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Structure of the Atom - Neutrons

Grade 9CBSE

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

🔑Concepts

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Neutrons were discovered by J. Chadwick in 1932 as a subatomic particle having no charge and a mass nearly equal to that of a proton.

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Neutrons are present in the nucleus of all atoms, except for Hydrogen (11H^1_1H), which contains only one proton and no neutrons.

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The neutron is represented by the symbol nn.

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Since protons and neutrons are both located in the nucleus, they are collectively known as nucleons.

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The mass of an atom is determined by the sum of the masses of protons and neutrons present in the nucleus, as the mass of electrons is considered negligible.

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The number of neutrons in an atom can be calculated if the mass number (AA) and the atomic number (ZZ) are known.

📐Formulae

A=Z+nA = Z + n

n=A−Zn = A - Z

Mass of Atom≈Mass of Protons+Mass of Neutrons\text{Mass of Atom} \approx \text{Mass of Protons} + \text{Mass of Neutrons}

💡Examples

Problem 1:

Calculate the number of neutrons in an atom of Aluminum (AlAl) which has an atomic number Z=13Z = 13 and a mass number A=27A = 27.

Solution:

Given: A=27A = 27, Z=13Z = 13. Using the formula n=A−Zn = A - Z, we get n=27−13=14n = 27 - 13 = 14.

Explanation:

The mass number AA is the total count of protons and neutrons. Since the atomic number ZZ represents the number of protons, subtracting 1313 from 2727 gives the total number of neutrons (1414).

Problem 2:

An isotope of Carbon has 66 protons and 88 neutrons. Represent this atom using the notation ZAX^A_Z X and identify its mass number.

Solution:

Mass number A=protons+neutrons=6+8=14A = \text{protons} + \text{neutrons} = 6 + 8 = 14. The atomic number Z=6Z = 6. The notation is 614C^{14}_{6}C.

Explanation:

The mass number is the sum of the nucleons (66 protons and 88 neutrons). Carbon always has an atomic number of 66 (Z=6Z=6). This specific isotope is Carbon-14.

Problem 3:

If an atom has a mass number A=4A = 4 and contains 22 neutrons, identify the element and the number of protons.

Solution:

Z=A−n=4−2=2Z = A - n = 4 - 2 = 2. Since Z=2Z = 2, the element is Helium (HeHe).

Explanation:

By rearranging the mass number formula, we find the number of protons (ZZ). An element with 22 protons is Helium.