The Periodic Table
ICSE Class 9 · Chemistry · Chapter 5
What this chapter covers
- Why elements needed to be classified
- Dobereiner's triads
- Newlands' law of octaves
- Mendeleev's periodic law
- Atomic number and the modern periodic law
- The modern periodic table
- Important groups
Key points
- Elements were classified so that similar ones could be studied together and properties predicted.
- Dobereiner's triads: in a set of three similar elements, the middle atomic mass is about the mean of the other two (Li, Na, K); only a few triads exist.
- Newlands' law of octaves: every eighth element resembles the first; it worked only up to calcium.
- Mendeleev's periodic law: properties are a periodic function of atomic masses; he left gaps, predicted new elements and corrected atomic masses, but could not place hydrogen or isotopes and had anomalous pairs.
- Moseley showed that atomic number is the fundamental property; the modern periodic law makes properties a periodic function of atomic numbers.
- Periodicity arises because similar outer electronic configurations recur as atomic number increases.
- The modern periodic table has 18 groups (same valence electrons) and 7 periods (same number of shells); transition elements fill groups 3–12, and the lanthanides and actinides sit in two rows at the bottom.
- Group 1 alkali metals (1 valence electron, very reactive), group 2 alkaline earth metals (2 valence electrons), group 17 halogens (7 valence electrons, salt-formers) and group 18 noble gases (full shells, inert).
Questions answered
Why was it necessary to classify elements?
Many elements were known, each with its own properties; classification groups similar elements so that they can be studied together and the properties of others predicted.
State Dobereiner's law of triads with one example. Give one limitation.
In a triad of similar elements arranged by atomic mass, the middle element's mass is about the mean of the other two, as with Li (7), Na (23), K (39). Only a few triads could be found.
State Newlands' law of octaves. Why was it rejected?
When elements are arranged by increasing atomic mass, every eighth element resembles the first. It held only up to calcium and could not accommodate newly discovered elements.
State Mendeleev's periodic law.
The physical and chemical properties of elements are a periodic function of their atomic masses.
Give two merits of Mendeleev's periodic table.
He left gaps and predicted the properties of undiscovered elements such as eka-silicon (germanium); he corrected wrong atomic masses such as that of beryllium.
What is meant by an anomalous pair? Give one example.
An anomalous pair is a pair of elements in which the heavier is placed before the lighter to keep similar elements together, for example argon (39.9) before potassium (39.1).
State the modern periodic law. Who showed that atomic number is the basis of it?
The properties of elements are a periodic function of their atomic numbers. Henry Moseley.
How does the modern periodic law remove the problem of isotopes?
Isotopes have the same atomic number, so they occupy the same place in the table.
Past board exam questions
ICSE 2019 sample paper · 1 mark
According to the modern periodic law, the properties of elements are a periodic function of their:
- Atomic volume
- Atomic number
- Atomic weight
- Mass number
Show answer
Atomic number
ICSE 2019 sample paper · 3 marks
(i) What is meant by ‘Group’ in the Periodic Table? (ii) Explain why the elements of the same group exhibit the same chemical behaviour. (iii) In which Group are the inert gases placed in the Periodic Table?
Show answer
(i) A group is a vertical column of elements in the Periodic Table; the elements of a group have the same number of valence electrons.
(ii) Chemical properties depend on the number of valence electrons; elements of the same group have the same number of electrons in their outermost shell (the same valency), so they show similar chemical behaviour.
(iii) Group 18 (zero group / Group VIII A).
ICSE 2019 sample paper · 4 marks
A part of the periodic table is shown below with one element missing: | | | | | | | | | |---|---|---|---|---|---|---|---| | H | | | | | | | He | | Li | Be | B | C | N | O | F | Ne | | Na | Mg | Al | Si | – | S | Cl | Ar | | K | Ca | | | | | | | Based on the above table, answer the following questions: (i) Name the element that has duplet structure. (ii) Name the lightest alkali metal. (iii) Name the halogen of period 2. (iv) Identify the missing element.
Show answer
(i) Helium (He)
(ii) Lithium (Li)
(iii) Fluorine (F)
(iv) Phosphorus (P)