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Structure of the Atom - Mass Number

Grade 9CBSE

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

🔑Concepts

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The mass of an atom is practically concentrated in its nucleus. The nucleus contains both protons and neutrons, which are collectively referred to as nucleons.

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The Mass Number (AA) is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.

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For any element XX, the notation used to represent the mass number and atomic number is \prescriptAZX\prescript{A}{Z}{X}, where AA is the mass number and ZZ is the atomic number.

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The number of neutrons (nn) in an atom can be calculated if the mass number (AA) and atomic number (ZZ) are known, using the relationship between these three values.

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Since protons and neutrons each have a mass of approximately 1 u1\text{ u}, the mass number is numerically equal to the atomic mass of the atom in unified atomic mass units (uu).

📐Formulae

A=Z+nA = Z + n

n=A−Zn = A - Z

Mass Number=Number of Protons+Number of Neutrons\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}

💡Examples

Problem 1:

An atom of Aluminum has an atomic number Z=13Z = 13 and contains 1414 neutrons. Calculate its mass number AA.

Solution:

A=13+14=27A = 13 + 14 = 27

Explanation:

To find the mass number, we add the atomic number (number of protons) to the number of neutrons. Using the formula A=Z+nA = Z + n, we get 13+14=2713 + 14 = 27.

Problem 2:

Calculate the number of neutrons in a Bromine atom represented as \prescript8035Br\prescript{80}{35}{\text{Br}}.

Solution:

80−3545\begin{array}{r} 80 \\ -35 \\ \hline 45 \end{array}

Explanation:

From the notation \prescriptAZBr\prescript{A}{Z}{\text{Br}}, we have the mass number A=80A = 80 and the atomic number Z=35Z = 35. The number of neutrons nn is calculated by subtracting the atomic number from the mass number: n=A−Zn = A - Z.

Problem 3:

The mass number of an element is 2323 and it has 1111 electrons. If the atom is neutral, identify the element and the number of neutrons.

Solution:

n=23−11=12n = 23 - 11 = 12

Explanation:

In a neutral atom, the number of electrons equals the number of protons (ZZ). Thus, Z=11Z = 11. The element with atomic number 1111 is Sodium (Na\text{Na}). The number of neutrons is found by A−ZA - Z, which is 23−11=1223 - 11 = 12.