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Atomic Structure - Particle Theory of Matter

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Matter is composed of very tiny particles called atoms or molecules. These particles are so small that they cannot be seen with the naked eye.

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The space between particles is known as Intermolecular SpaceIntermolecular\ Space, and the force of attraction between them is called Intermolecular ForceIntermolecular\ Force.

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In solids, particles are closely packed with negligible Intermolecular SpaceIntermolecular\ Space and very strong Intermolecular ForceIntermolecular\ Force.

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In liquids, particles are less closely packed compared to solids, allowing them to flow.

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In gases, particles have the maximum Intermolecular SpaceIntermolecular\ Space and the weakest Intermolecular ForceIntermolecular\ Force, moving randomly in all directions.

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An atom consists of three subatomic particles: Protons (p+p^+), Neutrons (n0n^0), and Electrons (e−e^-).

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The nucleus of an atom contains protons and neutrons, while electrons revolve around the nucleus in fixed paths called shells or orbits.

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An atom is electrically neutral because the number of protons (++) is equal to the number of electrons (−-).

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The Atomic Number (Z)Atomic\ Number\ (Z) of an element is the number of protons present in the nucleus of its atom.

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The Mass Number (A)Mass\ Number\ (A) is the total number of protons and neutrons in the nucleus.

📐Formulae

Atomic Number (Z)=Number of protons (p)Atomic\ Number\ (Z) = Number\ of\ protons\ (p) icon

Mass Number (A)=Number of protons (p)+Number of neutrons (n)Mass\ Number\ (A) = Number\ of\ protons\ (p) + Number\ of\ neutrons\ (n)

Number of neutrons (n)=A−ZNumber\ of\ neutrons\ (n) = A - Z

For a neutral atom: Number of protons (p)=Number of electrons (e)For\ a\ neutral\ atom:\ Number\ of\ protons\ (p) = Number\ of\ electrons\ (e)

💡Examples

Problem 1:

An atom of Sodium (NaNa) has an atomic number Z=11Z = 11 and a mass number A=23A = 23. Calculate the number of protons, electrons, and neutrons.

Solution:

Given: Z=11Z = 11, A=23A = 23.

  1. Number of protons p=Z=11p = Z = 11.
  2. For a neutral atom, number of electrons e=p=11e = p = 11.
  3. Number of neutrons n=A−Z=23−11=12n = A - Z = 23 - 11 = 12.

Explanation:

The atomic number gives the count of protons and electrons, while the difference between the mass number and atomic number gives the count of neutrons.

Problem 2:

Calculate the mass number of an element that has 1313 protons and 1414 neutrons.

Solution:

A=p+nA = p + n 13+1427\begin{array}{r} 13 \\ +14 \\ \hline 27 \end{array} So, A=27A = 27.

Explanation:

The mass number is the sum of the nucleons (protons and neutrons) present in the nucleus of the atom.

Problem 3:

If an atom has 88 electrons and 88 neutrons, what is its atomic number and mass number?

Solution:

Number of electrons e=8e = 8. Since it is a neutral atom, p=e=8p = e = 8. Therefore, Z=8Z = 8. Mass number A=p+n=8+8=16A = p + n = 8 + 8 = 16.

Explanation:

Since the number of protons defines the identity of the element, the atomic number is 88 (Oxygen).