# Samacheer Kalvi Class 7 Science Term 1 Unit 4 Solutions

## Samacheer Kalvi Class 7 Science Term 1 Unit 4 Atomic Structure Solutions

Welcome to NCTB Solutions. Here with this post we are going to help 7th class students by providing Solutions for Samacheer Kalvi Class 7 Science Term 1 Unit 4 Atomic Structure. Here students can easily find all the solutions for Atomic Structure Exercise questions. Also here our Expert Science Teacher’s solved all the questions with easily understandable language with proper guidance so that all the students can understand easily. Here on this post students will get Unit 4 exercise solutions. Here all the solutions are based on Tamil Nadu State Board latest syllabus.

#### Unit 4 Atomic Structure Exercise Questions Solutions :

(I) Choose the appropriate answer :

(1) The basic unit of matter is ____

(a) element

(b) atom

(c) molecule

(d) electron

Correct Option → (b)

The basic unit of matter is Atom.

(2) The sub-atomic particle which revolves around the nucleus is __.

(a) atom

(b) neutron

(c) electron

(d) proton

Correct Option → (c)

The subatomic particle revolve around the nucleus is Electron.

(3) ___ is positively charged

(a) Proton

(b) Electron

(c) Molecule

(d) Neutron

Correct Option → (a)

Proton is positively charged.

(4) The atomic number of an atom is the ___

(a) number of neutrons

(b) number of protons

(c) total number of protons and neutrons

(d) number of atoms

Correct Option → (b)

The atomic number of an atom is Number of protons.

(5) Nucleons comprises of _____

(a) protons and electrons

(b) neutrons and electrons

(c) protons and neutrons

(d) neutrons and positron

Correct Option → (c)

Nucleons comprises of Protons and Neutrons

(II) Fill in the blanks :

(1) The smaller particles found in the atom are called ___.

(2) The nucleus has ___ and ___

(3) The ____ revolve around the nucleus.

(4) If the valency of carbon is 4 and that of hydrogen is 1, then the molecular formula of methane is ___

(5) There are two electrons in the outermost orbit of the magnesium atom. hence, the valency of magnesium ___

(1) → Sub-atomic particles

(2) → Protons and Neutrons.

(3) → Electrons

(4) → CH4

(5) → 2

(III) Match the following :

 (1) Valency Fe (2) Neutral Particle Proton (3) Iron Electrons in the outermost Orbit (4) Hydrogen Neutron (5) Positively charged Particle Monovalent

(1) Valency → Electrons in the outermost Orbit.

(2) Neutral Particle → Neutron.

(3) Iron → Fe.

(4) Hydrogen → Monovalent.

(5) Positively charged Particle → Proton.

(IV) State true or false. If false, correct the statement.

(1) The basic unit of an element is molecule.

(2) The electrons are positively charged.

(3) An atom is electrically neutral.

(4) The nucleus is surrounded by protons.

Solution :

(1) → False.

Because, the basic unit of an element is atom.

(2) → False.

Because, they are negatively charged.

(3)  → True

(4) → False

Because, the nucleus is surrounded by electrons.

(V) Complete the analogy :

(1) Sun : Nucleus : : Planets : ___.

(2) Atomic number : ___ : : Mass number : Number of protons and neutrons.

(3) K: Potassium : : C : ___.

(1) Sun : Nucleus : planets : Electrons

(2) Atomic number : Number of protons, Mass number : number of protons and neutrons.

(3) K : Potassium : C : Carbon.

(VI) Consider the following statements and choose the correct option :

(1) Assertion : An atom is electrically neutral.

Reason : Atoms have equal number of protons and electrons.

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(2) Assertion : The mass of an atom is the mass of its nucleus.

Reason: The nucleus is at the centre.

Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(3) Assertion : The number of protons or the number of neutrons is known as atomic number.

Reason : The mass number is the sum of protons and neutrons.

Assertion is false but the reason is true statement.

The number of protons and neutrons is mass number.

(1) Define – Atom.

An atom is the smallest unit of matter that can retain it’s chemical identity in all the physical and chemical changes.

(2) Name the sub-atomic particles.

Proton, Electron, Neutron are the sub-atomic particles.

(3) What is atomic number?

Atomic number is a number of electrons and protons in an atom which is represented by letter Z.

(4) What are the characteristics of proton?

Characteristics of proton are

(a) They are positively charged particles.

(b) Located in the nucleus.

(c) It take only part in nuclear reaction.

(5) Why neutrons are called neutral particles?

Neutrons called neutral particles because they are not positive like protons and not negative like electrons and in its atoms they have no charge.

(1) Distinguish isotopes from isobar.

Isotopes :

(i) Isotopes have same atomic number.

(ii) Isotopes have different mass number.

(iii) Number of protons is same.

Isobars :

(i) Isobars have different atomic number.

(ii) Isobars have same mass number.

(iii) Number of protons is different.

(2) What are isotones? Give one example.

Atoms of different elements with same number of neutrons is called Isotones.

Example :

136C Neutrons

= 13 – 6

= 7

147C Neutrons

= 13 – 6

= 7

(3) Differentiate mass number from atomic number.

Atomic number :

(a) It is the number of protons present in an atom.

(b) Isotopes have same atomic number.

(c) Represented by the letter Z.

Mass number :

(a) It is the sum of the number of protons and the neutrons of an atom.

(b) Isotopes have different mass number.

(c) Represented by the letter A.

(4) The atomic number of an element is 9. It has 10 neutrons. Find the element from the periodic table. What will be its mass number?

As we know that the element number 9 is Fluorine

Its mass number (A) = number of proton + number of neutron

= 10 + 9

= 19

Therefore, Mass number of Fluorine = 19

(2) The atomic number and the mass number of an element is 26 and 56 respectively. Calculate the number of electrons, protons and neutrons in its atom. Draw the structure.

In this question we have:

Atomic number A = 26

Mass number A = 56

So, the given element is Iron

So, No. of proton (P) = 26

Mass number = no. of proton + no. of neutron

56 = n + 26

n + 26 = 56

n = 56 – 26

∴ No. of neutrons n = 30

In an element No. of protons and No. of electrons is equal we know that.

∴ No. of electrons,

= no. of protons

= 26

(3) What are nucleons? Why are they called so? Write the properties of the nucleons

As we know that the sub-atomic like protons and neutrons which are located in the nucleus of atoms are called nucleons. They are called nucleons because they exist within the nucleus of the atom together.

Properties of nucleons :

(a) Nucleons have same mass.

(b) The size nucleon is about 1 : 6 fm.

(c) The force that holds the nucleus together is very short range.

(4) Define valency. What is the valency of the element with atomic number 8? What is the compound format by this element with hydrogen?

As we know that the combining capacity of an element is called Valency.

We know that Oxygen is the element with atomic no. 8

So, It’s valency is 2

H2O is the most common compound by the element with hydrogen.

(X) Higher Order Thinking Skills :

(1) An atom of an element has no electron. Will that atom have any mass or not? Can an atom exist without electron? If so then give example.

As we know that mass does not depend on electron it depends upon number of protons and neutrons so the atom of an element has no electron have mass. Yes, atom exist without electron.

Example : He2+. It has 2 protons and 2 neutrons but no electrons.

(2) What is common salt? Name the elements present in it. Write the formula of common salt. What are the atomic number and the mass number of the elements? Write the ions in the compound.

Sodium chloride which are also called NaCl is the common salt where sodium and chlorine are the elements present in it.

Atomic number of Na = 1

Atomic number of Cl = 17

Mass number of Na = 23

Mass number of Cl = 35

Lons present in the compound are Na+ (1, 8, 1) and Cl (2, 8, 7).

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Updated: July 31, 2023 — 4:44 am