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Structure of the Atom - What Components Contribute to the

Grade 9CBSE

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

🔑Concepts

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The atom consists of three subatomic particles: protons, neutrons, and electrons.

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Protons and neutrons are located in the nucleus and are collectively known as nucleons. They contribute almost the entire mass of the atom.

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The mass of an electron is considered negligible (1/20001/2000 times the mass of a hydrogen atom), but it contributes the negative charge to the atom.

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Atomic Number (ZZ) is the total number of protons present in the nucleus of an atom. It determines the identity of the element.

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Mass Number (AA) is the sum of the total number of protons and neutrons in the nucleus.

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In a neutral atom, the number of protons is equal to the number of electrons, making the atom electrically neutral.

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Valency is the combining capacity of an atom, determined by the number of electrons in its outermost (valence) shell.

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Isotopes are atoms of the same element with the same atomic number but different mass numbers (different number of neutrons).

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Isobars are atoms of different elements with different atomic numbers but the same mass number.

📐Formulae

Z=p=eZ = p = e (For a neutral atom)

A=p+nA = p + n

n=A−Zn = A - Z

Maximum electrons in a shell=2n2\text{Maximum electrons in a shell} = 2n^2

Average Atomic Mass=∑(Mass of Isotope×Percentage Abundance100)\text{Average Atomic Mass} = \sum \left( \text{Mass of Isotope} \times \frac{\text{Percentage Abundance}}{100} \right)

💡Examples

Problem 1:

Calculate the number of neutrons in an atom of Sodium (NaNa) which has a mass number A=23A = 23 and atomic number Z=11Z = 11.

Solution:

Given: A=23A = 23, Z=11Z = 11. Using the formula for neutrons: n=A−Zn = A - Z n=23−11n = 23 - 11 n=12n = 12

Explanation:

Since the mass number is the sum of protons and neutrons, subtracting the atomic number (protons) from the mass number gives the number of neutrons.

Problem 2:

An element XX has 66 protons and 66 neutrons. Determine its atomic number, mass number, and identify the element.

Solution:

Z=Number of Protons=6Z = \text{Number of Protons} = 6 A=Protons+Neutrons=6+6=12A = \text{Protons} + \text{Neutrons} = 6 + 6 = 12 The element with Z=6Z = 6 is Carbon.

Explanation:

The atomic number is defined by the number of protons. The mass number is the total count of nucleons (protons + neutrons).

Problem 3:

Chlorine exists in two isotopic forms with masses 35u35u and 37u37u in the ratio 3:13:1. Calculate the average atomic mass of Chlorine.

Solution:

Average Mass=(35×75100)+(37×25100)\text{Average Mass} = \left( 35 \times \frac{75}{100} \right) + \left( 37 \times \frac{25}{100} \right) Average Mass=(35×34)+(37×14)\text{Average Mass} = \left( 35 \times \frac{3}{4} \right) + \left( 37 \times \frac{1}{4} \right) Average Mass=1054+374=1424=35.5u\text{Average Mass} = \frac{105}{4} + \frac{37}{4} = \frac{142}{4} = 35.5u

Explanation:

The average atomic mass is calculated by taking the weighted average of the masses of all naturally occurring isotopes of that element.