- (a) Modern periodic law
- (b) Hund’s rule
- (c) Law of octaves
- (d) Pauli’s exclusion principle
Answer:
(c) Law of Octaves
- (a) atomic numbers
- (b) atomic masses
- (c) similarities
- (d) anomalies
Answer:
(a) atomic numbers
- (a) 7 groups and 18 periods
- (b) 18 groups and 7 periods
- (c) 17 groups and 8 periods
- (d) 8 groups and 17 periods
Answer:
(b) 18 groups and 7 periods
- In Dobereiner’s triads, atomic mass of middle element is average of atomic masses of the first and third elements.
- Noble gases belong to Group 18.
- Dobereiner, Newlands and Mendeleev classified elements mainly based on atomic mass.
- Example of liquid metal is mercury.
| List I | List II |
|---|---|
| Triads | Dobereiner |
| Alkali metal | Sodium |
| Law of octaves | Newlands |
| Alkaline earth metal | Calcium |
| Modern periodic law | Henry Moseley |
If false, correct the statement.
Answer: True
Answer: False
Correct Statement:
Metals generally lose electrons.
Answer: False
Correct Statement:
Metalloids possess properties of both metals and non-metals.
Answer: True
Answer: True
Statement
Elements in same group generally show similar properties, whereas elements across a period show different properties.
Reason
Difference in electronic configuration causes variation in chemical properties across a period.
Answer:
(a) Statement is true and reason correctly explains the statement.
Answer:
“The physical and chemical properties of elements are periodic functions of their atomic numbers.”
Groups
Vertical columns in periodic table are called groups.
There are:
Periods
Horizontal rows in periodic table are called periods.
There are:
(i) Dissimilar Elements in Same Group
Example:
- Sodium
- Copper
- Silver
were grouped together.
(ii) Position of Hydrogen Uncertain
Hydrogen resembles both:
- alkali metals
- halogens
(iii) Increasing Atomic Mass Order Not Strictly Followed
Examples:
- Cobalt and Nickel
- Tellurium and Iodine
(iv) No Position for Isotopes
Isotopes have same chemical properties but different masses.
(i)
Elements arranged in increasing atomic number.
(ii)
Horizontal rows are called periods.
There are 7 periods.
(iii)
Elements in same period have same number of electron shells.
(iv)
Vertical columns are called groups.
There are 18 groups.
(v)
Elements with similar properties are grouped into families.
Answer:
Elements in same group possess same number of valence electrons.
Hence they show similar chemical properties.
Answer:
Across a period:
- atomic number increases
- valence electrons increase
Hence chemical properties gradually change.
Answer:
Group 1 elements are called alkali metals.
Examples:
- Lithium
- Sodium
- Potassium
Answer:
Group 17 elements are called halogens.
Examples:
- Fluorine
- Chlorine
- Bromine
They are highly reactive non-metals.
Answer:
Group 18 elements are called noble gases.
Examples:
- Helium
- Neon
- Argon
They are chemically inert because outer shell is completely filled.
Applying Newlands’ Law of Octaves
Set I:
F, Mg, C, O, B
Set II:
Al, Si, S, Cl, Ca
Similar Pairs
| Pair | Reason |
|---|---|
| F – Cl | Halogens |
| Mg – Ca | Alkaline earth metals |
| C – Si | Same group |
| O – S | Chalcogens |
| B – Al | Same group |
- Decreases across period
- Increases down group
- Decreases across period
- Increases down group
- Increases from 1 to 4
- Then decreases to 0
Metals:
Increase down group
Non-metals:
Decrease down group
Dobereiner
- Grouped elements into triads.
Newlands
- Proposed Law of Octaves.
Mendeleev
- Arranged by atomic masses.
Moseley
- Proposed modern periodic law based on atomic number.
Modern Periodic Table
- 18 groups
- 7 periods
- Based on atomic number
- Elements arranged systematically according to electronic configuration
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