Periodic Classification of Elements • Topic 2 of 3

The Modern Periodic Table & Periodic Law

Mendeleev's table had a few stubborn problems — reversed mass orders, isotopes and the position of hydrogen. The fix came when scientists realised that atomic mass was not the most fundamental property of an element. The real key was the atomic number.

Moseley's discovery

In 1913 the English physicist Henry Moseley studied the X-rays given off by different elements. He showed that the atomic number (Z) — the number of protons in the nucleus — is a more fundamental property than atomic mass. When elements were arranged by atomic number, the earlier anomalies disappeared. This led to the Modern Periodic Law.

The Modern Periodic Law

The properties of elements are a periodic function of their atomic numbers. Because the atomic number fixes the number of electrons and hence the electronic configuration, elements with similar outer electron arrangements naturally fall together.

Structure of the modern periodic table

  • Groups: the 18 vertical columns are called groups, numbered 1 to 18. Elements in the same group have the same number of valence (outer) electrons, so they show similar chemical properties.
  • Periods: the 7 horizontal rows are called periods. The period number tells you the number of shells in the atom. Period 1 has 2 elements, periods 2 and 3 have 8 each, periods 4 and 5 have 18 each, and so on.

How electronic configuration decides position

The position of an element follows directly from how its electrons are arranged in shells.

  • The number of occupied shells gives the period number.
  • The number of valence electrons usually gives the group (for the main-group elements). For example, sodium (2, 8, 1) has 3 shells and 1 valence electron, so it is in Period 3, Group 1.
  • Magnesium (2, 8, 2) sits in Period 3, Group 2; chlorine (2, 8, 7) sits in Period 3, Group 17.

Anomalies resolved

  • Reversed mass pairs: argon (Z = 18) correctly comes before potassium (Z = 19) by atomic number, even though argon is heavier. The same logic fixes the cobalt and nickel pair.
  • Isotopes: isotopes have the same atomic number, so they all occupy the same single position — the isotope problem vanishes.
  • Hydrogen: while its position is still debated, the modern table places hydrogen at the top of Group 1, and its unique behaviour is simply accepted.
Structure of the modern periodic table: 18 groups (columns) and 7 periods (rows)Groups 1 to 18 (columns) -- Periods 1 to 7 (rows)HHeLiNeNaArPeriod number = number of shellsGroup (1-2, 13-18) = valence electronsNa (2,8,1) -- Period 3, Group 1Cl (2,8,7) -- Period 3, Group 17
Solved Examples
1
Worked Example
State the Modern Periodic Law and the property on which it is based.
Solution
  1. Moseley showed atomic number is more fundamental than atomic mass.
  2. The law was reframed using atomic number.
  3. Modern Periodic Law: the properties of elements are a periodic function of their atomic numbers.

Answer: Properties of elements are a periodic function of their atomic numbers (Z).

2
Worked Example
An element has the electronic configuration 2, 8, 1. Find its period and group.
Solution
  1. Number of shells = 3, so the period number is 3.
  2. Number of valence electrons = 1, so it belongs to Group 1.
  3. The element is sodium (Na).

Answer: Period 3, Group 1 (sodium).

3
Worked Example
An element has atomic number 17. Write its electronic configuration and find its position in the periodic table.
Solution
  1. Z = 17, so the configuration is 2, 8, 7.
  2. Three shells are occupied, so the period number is 3.
  3. Valence electrons = 7, so it lies in Group 17.

Answer: Configuration 2, 8, 7; Period 3, Group 17 (chlorine).

4
Worked Example
How did arranging elements by atomic number remove the anomaly of argon and potassium?
Solution
  1. Argon (mass 40) is heavier than potassium (mass 39), so by mass argon should come after potassium.
  2. But argon has Z = 18 and potassium has Z = 19.
  3. Arranged by atomic number, argon (18) correctly comes before potassium (19), matching their properties.

Answer: By atomic number argon (18) comes before potassium (19), so the anomaly disappears.

5
Worked Example
How does the modern periodic table solve the problem of isotopes?
Solution
  1. Isotopes of an element have different atomic masses but the same number of protons.
  2. The same number of protons means the same atomic number.
  3. Since the modern table is arranged by atomic number, all isotopes of an element share one position.

Answer: Isotopes have the same atomic number, so they all occupy a single position.

6
Worked Example
Two elements have configurations 2, 8, 2 and 2, 8, 8, 2. State whether they are in the same group or the same period, with reason.
Solution
  1. The first element has 3 shells (Period 3) and 2 valence electrons (Group 2).
  2. The second has 4 shells (Period 4) and 2 valence electrons (Group 2).
  3. They have the same number of valence electrons but different numbers of shells.

Answer: Same group (Group 2) but different periods (Period 3 and Period 4).

Key Points

  • Moseley showed atomic number (number of protons) is more fundamental than atomic mass for classifying elements.
  • Modern Periodic Law: the properties of elements are a periodic function of their atomic numbers.
  • The modern table has 18 vertical groups and 7 horizontal periods; elements in a group have the same number of valence electrons.
  • Period number = number of occupied shells; group (for main-group elements) is decided by the number of valence electrons.
  • Using atomic number removes the anomalies of reversed mass pairs (Ar before K) and isotopes (which share one position).
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The scientist who showed that atomic number is more fundamental than atomic mass was:
Explanation: Henry Moseley's X-ray work established the atomic number as the basis of classification.
Q2.An element with configuration 2, 8, 8 belongs to:
Explanation: Three shells means Period 3; a complete octet (noble gas, argon) means Group 18.
Q3.The number of periods in the modern periodic table is:
Explanation: The modern table has 7 horizontal rows called periods and 18 vertical groups.
Q4.The period number of an element is equal to its:
Explanation: The period number equals the number of shells (energy levels) being filled in the atom.
Practice more MCQs → 📝 Assignment · 30 marks