10th Class Important Questions – Page 14

1. Newlands proposed the law of octaves. Mendeleev suggested eight groups for elements in his table. How do you explain these observations in terms of modern periodic classification? (AS – 1) (4 Marks)

A: According to Newland’s law of octaves every eighth element starting from a given element resembles in its properties to that of the starting element. If we start at Lithium, then the eighth element is sodium and next coming eighth is potassium and so on. These elements show similar physical and chemical properties.

According to modern periodic table elements lithium, sodium, potassium …. etc. are all placed in the same group starting that they have similar physical and chemical properties.

Mendeleev’s periodic table has eight vertical columns. These are groups. There are seven horizontal rows. These are periods. According to Mendeleev’s table elements present in a group have similar properties. What is proposed by Mendeleev was supported in modern periodic table?

For example Mendeleev gave the general formula for the first group of elements as R2O and for second group of elements as RO, the same thing was followed in modern periodic table.

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2. What are the limitations of Mendeleev’s periodic table? How could the modern periodic table over come the limitations of Mendeleev’s table? (AS – 1) (4 Marks)

A: Limitations of Mendeleev’s periodic table

1. Anomalous pair of elements: Certain elements of highest atomic weights precede those with lower atomic weights. For example, tellurium (atomic weight 127.6) precedes iodine (atomic weight 126.9).

2. Dissimilar elements placed together: Elements with dissimilar properties were placed in same group as sub-group A and sub-group B. For example, alkali metal like Li, Na, K etc., of I A group have little resemblance with coinage metals like Cu, Ag, Au of I B group. Cl is of VII A group and ‘Mn’ is of VII B, but chlorine is a non metal, where as manganese is a metal.

Modern periodic table: Over coming the above limitations:

The periodic law is changed from atomic weight concept to atomic number concept in the modern periodic law.

a) In Mendeleev’s periodic table tellurium (Te) (atomic weight 127.6) precedes iodine (atomic weight 126.9).

b) In modern periodic table this placement is made justified because the atomic number of tellurium is 52 and that of iodine is 53.

c) So in the long form of periodic table these dissimilar elements changed their place in groups and periods.

a) In the long form of periodic table ‘Cl’ falls in 3rd period and VII A group and ‘Mn’ falls in 4th period and VII B group.

b) Metals and non-metals are separated here.

Essay Type Questions (4 Marks)
Q. Write a brief account on axon.
A. 1. Axon is defined as one of the main parts of neuron. It is a long fibre arising from one corner of the cyton. Each neuron
possesses only one axon.
2. Axons are very long and are usually unbranched structures. The axon is generally called as nerve fibre.
3. The central cavity of the tubular axon is filled with axoplasm.
4. It is covered by a thin plasma membrane called axo lemma, Nissil granules are present in axon.
5. The terminal branches of axon are called telodendrites or nerve terminals.
6. Each of these nerve terminal makes contact with the dendrites or cell body or even axons of another neuron.
7. Axons covered by thin myelin sheath are called medullated fibres. Others without the sheath are the non medullated fibres.
8. The sheath prevents the leakage of impulses and transmits them at a faster rate.
9. Some of the axons also make contact with the cells in the effector organs such as muscles or glands. This site is called a
synapse.

Q. Give an account of various types of nerves found in human body.
A. Several thousands of nerve fibres are enclosed in a connective tissue sheath. This is called as a nerve. There are three types of
nerves in human body. They are motor, sensory, and mixed nerves.
1. Efferent nerves: the nerves that carry impulse from brain or spinal cord to effector organs are called Efferent nerves (efferent- out going).
2. Motor nerves: Stimulation of motor nerves makes the muscles to contract. These are also responsible for the movement of hands and legs. All these receptors are associated with the nervous system.
3. Sensory nerves: These nerves carry information from sense organs or receptors to brain and spinal cord. They are also called as afferent nerves.
4. Mixed nerves: Mixed nerves are provided with both sensory and motor nerve fibres.
Q. Describe the nerve cell with the help of a neat diagram.
A. 1. The chief components of any neuron are the cell body (Cyton), the axon and the dendrites.
2. Cell body or perikaryon or cyton has a large nucleus with one or two nucleali.
3. Nissil granules are present in the cytoplasm. These are groups of
ribosomes.
4. Dendrites are projections arising from the cell body. They receive information from other neurons.
5. Each neuron has one axon. The cytoplasm of axon is covered by
plasma membrane. At the end of axon nerve terminals are present.
6. Some neuron axons are covered by myelin sheath. The myelin sheath leaves small gaps called nodes of Ranvier.
7. The nerve terminals make contact with the dendrites or cytons of other neurons.
Q. What is a nerve impulse?
A. When a stimulus is applied to the neuron, a small electrical potential (Current) about 0.055 volts to 55 milli volts is generated. This is produced at the junction between axon and cell
body. This electrical potential is called action potential. This is commonly called as nerve impulse.
This potential travels all along the length of the axon until it reaches the nerve terminal. From here, the impulse is transferred to another neuron or to an effector organ.
Q. Describe the structure of cerebrum.
A. 1. Cerebrum is the forebrain. It is the largest part of brain.
2. A deep groove in the middle of the cerebrum divides it into two equal halves. Each half is called as cerebral hemisphere.
3. Right and left cerebral hemispheres are connected to each other by a budle of axons.
4. The outer portion of cerebral hemisphere is grey in colour and is called cerebral cortex.
5. There are several ridges called Gyri and the grooves called Sulci.
6. They increase the surface area of the cortex in order to accommodate more number of neurons.
7. Three very deep grooves present across each cerebral hemisphere divides it into four lobes- Frontal, Parietal, Temporal and
Occipital lobes.
Q. What are the functions of cerebrum?
A. 1. Cerebrum controls body functions and regulates voluntary activities.
2. Left cerebral hemisphere controls the right side of the body and right hemisphere regulates the left side of the body functions.
3. It is the seat of intelligence, thinking, judgement remembrance.
4. It receives, analyses and stores the information. It is retrieved whenever needed.
5. It can even generate impulse.
6. It has the visual (sight), auditory (hearing) affactory (smell) centres and has the capacity to associate itself with several
functions.

10th Physical Sciences Important Questions – Classification of Elements – 4 Marks

10th Biology Important Questions for AP Students