Living Science Chemistry Class 7 ICSE Solutions Chapter 1 Elements and Compounds

ICSE Living Science Chemistry for Class 7 Solutions Chapter 1 Elements and Compounds

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Answer these questions.
Question 1.
Name the smallest particle of an element that exhibits the properties of the element.
Answer:
Atom

Question 2.
Which other subatomic particle is usually found with protons in the nucleus ?
Answer:
Neutron

Question 3.
Which subatomic particles are negatively charged ?
Answer:
Electrons

Question 4.
What is the charge on a proton ?
Answer:
+1 (1 unit positive charge)

Question 5.
What is a molecule ?
Answer:
A molecule is a group of two or more atoms that are chemically joined together.

Question 6.
Name a polyatomic molecule that has 4 atoms in it.
Answer:
P₄ (Phosphorous molecule)

A. Tick the most appropriate answer

Question 1.
The earth’s crust is made up mainly of oxygen and –
a. Carbon
b. phosphorus
c. silicon
d. nitrogen
Answer:
c. silicon

Question 2.
Which of the following is a metalloid ?
a. Copper
b. Silver
c. Germanium
d. Lead
Answer:
c. Germanium

Question 3.
The non-metal which is liquid at room temperature is
a. sulphur
b. chlorine
c. iodine
d. bromine
Answer:
d. bromine

Question 4.
The symbol of radon is
a. Ra
b. Rn
c. Rd
d. Ro
Answer:
b. Rn

Question 5.
Which of the following is a triatomic molecule ?
a. Oxygen
b. Sulphur
c. Nitrogen
d. Ozone
Answer:
d. Ozone

Question 6.
Which among the following shows variable valency ?
a. Zinc
b. Sodium
c. Iron
d. Calcium
Answer:
c. Iron

Question 7.
Which of the following lose electrons to form cations ?
a. Metals
b. Non-metals
c. Both a. and b.
d. None of these
Answer:
a. Metals

B. Fill in the blanks.

Question 1.
An …………… is a pure substance which cannot be broken down into simpler substances by physical or chemical means.
Answer:
An element is a pure substance which cannot be broken down into simpler substances by physical or chemical means.

Question 2.
The …………… of an element is the one- or two-letter abbreviation of its name.
Answer:
The symbol of an element is the one- or two-letter abbreviation of its name.

Question 3.
The modern system of denoting the elements by symbols was introduced by ……………
Answer:
The modem system of denoting the elements by symbols was introduced by J.J.Berzelius.

Question 4.
…………… is the Latin name of copper.
Answer:
Cuprum is the Latin name of copper.

Question 5.
…………… have unit positive charge while …………… have unit negative charge.
Answer:
Protons have unit positive charge while electrons have unit negative charge.

Question 6.
A/An …………… has mass 1/183 time that of a hydrogen atom.
Answer:
An electron has mass 1/183 time that of a hydrogen atom.

Question 7.
The …………… is an electrically neutral particle present in the nucleus.
Answer:
The neutron is an electrically neutral particle present in the nucleus.

Question 8.
A/An …………… is the smallest particle of an element or a compound that is capable of an independent existence.
Answer:
A molecule is the smallest particle of an element or a compound that is capable of an independent existence.

C. State if the following statements are true or false. Correct the statement if it is false.

Question 1.
A pure substance is composed of the same kind of particle throughout.
Answer:
True

Question 2.
Metals are not ductile.
Answer:
False. Metals are ductile.

Question 3.
Metalloids have intermediate properties between those of metals and non-metals.
Answer:
True

Question 4.
The symbol of cobalt is C.
Answer:
False.
The symbol of carbon is C.

Question 5.
The smallest particle of an element that exhibits the properties of that element is called an atom.
Answer:
True.

Question 6.
The tiny central core of an atom consisting of protons and neutrons is called the nucleus.
Answer:
True.

Question 7.
A proton is electrically neutral.
Answer:
False.
A neutron is electrically neutral.

Question 8.
The particle formed by the loss or gain of electrons by an atom is called an ion.
Answer:
True

D. Match the following

Column A	Column B
1. Charged particle	a. Element
2. ZnS	b. Ion
3. Positively charged ion	c. Nucleus
4. Central core of an atom	d. Zinc sulphide
5. Argon	e. Cation
6. Valency	f. Anion
	g. Whole number

Answer:

Column A	Column B
1. Charged particle	b. Ion
2. ZnS	d. Zinc sulphide
3. Positively charged ion	e. Cation
4. Central core of an atom	c. Nucleus
5. Argon	a. Element
6. Valency	g. Whole number

E. Name the following.

Question 1.
The hardest natural substance known
Answer:
Diamond

Question 2.
The element whose Latin name is kalium
Answer:
Potassium

Question 3.
The smallest indivisible particle of an element
Answer:
Atom

Question 4.
A subatomic particle which is negatively charged
Answer:
Electron

Question 5.
A force of attraction that holds the atoms of a compound together
Answer:
Chemical bond

Question 6.
An atom that loses one or more electrons to form a positive ion
Answer:
Cation

Question 7.
An atom that gains one or more electrons to form a negative ion.
Answer:
Anion

Question 8.
The ion consisting of a group of atoms with an overall charge
Answer:
Polyatomic or compound ion

F. State the symbols for the following elements.

1. Sodium
2. Potassium
3. Iron
4. Copper
5. Silver

Answer:

1. Sodium – Na
2. Potassium – K
3. Iron – Fe
4. Copper – Cu
5. Silver – Ag

G Write the formula for the following.

1. Aluminium chloride
2. Magnesium hydroxide
3. Calcium chloride
4. Sodium carbonate
5. Copper(II) oxide
6. Iron (II) sulphide

Answer:
1. Formula of aluminium chloride Symbol :

Formula : Al₁, Cl₃ = AlCl₃

2. Formula of magnesium hydroxide –

Formula : Mg₁ (OH)₂ = Mg(OH)₂

3. Formula of calcium chloride –

Formula : Ca₁Cl₂ = CaCl₂

4. Formula of sodium carbonate –

Formula: Na₂(CO₃), = NaC0₃

5. Formula of copper (II) oxide –

Formula : Cu₂O₂ = CuO

6. Formula of iron (II) sulphide –

Formula : Fe₂S₂ = FeS

H. Write the names of the compounds represented by the following formula.

1. Al₂(SO₄)₃
2. K₂SO₄
3. KNO₃
4. CaCO₃
5. MgCl₂

Answer:

1. Al₂(SO₄)₃ – Aluminium sulphate
2. K₂SO₄ – Potassium sulphate
3. KNO₃ – Potassium nitrate
4. CaCO₃ – Calcium carbonate
5. MgCl₂ – Magnesium chloride

I. Define the following terms.

1. Atomicity
2. Chemical bond
3. Valency
4. Ion
Answer:
1. Atomicity – The number of atoms present in a molecule of an element is called its atomicity. Depending on the atomicity, molecules can be monoatomic (e.g. helium = He), diatomic (e.g. oxygen – O₂), triatomic (e.g. ozone = O₃) or polyatomic (e.g. phosphorous = P₄)

2. Chemical bond – A chemical bond is a force of attraction which bonds the atoms of a compound together.

3. Valency – Valency is the number of electrons an atom can donate or gain to form a chemical bond with another atom. It is the combining capacity of an atom of an element. For example, in the formation of sodium chloride’s the sodium atom donates one electron that is accepted by the chlorine atom. Therefore, the valency of sodium is 1 and that of chlorine is also 1.

4. Ion – When an atom loses or gains electrons, it becomes a charged particle called ion. When an atom loses one or more electrons, it forms a positive ion called cation, e.g. Na+ (sodium ion). When an atom gains one or more electrons, it forms a negative ion called anion e.g. CT (chloride ion).

J. Answer the following in short.

Question 1.
What are the four groups into which all the elements can be divided ?
Answer:
All elements can be divided into four groups, namely, metals, non-metals, metalloids and inert or noble gases.

Question 2.
Name the subatomic particles present in an atom.
Answer:
The subatomic particles present in an atom are electrons, protons and neutrons.

Question 3.
Give the charge and mass of an electron, a proton and a neutron.
Answer:

Question 4.
Where is the nucleus of an atom present ?
Answer:
Nucleus of an atom is present at the centre of an atom as a tiny central core containing protons and neutrons.

Question 5.
Out of atoms and molecules, which can exist independently?
Answer:
The atoms of many elements cannot exist independently but molecules can exist independently.

Question 6.
How is a compound formed ?
Answer:
A compound is formed when atoms of different elements – combine together chemically. When two atoms of different elements come closer to each other, one atom gives or donates one or more electrons, which are accepted or gained by the other atom. As a result, a chemical bond is formed, which holds the atoms together to form a compound. For example. When sodium atom combines with chlorine atom, sodium transfers one electron to chlorine atom and compound called sodium chloride is formed.

Question 7.
What is meant by the term ‘variable valency’?
Answer:
Variable valency of an element means that those elements have more than one valency. Example, Copper has two valencies – 1
and 2 written as copper (I) copper (II).

Question 8.
How is a cation formed ?
Answer:
When an atom loses one or more electrons, a positive ion called cation is formed.

Question 9.
How is an anion formed ?
Answer:
When an atom gains one or more electrons, a negative ion called anion is formed.

Question 10.
What is a polyatomic ion ?
Answer;
An ion that consists of a group of atoms with an overall charge is called a polyatomic or compound ion. For example, ammonium ion (NH⁺₄), hydroxide ion (OH^–), etc.

Question 11.
Name the polyatomic ion that carries a positive charge.
Answer:
Ammonium ion (NH⁺₂)

Question 12.
What is a chemical formula ?
Answer:
The chemical formula of a compound is the symbolic respresentation of its composition. It tells us the number of atoms of various elements present in one molecule of a compound. For example, the chemical formula of water is H₂O.

K. Answer the following in detail

Question 1.
What is the difference between an atom and a molecule?
Answer:
Atom :

• An atom of an element is the smallest particle that exhibits the properties of that element.
• Atoms generally cannot exist independently.
• Example : Atom of hydrogen is.H.
• It consists of sub atomic particles – protons, electrons and neutrons.

Molecule :

• A molecule is a group of two or more atoms that are chemically joined together.
• Molecules can exist independently.
• Example : Molecule of hydrogen is H₂
• It consists of two or more atoms of same or different elements.

Question 2.
Describe the structure of an atom.
Answer:
An atom consits of three subatomic particles : electrons, protons and neutrons. At the centre of an atom, the protons and neutrons form a tiny central core called nucleus. The electrons in an atom move around the nucleus in definite paths called oibits.

Electron : Negatively charged particle which carries one unit negative charge and has mass nearly 1/1837 time that of a hydrogen atom.

Mass : The mass of an atom and subatomic particles are expressed in atomic mass units (amu or u).The mass of a
proton is equal to 1 amu or u. The mass of a neutron is 1 u. The mass of an electron is negligible.
electron

Proton : Positively charged particle which carries one unit positive charge and has mass nearly equal to that of a hydrogen atom. Thus, a proton is 1837 times heavier than an electron.

Neutron : A neutron is electrically neutral, that is, it does not cany any charge. The mass of a neutron is equal to the mass of a proton.

Question 3.
What is the difference between a molecule of an element and a molecule of a compound ? Give an example of each.
Answer:
The molecules of an element are made up of the same type of atoms. Examples :
1. molecules (He), argon (Ar), etc are made up of only one atom of that element.
2. molecules of oxygen, are made up of two oxygen atoms combined together.

Depending on the atomicity, molecules of an element can be monoatomic (helium = He), diatomic (oxygen = O₂), triatomic (ozone = O₂) or polyatomic (phosphorous = P₄).
The molecules of a compound are made up of atoms of different elements combined together in a definite proportion.

Examples :
1. Water (H₂O) represent a molecule of the compound water in which two atoms of hydrogen combine with one atom of oxygen.
2. Hydrogen chloride (HCl) is a molecule of compound having one atom of hydrogen and one of chlorine.

Question 4.
What happens when atoms combine to form a compound? Explain with an example.
Answer:
A compound is formed when atoms of different elements ; combine together chemically. When two atoms of different
elements come closer to each other, one atom gives or donates one or more electrons, which are accepted or gained by the other atom. As a result, a chemical bond is formed, which holds the atoms together to form a compound.
For example. When sodium atom combines with chlorine atom, k sodium transfers one electron to chlorine atom and compound called sodium chloride is formed.

Question 5.
Write the difference between monoatomic ions and polyatomic ions with the help of examples.
Answer:
An ion that consists of only a single atom with a charge is called a simple ion or monoatomic ion. For example, a sodium i ion (Na⁺)) is a simple ion because it is formed from a single sodium ion. Magnesium ion (Mg²⁺), aluminium ion (Al³⁺), oxide ion (O^2-), chloride ion (Cl^–), etc are other monoatomic ions.

An ion that consists of a group of atoms with an overall charge 1 is called a polyatomic or compound ion. For example
ammonium ion (NH⁺₄) is a polyatomic ion which is made up of nitrogen and hydrogen with a positive charge. Carbonate ion (CO₃^2-), hydroxide ion (OH^–), etc are other polyatomic ions.

L. Answer the following questions.

Question 1.
What is the valency of copper in –
a. CuSO₄
b. Cu₂O?
Answer:
(a) Since sulphate has a charge of -2, copper would also have a charge/valency of +2, i.e., Cu(II).
(b) Since the valencies / charges of atoms are interchanged while writing the formulae, valency of copper in Cu₂O would be +1, i.e. Cu(I).

Question 2.
An element X has the valency 3 while element Y has the valency 2. Write the formula of the compound formed between X an Y.
Answer:

Formula: X₂Y₃

Question 3.
XCl₂ is the chloride of a metal X. State the formula of the sulphate and the hydroxide of metal X.
Answer:
From XCl₂, we know that the valency of X is +2 (as valencies of X and Cl are interchanged while writing the chemical formula) So, formula of die sulphate of X →

Formula: X₂(SO₄)₂ = XSO₄
Formula of hydroxide of metal X →

Formula: X(OH)₂

Question 4.
The formula of the nitride of a metal X is XN. State the formula of its a. Chloride b. Carbonate.
Answer:
Since charge of nitride is -3, the charge of X would be + 3 as the formula is XN.

(a) Formula of chloride of X

Formula: XCl₃

(b) Formula of Carbonate of X

Formula : X₂(CO₃)₃

M. Draw the diagram of the following

Question 1.
To show the structure of an atom.

Living Science Chemistry Class 7 ICSE Solutions