Selina Concise Chemistry Class 7 ICSE Solutions – Atoms, Molecules and Radicals

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 4 Atoms, Molecules and Radicals

Points to Remember :

  1. Atoms of the most elements are composed of still smaller particles known as fundamental particles or subatomic particles. They are protons, electrons and neutrons.
  2. Protons are the positively charged particles present in an atom.
  3. Electrons are the negatively charged particles present in an atom and its mass is 1/1837 the mass of one hydrogen atom.
  4. Neutrons are particles with no electrical charge. Its mass is equal to one atomic mass unit.
  5. The central part of an atom is called nucleus, which contains both protons and neutrons.
  6. Shells or orbits around the nucleus.
  7. Atomic number refers to the number of protons present in an atom. It is denoted by the alphabet Z.
  8. Mass number— Mass number is the sum of number of protons and neutrons present in the nucleus of an atom. Mass number (A) = Number of protons + Number of neutrons.
  9. Atomic mass— The mass of an atom is known as its atomic mass.
  10. Relative atomic mass ip the mass of an atom of an element as a multiple of the standard atomic mass unit.
  11. Atoms of different elements combine to form molecules so as to attain chemical stability.
  12. A positively charged ion is called a cation and a negatively charged ion is called anion.
  13. The valency of an element is the number of electrons donated or accepted by its ‘atom’ during chemical combination.
  14. There are some elements with more than one valency e.g., iron, copper, tin, lead.
  15. Two or more different non-metals that collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals.

EXERCISE

Question 1.
Define the following terms :
Answer:

  1. Atom : An atom is the smallest indivisible unit of an
    OR
    Atom is the smallest unit of matter.
  2. Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.
  3. Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
  4. Valency : It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
  5. Periodic table represents the tabular arrangment of elements in horizontal rows called periods and vertical columns called groups in order to classify the elements and their systematic study.

Question 2.
Write the names of the elements present in the following compounds.
Answer:

  1. Common salt : Sodium, chlorine.
  2. Ammonia : Nitrogen, hydrogen.
  3. Sulphuric acid : Hydrogen, sulphur, oxygen.
  4. Glucose : Carbon, hydrogen, oxygen.
  5. Sodium hydroxide : Sodium, oxygen, hydrogen.
  6. Acetic acid : Carbon, hydrogen, oxygen.

Question 3.
What does each of the following represent ?
Answer:

  1. 2C02 = 2 molecules of carbon dioxide.
  2. 2H2S = 2 molecules of hydrogen sulphide.
  3. 5H2S04 = 5 molecules of sulphuric acid.
  4. 6NaNO3 = 6 molecules of sodium nitrate.

Question 4.
Write the symbols and valencies of the following radicals:
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 4
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 4.
Question 5.
Name the following radicals :
Answer:

  1. SO42- = Sulphate
  2. HC03– = Bicarbonate
  3. OH- = Hydroxide
  4. Cr2072-  = Dichromate

Question 6.

  1. Name one ion for  each of the valencies +1, +2  and +3.
  2. Name one ion for  each of the valencies-1, -2  and -3.

Answer:

  1. +1 = Sodium Na+
    +2 = Calcium Ca+2
    +3 = Aluminium Al+3
  2. -1 = Chlorine Cl-
    -2 = Oxygen 0-2
    -3 = Nitrogen N3_

Question 7.
The valency of calcium is 2. Write the valencies of other radical in the following :

  1. CaO
  2. Ca(OH)2
  3. CaC03
  4. CaCl2

Answer:

  1. O= 2
  2. OH = 1
  3. CO3 = 2
  4. Cl = l

Question 8.
Write the names of the following compounds :
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 8

Question 9.
Write the molecular formulae of:
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 9

Question 10.
The valency of sodium is one, write the molecular formula for the following compounds of sodium.

  1. sodium oxide : Na20
  2. sodium sulphate : Na2S04
  3. sodium carbonate : Na2CO3
  4. sodium hydroxide : NaOH
  5. sodium nitrate : NaN03

 

Question 11.
What is variable valency ? Give two examples of elements showing variable valency.
Answer:
There are some elements with more than one valency. They are said to have variable valency, e.g. Iron, copper.

Question 12.
Give the group number of following elements present in periodic table

  1. Magnesium : IIA
  2. Carbon : IVA
  3. Sulphur : VIA
  4. Neon : Zero

 

Question 13.
An element belongs to group VA. What would be its valency? Name two such elements.
Answer:
Elements of group VA has valency 3.
Two elements : Nitrogen and phosphorus.

Question 14.
An element belongs to group II. What would be its valency? Write the formula of molcules of compounds it will form with elements in VA, VIA and VIIA groups.
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 14
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 14 a

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
Answer:

  1. Atoms are neutral.
  2. An ion with positive charge is called cation.
  3. An ion with negative charge is called anion.
  4. 2H2 means two atoms of hydrogen.
  5. Ozone is a triatomic molecule.
  6. Metals have variable valency.
  7. Chemical name of caustic soda is sodium hydroxide NaOH.

2. Tick (√) the correct answer.

(a) The valency of iron in Fe203 is

  1. 1
  2. 2
  3. 3
  4. 6

(b) Which of the following has valency 4 ?

  1. aluminium
  2. oxygen
  3. carbon
  4. phosphorus

(c) The sulphate radical is written as S042-. What is the formula of calcium sulphate ?

  1. Ca(S04)2
  2. Ca2(S04)
  3. Ca(S04)3
  4. CaS04

(d) Which of the following exhibit variable valency ?

  1. calcium
  2. copper
  3. carbon
  4. chlorine

3. State the term for the following:

  1. The number of atoms present in a molecule of an element atomicity.
  2. The symbolic representation of a molecule molecular formula.
  3.  A group of atoms that react as a single unit molecule.
  4. The combining capacity of an element valency.
  5. The tabular arrangement of elements in horizontal rows and vertical columns periodic table.

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