4.1.1 Periods and Groups of Elements

This topic explains the horizontal rows (periods) and vertical columns (groups) of the periodic table in detail.

Definition

Periods The horizontal rows of the periodic table are called periods.

Groups Each vertical column of elements in the periodic table is called a group or family.

Simple explanation: Periods go left to right; groups go top to bottom.

Main Idea

The most commonly used form of the periodic table shows elements listed in order of increasing atomic numbers, from left to right and from top to bottom.

• Hydrogen (H) is in the top left corner.
• Helium (He), atomic number 2, is at the top right corner.
• Lithium (Li), atomic number 3, is at the left end of the second row.

Simple explanation: The periodic table is arranged like a grid — periods are rows, groups are columns.

Core Concept

Periods

• There are seven periods in the periodic table.
• The number of elements per period ranges from 2 in period 1 to 32 in period 6.
• First three periods are called short periods.
• The remaining periods are called long periods.
• The properties of elements within a period change gradually as you move from left to right.
• But when you move from one period to the next, the pattern of properties within a period repeats. This is in accordance with the periodic law.

Groups

• Each vertical column is called a group or family.
• Two numbering systems are used:
• Traditional system (old IUPAC): letters A and B are used.
• First two groups: IA and IIA
• Last six groups: IIIA to VIIIA
• Middle groups: B (transition elements)
• Modern IUPAC system: groups numbered 1–18 from left to right.

Simple explanation: Groups are numbered differently in different systems, but they represent the same columns.

Detailed Explanation

Number of Elements in Each Period

Simple explanation: The first three periods are short (few elements), and the later periods are long (many elements).

Group Numbering Systems

Traditional System (A and B):

Modern IUPAC System (1–18):

Names of Some Groups

Some groups of elements in the periodic table have been given group names:

• Group 1 (IA): Alkali metals
• Group 2 (IIA): Alkaline earth metals
• Group 17 (VIIA): Halogens
• Group 18 (VIIIA): Noble gases (because they do not readily undergo chemical reactions)

Simple explanation: These names help us remember the properties of elements in those groups.

Group Number and Valence Electrons

• The elements in the same group have the same number of valence electrons.
• Group number indicates the number of valence electrons in an element.

• Group 1 and Group 2 elements have 1 and 2 valence electrons respectively.
• Group 13 elements have 3 valence electrons.
• Group 14 elements have 4 valence electrons.
• Group 15 elements have 5 valence electrons, and so on.

Important note for Groups 13–18 (p-block elements): - The number of valence electrons is equal to group number minus 10.

• Group 13 → 13 − 10 = 3 valence electrons
• Group 14 → 14 − 10 = 4 valence electrons
• Group 17 → 17 − 10 = 7 valence electrons

Normal/Representative Elements and Transition Elements

• Group A elements are called normal or representative elements. They are also called main group elements.
• Group B elements are called transition elements.

Simple explanation: Group A elements show clear periodic trends; Group B elements (transition metals) have more complex behaviour.

Intermediate Level

Why Periods Have Different Lengths

The length of a period depends on the type of sub-shell that fills:

• Period 1: fills 1s (2 electrons) → 2 elements
• Period 2: fills 2s, 2p (2 + 6 = 8) → 8 elements
• Period 3: fills 3s, 3p (2 + 6 = 8) → 8 elements
• Period 4: fills 4s, 3d, 4p (2 + 10 + 6 = 18) → 18 elements
• Period 5: fills 5s, 4d, 5p (2 + 10 + 6 = 18) → 18 elements
• Period 6: fills 6s, 4f, 5d, 6p (2 + 14 + 10 + 6 = 32) → 32 elements
• Period 7: fills 7s, 5f, 6d, 7p (2 + 14 + 10 + 6 = 32) → 32 elements

Simple explanation: The length of a period depends on how many electrons can fit in the sub-shells being filled.

Groups and Valence Electrons

Rule for Groups 13–18: Valence electrons = Group number − 10.

Group A vs Group B Elements

Important Points

• - Periods: horizontal rows; there are 7 periods.
• - Groups: vertical columns; there are 18 groups in the modern IUPAC system.
• - Number of elements per period: 2, 8, 8, 18, 18, 32, 32.
• - First three periods are short periods; the rest are long periods.
• - Group number indicates the number of valence electrons (for Groups 1, 2, and 13–18).
• - For Groups 13–18: valence electrons = group number − 10.
• - Group A elements = normal/representative/main group elements.
• - Group B elements = transition elements.
• - Names of groups: Group 1 (alkali metals), Group 2 (alkaline earth metals), Group 17 (halogens), Group 18 (noble gases).
• - Properties repeat periodically — periodic law.

Remember

• - **Periods** = rows (7).
• - Groups = columns (18 in IUPAC).
• - Elements per period: 2, 8, 8, 18, 18, 32, 32.
• - First 3 periods = short; rest = long.
• - Group number indicates valence electrons (Groups 1, 2, 13–18).
• - Groups 13–18: valence electrons = group number − 10.
• - Group A = representative elements; Group B = transition elements.
• - Group names: alkali metals (1), alkaline earth metals (2), halogens (17), noble gases (18).
• - Periodic law: properties repeat periodically.

Common Mistakes

• 1. Confusing periods and groups. Periods are rows; groups are columns.
• 2. Thinking all periods have the same number of elements. They range from 2 to 32.
• 3. Forgetting the Group 13–18 rule. Valence electrons = group number − 10.
• 4. Confusing Group A and Group B. Group A = representative; Group B = transition.
• 5. Thinking group numbers are always the same as valence electrons. This is true for Groups 1, 2, and 13–18, but not for transition elements.
• 6. Forgetting the special names. Alkali metals (1), alkaline earth metals (2), halogens (17), noble gases (18).

Quick Revision

• Periods: horizontal rows; there are 7 periods.
• Groups: vertical columns; there are 18 groups in the modern IUPAC system.
• Number of elements per period: 2, 8, 8, 18, 18, 32, 32.
• First three periods are short; the rest are long.
• Group number indicates the number of valence electrons (for Groups 1, 2, 13–18).
• For Groups 13–18: valence electrons = group number − 10.
• Group A elements = normal/representative/main group elements.
• Group B elements = transition elements.
• Names of groups: alkali metals (1), alkaline earth metals (2), halogens (17), noble gases (18).
• Properties repeat periodically — periodic law.

60-Second Revision

• **Periods** = rows (7).
• **Groups** = columns (18).
• Elements per period: 2, 8, 8, 18, 18, 32, 32.
• Group number = valence electrons (Groups 1, 2, 13–18).
• Groups 13–18: valence electrons = group − 10.
• Group 1 = alkali metals; Group 2 = alkaline earth metals; Group 17 = halogens; Group 18 = noble gases.
• Warning: don't confuse periods (rows) with groups (columns).