1. Gist of Lesson for Quick Revision (By JSUNIL)
2. In 1789, Lavoisier first attempted to classify the elements into two divisions namely Metals and Non-metals.
3. Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group. On the basis of this, he named Gallium as Eka-Almunium, Germanium as Eka-Silicon and Scandium as Eka-Boron.
4. Mendeleev's periodic table helped in correcting the atomic masses of some of the elements, based on their positions in the periodic table. For example, atomic mass of beryllium was corrected from 13.5 to 9.0. Atomic masses of indium, gold and platinum were also corrected.
5. In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.
6. The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).
7. Lithium, Sodium, Potassium are called Alkalies metal as they react with metal and form alkalies (Caustic solution)
8. Calcium, Strontium, Bromine are called Alkalies Earth metal as they react with metal and form salt as their oxide are alkalies in nature and exist in the Earth
9. Chlorine, Bromine, Iodine are called Halogen as their oxide are alkalies in nature and exist in the Earth
10. Alkali metal and coinage metals (Cu, Ag and Au) which differ widely in properties are placed into the same group.
In 1913, an English physicist MOSELEY, as a result of his work on characteristics X-rays of elements discovered a fundamental and most accurate relation between chemical properties and atomic number of elements, known as "Modern periodic law".
ACCORDING TO THE MODERN PERIODIC LAW:
1. The chemical and physical properties of elements are the periodic function of their atomic numbers".
2. The properties of elements depend upon their electronic configuration which varies with increasing atomic number in a periodic way".
1. Atomic size decreases in a period.
2-Nuclear charge increases in a period.
3-Ionization potential increases in a period.
4-Electronegativity increases in a period.
5-Electropositivity and metallic character decreases in period.
6-Each period starts with Alkali metal and ends on a Noble gas.
2nd period : Li .................................. Ne
3rd period : Na ..................................Ar
7-First element of each period is most reactive (electropositive), and last element is chemically inert.
1-Group number of an element shows the number of electrons in the outermost shell of that element
2-Atomic size increases in group.
3-Ionization potential of elements decreases down the group.
4-Electronegativity decreases down the group.
5-Metallic character increases down the groups.
On the basis of valence shell electronic configuration, periodic table is divided into four distinct blocks of elements.
All the elements in which last electron enters "ns" orbital are known as s-block elements.
Elements of group I-A and II-A belong to s-block.
Their valence shell electronic is ns1 to ns2.
All the elements of s-block are very reactive and electropositive in nature.
S-block elements are good reducing agents.
They form ionic compounds.
They are all metals and solid. They are low density metals.
For example :
Li (z=3) 1s2, 2s1
Na (z=11) 1s2, 2s2, 2p6, 3s1
Be (z=4) 1s2, 2s2
The elements in which valence electrons enter p-orbital and this orbital is progressively filled are
called p-block elements.
Their valence shell electronic configuration is from ns2, np1 to ns2, np6.
P-block consists of the elements of group III-A to VIII-A (zero-group).
They include metals, nonmetals and metalloids.
There are 30 elements in six sub-groups of p-block.
The elements in which last electron enters (n-1) d-orbital are called d-block elements or outer transition elements.
Valence shell electronic configuration of these elements is ns2, (n-1) d1-10.
All these elements are metals.There are four series of outer transition elements.
Fourth series is incomplete.
The elements in which last electron enters (n-2) f-orbital are called f-block elements or inner transition elements. Their outer electronic configuration is ns2,(n-1)d1,(n-2) f1-14
f-block elements constitute 2 series of elements.
Visit fore full study: CBSE Class 10th Periodic Classification
Council for the Indian School Certificate Examinations
Indian Certificate of Secondary Education Examination, March 2015
A1.Is light year a unit of time or distance? Define `1 light year?
Ans: Light year is a unit of distance .one light year is a distance covered by light in 1 year.
1 light year = 9.46 x 10^12 Km
A2.What is the source of stars energy?
Ans: The source of stars energy is nuclear fusion in which hydrogen atoms split up into helium atoms and large amount of energy is librated.
A3.What gives galaxies different shapes? Name a few common shapes of galaxies.
Ans: The arrangement of stars gives galaxies different shapes of galaxies. About 100 billians galaxies in the universe some of them are in shape of spiral, ring like or elliptical.
A4.What is the big Dipper?
Ans: The seven brightest stars in between the month of February and may form a shape of dipper in northern sky. This constellation is called Big deeper or Saptarshi.
A5. Why are Mercury venues, and Mars called terrestrial planets?
Ans: This is because they are closest to sun and rocky like earth.
A 6. Name the planet which have ring around them?
Ans: Saturn, Jupiter , Uranus and Neptune
A7. What is the huge spot on Jupiter?
Ans: This spots are actually a huge storm, which has been ranging on Jupiter for more than 300 years
A8. Name two dwarf planets?
Ans: Ceres and Eris
A9. How do dwarf planets different from other planets?
Ans: dwarf planets are not able to pull in the object (gravitational force) near its orbit at the time of its formation
A10.Why do scientists think that Mars had liquid water in the past?
Ans: Scientists think that Mars had liquid water in the past as the channels visible on its surfaces.
B1. What re the difference between stars and planets?
Stars are: a. luminous b. Stationary
Planets are : a. Non luminous b. Movable
B2. What re the difference between galaxy and constellation?
Ans: (i) The first difference is that a galaxy is a collection of billions of stars whereas a constellation is a collection of only a very few stars.
(ii) The second difference is that a galaxy does not form a definite pattern which resembles the shape of an animal or a human being but a constellation appears to resemble an animal or human being in its shape
B3. What are the asteroids?
Answer: There is a large gap in between the orbits of the Mars and Jupiter. This gap is occupied by a large number of small bodies that revolve around the sun. These are called asteroids
B.4.What is meteorites? In which bodies are meteorites craters more commonly found and why?
Ans: meteorites are rocky bodies revolving around the sun. Large meteors when hit the surface of planet create craters. Meteorites craters more commonly found on those planets and moon which have no or little atmosphere to burn off the falling rocks. E.g. Moon and mercury.
B5. What is comet? How is its tail formed?
Ans: Comet is small body of ice and dust revolving around the sun in its elongated orbit. When comet approaches the sun, it heats up and leaves behind a stream of hot glowing gases and dust particles. We see it as a tail.
B6. Why does geostationary satellite seem fixed above a point on the earth?
Ans: A satellite placed at about 3600 km above the equator completes one revolution around the sun in 24 hrs . This makes its position fixed above a point on the earth
B.7. why do other stars appear to move from east to west whereas the pole star appear to be stationary?
Ans: Pole star lies exactly above axis of the rotation of earth while other star does not. Therefore when earth rotates from west to east the star will appear to move from east to west to an observer on the earth but pole star appear stationary to same observer.
This property of pole star makes it useful to the sailors as they can locate north with the help of pole star
B.8. Why Venus is hotter than mercury even if it is far away from the Sun?
Ans: Venus has highest percentage of CO2 in its atmosphere that result into greenhouse effect.
B.9. why is mercury so hot during the day and cold at night?
Ans: Being close to sun, mercury turns extremely hot about 400 degree centigrade during day. As gravitational force is not enough to support hot atmosphere the temperature drops to - 200 degree centigrade.
B. 10. How far away is earth from the sun if sunlight takes 8.3 minutes to reach earth?
Ans: Time taken by sunlight to reach earth = 8.3 min = 498 sec
Speed of light = 3 x 10^8 m/s
Distance b/w earth and sun = 3 x 10^8 m/s x 498 sec = 1494 x 10^8 m
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Solution for Basic Science for class 8 Publication Bharati Bhawan [Central public School]
Multiple Choice Question
1. The male hormone testesterone is produced by _________.
(a) Leydig cells
(b) Seminiferous tubules
(d) Vas deferens
2. The process by which the sperms are produced is known as ____________.
3. The process by which the sperms are released is called __________________.
4. The attachment of the embryo to the uterus is called ____________________.
5. The sperms are temporarily stored in ___________.
(a) vas deferens
(b) vas efferens
Q. Name the following:
(a) The organ which produces sperm
(b) The organ which produces egg
(c) The place where embryo develops in human female
(d) The organ in reproductive system which receives the sperm
(e) A muscular tissue which delivers the sperm into vagina of female
(f) Name the method of asexual reproduction in which individuals develops from the bud.
(g) Name the process of reproduction in Amoeba.
(a) Testes (b) Ovaries (c) Uterus (d) Vagina (e) Penis (f) Budding (g) Binary fission
Q. Fill in the blanks:
(a) Male and female have different reproductive part testes and Ovaries respectively.
(b) Each parent produce special sex cell called gamete.
(c) The gamete produced by male and female is sperm and egg (Ovum) respectively.
(d) In most aquatic animal, external fertilization takes place.
(e) On reaching the uterus, the embryo attached to the wall of uterus by placenta. This is called implantation.
(f) In human fertilization of sperm and ovum takes place in the interior part of Oviduct
(g) The process of transformation of larva or tadpole into an adult through drastic changes is called metamorphosis.
(h) A hermaphrodite is a person who possesses the sexual organs of both male and female.
Solution for Basic Science Publication Bharati Bhawan [Central public School]
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