patterns together

Final practice questions

Question 7 of 8

Which statement about the labelled positions is not correct?

V, W, X, Y and Z are placeholders, not element symbols. Calcium (Ca) is shown only to keep the position map complete.

Positions used in Question 7
Period Group 1 Group 2 Group 17
3 V W X
4 Y Ca Z
  1. V is less reactive than Y.
  2. W has less metallic character than V.
  3. Y has a higher melting point than V.
  4. Z is less reactive than X.
Show answer and explanation

Answer: C. V and Y are in Group 1, with Y below V. Group 1 melting points decrease down the group, so Y has a lower melting point. A is true because Group 1 water reactivity rises down the group; B follows the decrease in metallic character across Period 3; D follows the decrease in Group 17 reactivity down the group.

Question 8 of 8

Which statement about groups in the Periodic Table is correct?

  1. All groups contain both metals and non-metals.
  2. Atoms of elements in the same group have the same total number of electrons.
  3. In Group 1, reactivity decreases from lithium to potassium.
  4. In Group 17, melting point increases from chlorine to iodine.
Show answer and explanation

Answer: D. Melting point increases from Cl2 to Br2 to I2. A is false because Group 18 contains non-metals; B confuses total electrons with outer-shell electrons; C reverses the Group 1 water-reaction trend. Hydrogen's position in Group 1 is another reason not to call every Group 1 element an alkali metal.

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