CBSE Class X

Question 3:

Which element has

(a) two shells, both of which are completely filled with electrons?

(b) the electronic configuration 2, 8, 2?

(c) a total of three shells, with four electrons in its valence shell?

(d) a total of two shells, with three electrons in its valence shell?

(e) twice as many electrons in its second shell as in its first shell?

Answer:

(a) Neon has two shells, both of which are completely filled with electrons (2 electrons

in K shell and 8 electrons in L shell).

(b) Magnesium has the electronic configuration 2, 8, 2.

(c) Silicon has a total of three shells, with four electrons in its valence shell (2

electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).

(d) Boron has a total of two shells, with three electrons in its valence shell (2

electrons in K shell and 3 electrons in L shell).

(e) Carbon has twice as many electrons in its second shell as in its first shell (2

electrons in K shell and 4 electrons in L shell).

Question 2:

Element X forms a chloride with the formula XCl₂, which is a solid with a high melting

point. X would most likely be in the same group of the Periodic Table as

(a) Na (b) Mg (c) Al (d) Si

Answer:

(b) X would most likely be in the same group of the Periodic Table as magnesium

(Mg).

Question 1:

Which of the following statements is not a correct statement about the trends when

going from left to right across the periods of periodic Table.

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(c) The atoms lose their electrons more easily.

(d) The oxides become more acidic.

Answer:

(c) The atoms lose their electrons more easily.

(On moving from left to right across the periods of the periodic table, the non-metallic

character increases. Hence, the tendency to lose electrons decreases.)

Question 6:

By considering their position in the Periodic Table, which one of the following elements

would you expect to have maximum metallic characteristic?

Answer:

Since Be lies to the extreme left hand side of the periodic table, Be is the most

metallic among the given elements.

Question 5:

In the Modern Periodic Table, which are the metals among the first ten elements?

Answer:

Among the first ten elements, lithium (Li) and beryllium (Be) are metals.

Question 4:

(a) Lithium, sodium, potassium are all metals that react with water to liberate

hydrogen gas. Is there any similarity in the atoms of these elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What,

if anything, do their atoms have in common?

Answer:

(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have

one electron in their outermost shells.

(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet

in its K shell, while neon has an octet in its L shell.

Question 3:

Name

(a) three elements that have a single electron in their outermost shells.

(b) two elements that have two electrons in their outermost shells.

(c) three elements with filled outermost shells.

Answer:

(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their

outermost shells.

(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.

(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.

Question 2:

Name two elements you would expect to show chemical reactions similar to

magnesium. What is the basis for your choice?

Answer:

Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to

magnesium (Mg). This is because the number of valence electrons (2) is same in all

these three elements. And since chemical properties are due to valence electrons,

they show same chemical reactions.

Question 1:

How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic

Table?

Answer:

Mendeleev was unable to give fixed position to hydrogen and isotopes in the periodic

table. In Mendeleev’s periodic table, the increasing manner of atomic mass of the

elements is not always regular from one to its next. It was believed that a more

fundamental property than atomic mass could explain periodic properties in a better

manner.It was Henry Moseley who demonstrated that atomic number of an element

could explain periodic properties in a better way than atomic mass of an element and

arranged the elements in increasing order of their atomic numbers. Then it was found

that the various anomalies of Mendeleev’s periodic table were removed by the modern

periodic table.

Question 4:

Why do you think the noble gases are placed in a separate group?

Answer:

Noble gases are inert elements. Their properties are different from the all other

elements. Therefore, the noble gases are placed in a separate group.

Question 3:

What were the criteria used by Mendeleev in creating his Periodic Table?

Answer:

Mendeleev’s periodic table was based on the observation that the properties

of elements are a periodic function of their atomic masses. This means that if

elements are arranged in the increasing order of their atomic masses, then their

properties get repeated after regular intervals.

Question 2:

Besides gallium, which other elements have since been discovered that were left by

Mendeleev in his Periodic Table? (any two)

Answer:

Scandium and germanium

Question 1:

Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following

elements:

K, C, Al, Si, Ba.

Answer:

K is in group 1. Therefore, the oxide will be K₂O.

C is in group 4. Therefore, the oxide will be CO₂.

Al is in group 3. Therefore, the oxide will be Al₂O₃.

Si is in group 4. Therefore, the oxide will be SiO₂.

Ba is in group 2. Therefore, the oxide will be BaO.

Question 3:

What were the limitations of Newlands’ Law of Octaves?

Answer:

Limitations of Newlands’ law of octaves:

(i) It was not applicable throughout the arrangements. It was applicable up to

calcium only. The properties of the elements listed after calcium showed no

resemblance to the properties of the elements above them.

(ii) Those elements that were discovered after Newlands’ octaves did not follow the

law of octaves.

(iii) The position of cobalt and nickel in the group of the elements (F, Cl) of different

properties could not be explained.

(iv) Placing of iron far away from cobalt and nickel, which have similar properties as

iron, could also not be explained.

Question 2:

What were the limitations of Dobereiner’s classification?

Answer:

Limitation of Dobereiner’s classification:

All known elements could not be classified into groups of triads on the basis of their

properties.

Question 1:

Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and

find out.

Answer:

Only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The

triad formed by the elements Li, Na, and K of Dobereiner’s triads also occurred in the

columns of Newlands’ octaves.

Dobereiner’s triads

Li     Ca     Cl

Na    Sr    Br

K     Ba     I

Newlands’ octaves

Question 15:

Explain the mechanism of the cleaning action of soaps.

Answer:

Cleansing action of soaps:

The dirt present on clothes is organic in nature and insoluble in water. Therefore, it

cannot be removed by only washing with water. When soap is dissolved in water, its

hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then,

the molecules of soap arrange themselves in micelle formation and trap the dirt at

the centre of the cluster. These micelles remain suspended in the water. Hence, the

dust particles are easily rinsed away by water.

Question 14:

Give a test that can be used to differentiate chemically between butter and cooking

oil.

Answer:

Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other

hand,oil has unsaturated fats. That is why it can be hydrogenated to saturated fats

(solids).

Question 13:

Which of the following hydrocarbons undergo addition reactions:

C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄.

Unsaturated hydrocarbons undergo addition reactions. Being unsaturated

hydrocarbons, C₃H₆ and C₂H₂ undergo addition reactions.

Question 12:

What is hydrogenation? What is its industrial application?

Answer:

Hydrogenation is the process of addition of hydrogen. Unsaturated hydrocarbons are

added with hydrogen in the presence of palladium and nickel catalysts to give

saturated hydrocarbons.

This reaction is applied in the hydrogenation of vegetables oils, which contain long

chains of unsaturated carbons.