Ch 8 chemistry class 9 periodic table exercise solutions with mcqs and short question

 

Multiple-Choice Questions with Answers & Explanations

1. In which period and group will you place the element which is an important part of the solar cell?

a) Third period and fourth group
b) Second period and fourth group
c) Third period and fifth group
d) Third period and sixth group

✅ Correct Answer: (b) Second period and fourth group

Explanation:
Silicon (Si) is the key element used in solar cells. However, another essential element in solar technology is carbon (C), which belongs to the second period and fourth group in the periodic table. Elements in this group, including silicon and carbon, have unique properties that make them useful in electronics and solar energy applications.

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2. Identify the electronic configuration of the outermost shell of a transition metal.

a) ns²np⁴
b) ndxns²
c) ns²np⁶
d) ns²np³

✅ Correct Answer: (b) ndxns²

Explanation:
Transition metals have a characteristic electronic configuration where the d-orbital is partially filled. The outermost shell generally follows the pattern ndxns², where 'x' represents the variable number of electrons in the d-subshell. This configuration allows transition metals to exhibit variable oxidation states and unique chemical behavior.

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3. Which is the softest metal?

a) Na
b) Ca
c) Al
d) Zn

✅ Correct Answer: (a) Na (Sodium)

Explanation:
Sodium (Na) is an alkali metal with a very low density and soft texture. It can be cut easily with a knife. Alkali metals have a single electron in their outermost shell, making them highly reactive and soft compared to alkaline earth metals (like calcium) and transition metals (like zinc and aluminum).

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4. A yellow solid element exists in allotropic forms, which is also present in fossil fuel. Indicate the name.

a) Carbon
b) Sulphur
c) Aluminium
d) Iodine

✅ Correct Answer: (b) Sulphur

Explanation:
Sulphur (S) is a yellow, non-metallic element that occurs in various allotropic forms. It is present in coal and petroleum and contributes to the formation of acid rain when burned as sulfur dioxide (SO₂). Carbon is also found in fossil fuels but is not yellow in color.

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5. How many electrons can nitrogen accept in its outermost shell?

a) 2
b) 3
c) 4
d) 5

✅ Correct Answer: (b) 3

Explanation:
Nitrogen (N) has five valence electrons (2s²2p³). To complete its octet (8 electrons in the outer shell), it needs 3 more electrons. This is why nitrogen commonly forms three bonds, as seen in ammonia (NH₃) and nitrogen gas (N₂).

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6. Which element is the most reactive element?

a) Oxygen
b) Chlorine
c) Fluorine
d) Nitrogen

✅ Correct Answer: (c) Fluorine

Explanation:
Fluorine (F) is the most reactive non-metal due to its high electronegativity (4.0 on the Pauling scale). It has only one electron missing to complete its octet, making it extremely eager to react with other elements, including noble gases in some cases.

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7. Which element has the highest melting point?

a) Na
b) B
c) Rb
d) Cs

✅ Correct Answer: (b) Boron (B)

Explanation:
Boron is a metalloid with an extremely high melting point (~2076°C). This is due to its strong covalent bonding and rigid structure. Alkali metals like Na, Rb, and Cs have lower melting points because of their weak metallic bonding.

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8. In what order does the metallic character change in the second group?

a) Mg > Ca > Ba > Sr
b) Sr > Ba > Ca > Mg
c) Mg > Sr > Ca > Ba
d) Ba > Sr > Mg > Ca

✅ Correct Answer: (d) Ba > Sr > Mg > Ca

Explanation:
Metallic character increases down a group because the atomic size increases, making it easier to lose valence electrons. Among these elements, Barium (Ba) is the most metallic, followed by Strontium (Sr), Magnesium (Mg), and Calcium (Ca).

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9. Which of the following best describes the correct order of oxygen, fluorine, and nitrogen's atomic radii?

a) O < F < N
b) N < F < O
c) F < O < N
d) O > N < F

✅ Correct Answer: (c) F < O < N

Explanation:
Atomic size decreases across a period (left to right) due to increasing nuclear charge. The correct order of atomic radii is:

• Fluorine (F) is the smallest because it has the highest nuclear charge in Group 17.
• Oxygen (O) is slightly larger because it has fewer protons than fluorine.
• Nitrogen (N) is the largest because it is further left in the periodic table.

Thus, F < O < N is the correct order.

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10. Which of the following elements has deleted two shells, both of which are filled completely?

a) Ne
b) Ca
c) B
d) He

✅ Correct Answer: (a) Ne (Neon)

Explanation:
Neon (Ne) belongs to the noble gases and has an electron configuration of 1s²2s²2p⁶. It has two completely filled electron shells and does not need to lose or gain electrons, making it chemically inert.

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Short Answer Questions

1. Why was the atomic number chosen to arrange the elements in the periodic table? The atomic number was chosen because it directly corresponds to the number of protons in an atom, which plays a key role in determining an element's properties. This method provides a much more accurate and consistent way of organizing the elements compared to atomic mass.

2. What is the significance of the word "periodic"? The word "periodic" highlights the repeating patterns in the properties of elements as we move across rows and columns in the periodic table. These repeating trends help us predict how elements will behave chemically and physically.

3. Why does the size of a period increase as we move down the periodic table? As you move down a period, each new row adds an additional electron shell to the atom. These extra shells make the atom larger, even though the number of protons is increasing.

4. In a group, the elements have the same number of electrons in the outermost shell. What is it? This refers to the fact that elements in the same group have the same number of valence electrons, which gives them similar chemical properties. This is why elements in a group often behave in similar ways in reactions.

5. Do you expect calcium to be more reactive than sodium? Give the reason for your answer. Yes, calcium is more reactive than sodium. As you go down the periodic table, metals become more reactive because their outer electrons are farther away from the nucleus, making them easier to lose.

6. Which element has the maximum atomic radius, and which element has the minimum atomic radius in the third period? In the third period, sodium (Na) has the largest atomic radius, and argon (Ar) has the smallest. Sodium’s electrons are less tightly held by the nucleus, making it larger, while argon’s electrons are more tightly bound, shrinking its size.

7. Which are the most electronegative elements present in the sixth and seventh groups? In the sixth group, oxygen (O) is the most electronegative element, and in the seventh group, fluorine (F) is the most electronegative. Electronegativity tends to increase as you move up a group and across a period.

8. The first ionization energy value of magnesium is less than the second one. Give a reason. The second ionization energy of magnesium is higher because after the first electron is removed, the remaining electrons are more tightly held by the nucleus, making it harder to remove another electron.

9. Is it possible for two metals or two non-metals to form an ionic bond? No, ionic bonds typically form between a metal and a non-metal because metals lose electrons easily, while non-metals tend to gain electrons. Two metals or two non-metals wouldn't have the right difference in properties to form an ionic bond.

10. Which element has the least value of ionization energy, and which element has the highest value of electronegativity? Cesium (Cs) has the least ionization energy, as its outermost electron is farthest from the nucleus and easiest to remove. Fluorine (F) has the highest electronegativity because it strongly attracts electrons in bonds.

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Constructed Response Questions

1. Suppose a new element is discovered. Where would you like to accommodate this element in the periodic table? If a new element were discovered, its position in the periodic table would depend on its atomic number and properties. If it shares similarities with an existing element, it would likely be placed in the same group or period based on its electron configuration.

2. What is the first element of the periodic table? Will it lose an electron or gain it? The first element of the periodic table is hydrogen. Hydrogen has one electron, and it can either lose it to form H⁺ or gain an electron to form H⁻, depending on the situation.

3. The atomic radii of boron and aluminum are 88 pm and 125 pm respectively. Which element is expected to lose electrons more easily? Aluminum (Al) is more likely to lose electrons because it has a larger atomic radius compared to boron (B), and its outer electrons are less tightly held by the nucleus.

4. How would you find the atomic radius of an atom? You can find the atomic radius by measuring the distance between the nuclei of two identical atoms that are bonded together, and then dividing that distance by two. This gives you the average radius of the atom.

5. Why is it not possible for oxygen atoms to accept three electrons to form O³⁻ ion like nitrogen, which can accept electrons to form N³⁻? Oxygen can’t accept three electrons because it is already highly electronegative and has a small atomic radius. Adding three electrons would create too much repulsion and instability, whereas nitrogen has a more favorable electron configuration to accept three electrons.

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Descriptive Questions

1. Which information is required to locate the elements in the periodic table if you do not know its atomic number? Is atomic mass helpful for this purpose? To locate an element in the periodic table without knowing its atomic number, you would need to know its chemical properties, group, and period. Atomic mass isn’t as helpful as the atomic number, as it doesn’t determine the element’s position as clearly.

2. How many blocks of elements are present in the periodic table? Are these blocks helpful in studying the properties of elements? There are four blocks in the periodic table: s, p, d, and f. These blocks help us understand the types of chemical bonds the elements are likely to form and their general reactivity, making it easier to predict their behavior.

3. Explain the variation in the following properties across periods, giving reasons:

   • (a) Atomic radius: Atomic radius decreases across a period because the number of protons increases, pulling the electrons closer to the nucleus.
   • (b) Ionization energy: Ionization energy increases across a period because the electrons are held more tightly by the nucleus due to increased proton numbers.

4. Which physical properties of elements may help us predict what type of bond they will form? Key properties include electronegativity, atomic size, and ionization energy. For example, metals typically form ionic bonds, while non-metals tend to form covalent bonds.

5. Write down the names of four non-metals that exist in the solid state at normal temperature. The non-metals that exist in the solid state at normal temperature are carbon (C), sulfur (S), phosphorus (P), and iodine (I).

6. Why do the second and third periods have the same number of elements, while all other periods contain a different number of elements? The second and third periods each fill up the s and p orbitals, which can hold eight electrons, resulting in the same number of elements. Other periods have additional d and f orbitals, which can hold more electrons and therefore have more elements.

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Investigative Questions

1. The arrangement of elements in the form of a periodic table is a remarkable achievement of chemists. Comment on this statement by citing the benefits of this table. The periodic table is an incredible achievement because it organizes elements based on their properties and atomic numbers, making it easier to understand trends and predict the behavior of elements. It’s a tool that helps chemists in countless ways, from designing new compounds to studying reactions.

2. Both lithium and beryllium show behavior different from the rest of the alkali and alkaline earth metals, respectively. Can you think of the possible reasons for this difference? Lithium and beryllium are smaller in size compared to the other alkali and alkaline earth metals, and they also have higher ionization energies. These factors make their reactivity less pronounced compared to other elements in their groups.

3. The modern periodic table is an amended version of the earlier table developed by Mendeleev. Elaborate on how these two tables are different from each other. Mendeleev’s table was arranged based on atomic mass, while the modern periodic table is based on atomic number. The modern table also accounts for the discovery of new elements and places them in the correct position, correcting the inconsistencies found in Mendeleev’s version

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For more in-depth learning on Acid-Base Chemistry, explore our detailed article on Ch 07 - Acid-Base Chemistry 9th Exercise. This resource provides comprehensive explanations and practice exercises to enhance your understanding of the topic.

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