i. Which information is needed to locate the elements in the periodic table if you do not know its atomic number? Is atomic mass helpful for this purpose?

Answer:
To locate an element in the periodic table, the most important information is its atomic number, which tells us the number of protons and the arrangement of electrons in shells.

If atomic number is unknown, we can use:

• Electronic configuration

• Atomic mass (as an approximation)

Although atomic mass was used in early versions of the periodic table (like Mendeleev’s table), it is not always reliable because some elements do not follow the mass order (e.g., Argon and Potassium).
Hence, atomic mass may give a rough idea, but atomic number is essential for accurate placement.

ii. How many blocks of elements are present in the periodic table? Are these blocks helpful in studying the properties of elements?

Answer:
There are four blocks in the periodic table:

1. s-block (Groups 1 and 2)

2. p-block (Groups 13 to 18)

3. d-block (Transition elements, Groups 3 to 12)

4. f-block (Lanthanides and Actinides)

These blocks are based on the type of subshell (orbital) in which the last electron enters.

Yes, blocks are very helpful in understanding:

• Chemical reactivity

• Valency

• Oxidation states

• Physical and chemical properties

For example:

• s-block elements are highly reactive metals

• p-block contains both metals and non-metals

• d-block elements show variable oxidation states

• f-block elements are rare earth metals

iii. Explain the variation in the following properties in the periods giving reasons.

(a) Atomic radius

Answer:
As we move left to right across a period, the atomic number increases, and more protons are added to the nucleus, increasing the nuclear charge.

Although electrons are added too, they go into the same shell, so the increased nuclear pull draws the electron cloud closer to the nucleus.
As a result, atomic radius decreases across a period.

(b) Ionization energy

Answer:
Ionization energy is the energy required to remove an electron from an atom.

As we move across a period from left to right, the nuclear charge increases and the atomic radius decreases. Electrons are held more tightly.

So, it becomes harder to remove an electron, and hence, ionization energy increases across a period.

iv. Which physical properties of elements may lead us to know what type of bond it will form?

Answer:
Some key physical properties that help predict the type of bond are:

1. Electronegativity

   • High difference → Ionic bond

   • Low or no difference → Covalent bond

2. Ionization energy

   • Low ionization energy → atom can lose electrons → forms ionic bonds

3. Electron affinity

   • High affinity → atom gains electrons → favors ionic or covalent bonding

4. Metallic or non-metallic character

   • Metal + non-metal → ionic bond

   • Non-metal + non-metal → covalent bond

   • Metal + metal → metallic bond

These properties help chemists predict bond type and the chemical behavior of elements.

v. Write down the names of four non-metals which exist in solid state at normal temperature.

Answer:
Four non-metals that exist as solids at room temperature (25°C) are:

1. Sulphur (S)

2. Phosphorus (P)

3. Iodine (I₂)

4. Carbon (C) (in the form of graphite or diamond)

These are exceptions among non-metals, many of which are gases (e.g., O₂, N₂).

vi. Why do second and third periods have equal number of elements while all other periods contain different number of elements?

Answer:
The number of elements in a period depends on the number of electrons that can be accommodated in the shells.

• The 2nd and 3rd periods both contain 8 elements because the 2nd and 3rd shells can accommodate a maximum of 8 electrons in their s and p orbitals only.

• In higher periods, additional orbitals (d and f) become available, which allows more elements to fit in.

Therefore, 2nd and 3rd periods have equal elements (8), while later periods have 18 or 32 elements due to availability of d and f subshells.