The modern periodic table and its periods

Line them up by atomic number

The modern periodic table arranges elements in order of increasing atomic number. When you do this, elements with similar properties turn up again at regular intervals, so they land in the same vertical column.

The horizontal rows are called periods and the vertical columns are called groups. Along a period the outer electrons keep changing, so properties change step by step. Down a group the outer-shell arrangement is the same, so chemical properties are similar. This repeating pattern is called periodicity.

There are seven periods. Each period except the first starts with an alkali metal. Because each shell holds a fixed number of electrons, each period holds a fixed number of elements. Period 1 has 2, periods 2 and 3 have 8, periods 4 and 5 have 18, and periods 6 and 7 have 32.

Periods 6 and 7 each contain a series of 14 extra elements. To keep the table a manageable shape, these two rows sit at the bottom: the lanthanides (after lanthanum, 57) and the actinides (after actinium, 89). Together they are called rare earths or f-block elements.

Periods are rows (properties change along them); groups are columns (properties repeat down them).

Key terms

Period

Filling one shell.

Group

Elements with similar properties.

Periodicity

Why the table works.

Lanthanides

Bottom row, period 6.

Actinides

Bottom row, period 7.

Short questions with model answers

1. Q1. What is the significance of the word 'periodic'? (Short question ii, p. 117)

   • Meaning
   • In the table

   It tells us that properties repeat at regular intervals when elements are arranged by atomic number.

2. Q2. Why were the lanthanides and actinides placed at the bottom of the table? (p. 106)

   • Too many to fit
   • Keep the shape

   Because of the space problem: it keeps the table manageable and presentable.

Common mistakes

• ✗ The modern table is arranged by atomic mass.

  ✓ It uses atomic number (Moseley); Mendeleev used atomic mass (p. 107).

• ✗ Groups are the horizontal rows.

  ✓ Groups are columns; periods are rows (p. 105).

• ✗ Every period has 8 elements.

  ✓ 2, 8, 8, 18, 18, 32, 32 (Table 8.1).

MCQs

1. 1. The modern periodic table is arranged by increasing:

   1. (a) Atomic mass
   2. (b) Atomic number
   3. (c) Density
   4. (d) Melting point
   Show answer

   (b) p. 105.

2. 2. How many periods are there?

   1. (a) 7
   2. (b) 8
   3. (c) 18
   4. (d) 32
   Show answer

   (a) p. 106.

3. 3. How many elements are in period 4?

   1. (a) 2
   2. (b) 8
   3. (c) 18
   4. (d) 32
   Show answer

   (c) Table 8.1: long period.

4. 4. The lanthanides start after:

   1. (a) Actinium (89)
   2. (b) Lanthanum (57)
   3. (c) Barium (56)
   4. (d) Uranium (92)
   Show answer

   (b) p. 106.

5. 5. Elements in the same group have similar chemical properties because they have the same:

   1. (a) Atomic mass
   2. (b) Number of shells
   3. (c) Outer-shell configuration
   4. (d) Density
   Show answer

   (c) p. 106.

Quick revision

• The modern table orders elements by atomic number; periods are rows, groups are columns.
• Properties repeat at regular intervals: periodicity.
• Seven periods with 2, 8, 8, 18, 18, 32 and 32 elements.
• Lanthanides and actinides sit at the bottom as the f-block.