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(a) Lithium, Sodium, Potassium .

(b) Magnesium, Calcium .

(c) Helium, Neon, Argon .

Magnesium and calcium are the two elements which have two electrons in their outermost orbit.

• Metals give electrons. Sodium has electronic configuration (2, 8, 1). It has only one electron in the outermost shell.
• It can easily give a single electron in the outermost shell. Hence, Sodium is a strong metal.
• Aluminium has an electronic configuration (2,8,3). It has three electrons in the outermost shell.
• Donation of three electrons is more difficult than the donation of one electron. Hence, sodium is more metallic than aluminium.

(i) Number of electrons in the outermost orbit of 

X=1 and Y=2.

(ii) Valency of X=1 and Y=2.

(iii) Metal X is more metallic than Y.

(iv) Atomic size of X is bigger than that of Y.

(v) Chloride XCl; YCl₂

(vi) Sulphate X₂SO₄;YSO₄ 

(i) Z.

(ii) Halogens.

(iii) Magnesium and nitrogen.

(iv) Silicon.

(v) X has bigger size than P because X has less effective nuclear charge. 

Mendaleev’s periodic table:

1. In this table, the elements are arranged in the order of their increasing atomic weights 

2. In this table, the position of an element is based on its properties and atomic weight. 

3. There are 8 groups in this table. 

4. In this table, some elements having similar properties are found in different groups, while those having different properties are sometimes found in the same group. 

5. Isotopes do not find separate places in this table.

Modern periodic table:

1. In this table, the elements are arranged in the order of their increasing atomic number. 

2. In this table, the position of an element, is based on its electronic configuration. 

3. There are 18 groups in this table. 

4. In this table, the elements belonging to the same group show similar chemical properties. 

5. Isotopes of an element can be placed at the same place as their atomic number is the same.

Answer is Hydrogen, sodium.

Answer is Periods.

The answer is (c) It has 18 vertical columns known as Groups

• Modern periodic table have 18 groups and 7 periods. 
• Columns are called groups and rows are called periods.

(i) On moving from left to right along a period of the Periodic Table, atomic number increases, i,e., number of protons and electrons increases which ultimately increases the nuclear charge. Hence, the electrons are pulled in closer to the nucleus which leads to contraction of the atom and thus atomic radius is decreased.
(ii) On moving from left to right in a Periodic Table, metallic character decreases and non-metallic character increases. This is because the capacity to lose electrons from outermost orbit of an atom (called ionisation energy) decreases. Hence, among the above elements, Li being placed on extreme left side is most metallic and F being placed on extremely right side is most non-metallic element.

(a) Position of isotopes: All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since, all the isotopes of an element have the same atomic number; they can be put at one place in the same group of the periodic table.

(b) Position of Cobalt and Nickel: The atomic number of cobalt is 27 and that of nickel is 28. According to modern periodic law, the elements are arranged in order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.

(c) Position of hydrogen: Hydrogen has been placed at the top of group 1, above the alkali metals in the modern periodic table because the electronic configuration of hydrogen is similar to those of alkali metals. Both, have 1 valence electron each.

(1) In the modern periodic table, there are seven horizontal rows called periods. In a periods elements are arranged in an increasing order of their atomic numbers. The third row contains 8 elements and the electron capacity of the third shell is 18.

(2) In the third period, while moving from left to right, atomic number increases, number of electrons increases in the shell. The number of elements present in 3rd period is decided on the basis of electronic configuration and octet rule.

Argon (Ar) is the last element of the third period and has a capacity of maximum 18 electrons. Its octet of electrons is completed and as argon belongs to zero group, the third shell contains 18 electrons.

In the modern periodic table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods. 

Metallic character increases on moving down a group in the Modern Periodic table. As we move down the Soup, the electrostatic attraction between the nucleus and the outermost electron decreases due to an increase in the distance between them. This happens because; on moving down the group a new shell an added. So, the valence electron can be easily lost by the element, thereby metallic character increases on moving down a group. 

The size of atomic radius decreases on moving left of right in a horizontal row. When, we move across a period, the number of electrons in the same shell increases. This leads to the greater electrostatic attraction between the nucleus and the outermost electron. This increased attraction pulls the outermost electron closer to the nucleus, thereby decreasing the atomic size.

(a) The atomic radii decreases from Li to F because of increase in nuclear charge which tends to pull electrons closer to nucleus and reduces the size of an atom.

(b). (i) As we go down the group metallic character increases and decreases along the period. So, the most metallic element is Li (Lithium)

(ii) As the metallic character decreases along the period, the non-metallic character increases. 

The most non-metallic element is F (fluorine).

(1) Metals have a tendency to lose the valence electrons to form cations. This tendency of an element is called the metallic character of the element.

(2) while going from left to right within a period the outermost shell remains the same and electrons are added to the same shell. However, the positive charge on the nucleus goes on Increasing while the atomic radius goes on decreasing and thus the effective nuclear charge goes on increasing. As a result of this the tendency of atom to lose electrons decreases, i.e., electropositivity decreases. Thus, metallic character goes on decreasing within a period from left to right.

The size of an atom is indicated by its radius. while going down a group a new shell is added. Therefore, the distance between the outermost electron and the nucleus goes on increasing. These electrons experience lesser pull from the nucleus. Thus, atomic radius goes on increasing down a group.

(i) E
(ii) B
(iii) C
(iv) B, because atomic radius decreases from left to right due to increase in the nuclear charge.
(v) Noble gases

The modern periodic table contains of groups and periods. 

The atoms with identical outer shell electronic cell electronic configuration are in the same groups. 

The atoms with same number of shells are in same period.

Second period

Group 17 contains highly reactive non-metals, namely, fluorine, chlorine, bromine, and iodine.

1. Various minerals, rocks, fine I soil particles are found on the sea floor. 

2. Remains of marine plants and animals are also found on the ocean floor. 

3. To study the marine life, mineral wealth as well as the process of volcanic eruption, the study of ocean floor is useful to man.

The landforms on the ocean bed that are comparatively less extending and shallower ones are called marine deep.

The following measures can be taken to control deposition of non-perishable items on the coast:

1. Plastic bags, plastic or glass containers must be prohibited on the seacoasts. 

2. Strict action must be taken against offenders carrying the banned products on the seacoasts. 

3. Hoardings explaining the importance of clean seacoasts must be displayed on all the seacoasts.

Correct option is (iii) Volcanic ash, lava, fine particles of soil

Continental shelf is appropriate to be used for the protection of marine borders and naval base building.

1. The average of the highest high tide and the lowest low tide is considered to calculate the mean sea level. 

2. This average is considered to be zero sea level.

3. The altitude of any place above the sea level is measured and expressed in positive value. (For example, Mount Everest is 8848 metres high.) 

4. The depth of any place below the sea level is measured and expressed in negative value. (For example, Mariana Trench is 11034 metres below sea level.)

We will run a campaign of environmental conservation to keep the coasts clean as follows:

1. We will display hoardings explaining the importance of clean seacoasts on the seacoasts. 

2. We will arrange street plays, dramas, etc. highlighting the importance of clean seacoasts. 

3. We will arrange special programmes during the festivals like Ganeshotsav, Diwali, Christmas, etc. to keep the seacoasts clean.

Andaman and Nicobar islands in our country are examples of peaks of submerged mountains.

The region near Madagascar island would be ideal for fishing. Madagascar island has a shallow continental shelf. The warm stream and cold stream also meet near Madagascar island. Therefore, the region near Madagascar island would be ideal for fishing

Submarine plateau and marine trench are the landforms useful for deep sea research.

Madagascar and Sri Lanka are related to island of the ocean floor.

(i) Altitude and the shape of the land were the parameters used for classifying the landforms on the earth.

(ii) Depth from the seafloor and the shape of the land were the parameters used for naming the landforms below water.

Madagascar is located near the continent of Africa and Sri Lanka is located near the continent of Asia.

Humidity is generally more in monsoon.

The rate of respiration and sweating increases due to humidity.

(iv) though land is continuous, its level is not the same everywhere like that of water

1. If the temperature of saturated air at 20° C drops to 10° C abruptly, the relative humidity of the air will increase abruptly. 

2. Rise in the relative humidity will lead to condensation or sublimation of water vapour in the air. This in turn, will lead to precipitation or snowfall.

Due to humidity, fungus flourish on food materials at our home and it gets spoiled.

1. High humidity increases the rate of sweating and respiration. This adversely affects the daily functioning. 

2. Low humidity with pleasant atmosphere, have favourable effects on the daily routine.

3. High humidity leads to late drying up of clothes and early spoiling of food materials. 

4. Low humidity leads to early drying up of clothes and late spoiling of food materials.

As the moisture-holding capacity is found to be high in low humidity air, it leads to early drying up of clothes. As the moisture-holding capacity is found to be low in high humidity air, it leads to late drying up of clothes.

There is a direct relationship between formation of fungus and humidity. Humidity supports the growth of fungus.

(i) vapour-holding capacity

(ii) absolute humidity

(iii) relative humidity

(iv) Cumulonimbus

(v) condensation

1. In case of any violation of Fundamental Right, the aggrieved person can approach the High Court or the Supreme Court for remedy.

2. Moreover, any person can go to court against the violation of the Fundamental Right, if it is of social or public interest. It is called the Public Interest Litigation (PIL).

3. Under this any citizen or group of citizens can approach the Supreme Court or the High Court for the protection of public interest against a particular law or action of the government.

4. One can write to the judges even on a postcard. The court will take up the matter if the judges find.

(a) Ganga-Brahmaputra

Folding, Faulting, volcanic activity, landslides are the major causes for earthquakes. Earth quakes are also caused by human activities such as mining, nuclear explosion.

These are associated with the processes of falling of rocks and avalanches in mountain, large landslides, and the collapse of natural caves and so on. These earthquakes are confined to earth’s surface and constitute only a small number of the total earthquakes.

The following precautions can be taken to reduce the damage caused by earthquakes:

1. Taking cover under a desk or a table or sturdy piece of furniture to protect oneself. 

2. Finding clear spot away from buildings, trees and power lines. Dropping to the ground and staying there until the shaking stops. 

3. Switching off the power supply, gas inhome/office, etc. 

4. Helping the victims at the earliest.

1. The average temperature of the outer core of the earth is around 5000° C. The average temperature of the inner core of the earth is around 6000° C. This difference between the temperature results in formation of vertical currents.

2. The rotation of the earth gives eddy (circular) motion to these currents. 

3. Electric currents develop in these spiral eddies of liquid iron leading to generation of the magnetic field of the earth. This magnetic field is called magnetosphere. In this way, the magnetosphere is formed due to the rotation of the earth.