PERIODIC TABLE questions

Q.1. Why does silicon is classified as Metalloid?

Ans: Silicon is gray color solid at room temperature with very high melting point and boiling point that lose or gain 4 electrons  having both metallic and non metallic properties so it is classified as Metalloid eg. Sio_2.

Q.2.Why inert gases have zero valencies?

Ans: It is because Inert gases have 2 or 8 electrons in valence shell.

Q.3. How is valency of an element determined?

Ans: The valency of an element measures its ability to combine with other elements. The valency is determined by knowing place in periodic table. All the elements in group VIII have eight electrons in their outer shells, and thus have a valency of zero (highly stable). Elements in group I just have one valence electron in their outer shells and thus have a valency of one.

Q.4. why could no fixed position be given to hydrogen in Mendeleev’s Periodic table?

Ans: Hydrogen placed in the first column above alkali metals. it is because hydrogen and alkali metals have similar properties.

However, hydrogen also resembles halogens. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H₂). It forms covalent compounds like halogens unlike alkali metals. Hence, it can also be placed above the halogen group .Therefore, Mendeleev was not able to explain the position of hydrogen.

Q.5. In modern periodic table, the isotopes of Chlorine Cl-35 and Cl-37 having different atomic masses will 

be kept in different slots or they would be assigned same position on the basis of their chemicalproperties? Give reason in support of your answer.

Ans: It is because periodic table is based on the atomic numbers of the element and both the isotopes of chlorine have the same atomic number (Z = 17).

Q.6. Why was Dobereiner’s system of classification of elements into triads not found to be useful?

Ans: it is because he could not arrange all elements in triads.

Q.7. Oxygen (O, 8) and sulphur (S, 16) belong to group 16 of the periodic table :-

(i) Write the electronic configuration and valency of these two elements?

(ii) Which among these will be more electronegative? Why?

Ans: (i) Oxygen (O, 8) = 2,6  ; Vacancy =2           Sulphur (S, 16) = 2,8,6 ; Vacancy =2  

(ii) Oxygen will be more electronegative due to electro negativity decreases from top to bottom because atomic size increases.

Q.8. How does electronic configuration of atoms change in a period with increase in atomic number?

Ans. On moving across a period from left to right, the atomic number of the elements increases, therefore, the number of electrons in the valence shell increases from 1 to 8, i.e, the first element in the given period will have one electron in its valence shell and the last element in the same period will have eight electrons.

Q.9. What happens to the melting points and boiling points of elements while moving down in a group?

Ans. The melting points and boiling points decrease while moving down in group of metals.

The melting points and boiling points increases while moving down in group of non-metals.

Q10. Why Chlorine (atomic number 17) is more electronegative than sulphur (atomic number 16)

Ans: The nucleus of chlorine has more tendency to attract an extra electron than the nucleus of sulphur because chlorine needs only one 1 electron to complete its shell. Hence, chlorine is more electronegative than sulphur.

  

                                           source-cbseadda.blogspot.in

CARBON AND ITS COMPOUNDS -important questions and answers

1.Q. Why carbon form covalent bond by sharing electron? 
Ans: Carbon has four electrons in its outermost shell and needs to gain or lose four
electrons to attain noble gas configuration.
(i) it could gain four electrons forming C4-
 anion. But it would be difficult for the nucleus with six protons to hold on to ten
electrons, that is, four extra electrons.
 (ii) It could lose four electrons forming C4+ cation.
But it would require a large amount of energy to remove four electrons leaving behind a
carbon cation with six protons in its nucleus holding on to just two electrons.
Hence, it is difficult for the carbon to lose or gain four electrons therefore carbon form
covalent bond by sharing electron.

2.Q. Why do covalent compounds are in gaseous or liquid state at normal temperature 
and pressure. 
Ans: due to Weak Vanderwaal’s forces present between the molecules of covalent
compounds.

3.Q. Why do covalent compounds have less melting and boiling points? 
Ans: As energy required breaking the crystal is less due to the presence of weak
Vanderwaal’s force.
4.Q. Why do covalent compounds are bad conductors of electricity?
Ans: Due to the absence of free electrons or free ions.

5.Q. Why graphite has high melting point? 
Ans: Within each layer of graphite, every carbon atom is joined to three others by strong
covalent bonds. This forms a pattern of interlocking hexagonal rings. The carbon atoms
are difficult to separate from one another. So graphite also has high melting point.

6.Q.Why is diamond known as the hardest metal?
 Ans: Each carbon is linked to four other carbon atoms forming regular and tetrahedral
arrangement and this network of carbon atoms extends in three dimensions and is very
rigid. This strong bonding is the cause of its hardness and its high density.

7.Q.Define term allotropy? 
Ans: Allotropy is the property by virtue of which an element exist in more than one form
and each form has different physical properties but identical chemical properties. These
different forms are called allotropes. The two common allotropic forms of carbon are
diamond and graphite.

                                                                                 src-mountabupublicschool.net

our environment (part-3)

Our environment (imp terms)

Environment:

Our surrounding is called environment.

Ecosystem:-

This is a system of interdependencies among various living beings and non-living things in a given habitat.

Components of Ecosystem:

An ecosystem has two types of components, viz. biotic component and abiotic component.

Abiotic Component:-

All the non-living things make the abiotic component of an ecosystem. Air, water and soil are the abiotic components. 

Biotic Component:

All living beings make the biotic component of an ecosystem. Green plants play the role of producers; because they prepare the food by photosynthesis. Animals and other living beings play the role of consumers; because they take food (directly or indirectly) from plants. Bacteria and fungi play the role of decomposers; as they decompose dead remains of plants and animals so that raw materials of organisms can be channelized back to the environment.

Food Chain:

A food chain is a simple representation of transfer of energy from the sun to different biotic components of an ecosystem. Sun is the ultimate source of energy. Green plants convert solar energy into chemical energy during photosynthesis. When an animal takes food, this energy is supplied to the animal and the process goes on. A simple food chain can be shown as follows:

Producer → Primary Consumer → Secondary Consumer

Grass → Deer → Lion

Food Web:-

Real life cannot be as simple as a food chain shown above. In any ecosystem, there can be many food chains which are interlinked at various levels. Thus, many food chains form a network which is called food web.

Transfer of Energy through a food chain: Different levels in the food chain are called trophic level. Out of the energy consumed by an organism at a particular trophic level, 90% is utilised for its own need and rest 10% is left for the organism of the next trophic level. So, very little energy is left for the organism which is at the tertiary level. Let us assume that a green plant makes 100% energy in the form of chemical energy. 90% of this energy would be utilised for its own purpose. This would leave just 10% energy for the primary consumer. Now, primary consumer shall also utilize 90% of energy which was consumed by it. This would leave just 1% energy for (10% of 10 = 1) for the secondary consumer. By this logic, the tertiary consumer would get just 0.1% of energy which was originally made by the green plant. This is the reason, there can be just one or two organisms at the top of the food pyramid.

This explains why the population of producers is always the largest in an ecosystem; followed by the population of herbivores and then that of carnivores. Moreover, an herbivore needs to eat many plants in its lifetime to fulfill its energy need. Similarly, a carnivore needs to eat many herbivores in its lifetime.

Balance in the Ecosystem:

There is a delicate balance in an ecosystem; as far as number of organisms at a particular trophic level is concerned. An increase or decrease in population of any organism can disturb this balance. Let us take a hypothetical example to understand this. If all the deer are killed in a jungle, the lions would be left with no food. This would endanger the existence of lions. Once the lions and deer would be finished, it would result in population explosion of green plants. If all the lions die in a jungle, it would create another problem. Since no lion would be left to kill the deer, the population of deer would increase substantially. This will finish off all the green plants and finally even the deer would be left with no food for them.

Biodegradable Substance:-

Substances which can be decomposed by microorganisms are called biodegradable substances. All the organic substances are biodegradable.

Non-biodegradable:

Substances which cannot be decomposed by microorganisms are non-biodegradable. All inorganic substances are non-biodegradable. Many synthetic substances are also non-biodegradable.

Ozone Layer Depletion:

Ozone layer is also known as stratosphere. When ultraviolet radiations act on oxygen, the oxygen gets converted into ozone.

Ozone layer works like a protective shield for living beings. The ozone layers wards off harmful ultraviolet radiations from the sun.

Use of CFCs (Chlorofluorocarbon) has damaged the ozone layer. As a result, the ozone layer has become thinner at certain parts. In 1987, the UNEP (United Nations Environment Programme) succeeded in forging an agreement among different nations to freeze the CFC production at 1986 level. Later, an agreement was signed among different nations to phase out CFCs. It is important to note that CFC is used in refrigerators and aerosol spray. India is also a signatory of that agreement and thanks to the efforts by the United Nations and different environmentalists, the CFC emission has been put under some control.

Problems of Waste Disposal: During our day to day activities, we produce lot of waste. While some of the waste is biodegradable, a large chunk is composed of non-biodegradable substances. Plastic waste is a serious concern because plastic is non-biodegradable. We need to respect our environment and find out ways to reduce the burden on our environment.

(source- http://www.excellup.com)

part 2 our environment.........

our environment

RECAPTULATE: NAMING ALKANES

ENVIRONMENT

ENERGY TRANSFER IN BIOSPHERE

The following are the characteristics of energy transfer in the biosphere:

(i) Energy is supplied by the sun and it is not created in the biosphere. Energy is only converted from one form to another in the biosphere.

(ii) There is a continuous transfer of energy from one trophic level to the next in a food chain.

(iii)At each trophic level, some of the energy is utilized by the organisms for their metabolic activities.

(iv)  At each trophic level, some amount of energy is utilized for the composition of decomposers.

(v) At each trophic level, there is loss of energy, which goes into the environment and remains un-utilized.

(vi) At each trophic level, the amount of energy available is less than that available at the previous level.
ENERGY FLOW CONSIDERED AS UNIDIRECTIONAL
The flow of energy is unidirectional and non cyclic. The energy captured by the autotrophs does not return to the sun, and the energy which passes to the subsequent levels does not come back to autographs. The amount of energy goes on successively decreasing from the producers to the carnivores.