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Structure of Atom - Discovery of Neutrons-advanced

Grade 9CBSE

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

🔑Concepts

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The neutron was discovered by James Chadwick in 1932 by bombarding a thin sheet of Beryllium (49Be^{9}_{4}Be) with high-energy alpha particles (24He^{4}_{2}He).

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Neutrons are subatomic particles that carry no electrical charge (neutral) and are located within the atomic nucleus alongside protons.

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The mass of a neutron is approximately 1.675×10−27 kg1.675 \times 10^{-27}\text{ kg}, which is nearly equal to the mass of a proton (1.672×10−27 kg1.672 \times 10^{-27}\text{ kg}) or approximately 1 u1\text{ u}.

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In the symbolic notation of an element ZAX^{A}_{Z}X, AA represents the mass number and ZZ represents the atomic number.

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The sum of the number of protons and neutrons in an atom is called its Mass Number (AA). Thus, the number of neutrons is given by n=A−Zn = A - Z.

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Except for Hydrogen (11H^{1}_{1}H), which contains only one proton and no neutrons, atoms of all other elements contain neutrons in their nuclei.

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Advanced insight: A free neutron is unstable and undergoes beta decay into a proton, an electron, and an antineutrino with a half-life of about 10.310.3 minutes.

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Neutrons play a crucial role in providing stability to the nucleus by overcoming the electrostatic repulsion between positively charged protons via the strong nuclear force.

📐Formulae

A=Z+nA = Z + n

n=A−Zn = A - Z

49Be+24He→612C+01n^{9}_{4}Be + ^{4}_{2}He \rightarrow ^{12}_{6}C + ^{1}_{0}n

mn≈1.00866 um_n \approx 1.00866\text{ u}

Mass of Atom≈(Number of protons+Number of neutrons)×1.67×10−27 kg\text{Mass of Atom} \approx (\text{Number of protons} + \text{Number of neutrons}) \times 1.67 \times 10^{-27}\text{ kg}

💡Examples

Problem 1:

An atom of an element has a mass number A=23A = 23 and an atomic number Z=11Z = 11. Calculate the number of neutrons present in its nucleus and identify the element.

Solution:

Given: Mass number A=23A = 23 Atomic number Z=11Z = 11

Using the formula: n=A−Zn = A - Z 23−1112\begin{array}{r} 23 \\ - 11 \\ \hline 12 \end{array} n=12n = 12

The number of neutrons is 1212. Since the atomic number Z=11Z = 11, the element is Sodium (NaNa).

Explanation:

The atomic number represents the number of protons. The mass number is the sum of protons and neutrons. By subtracting the atomic number from the mass number, we find the count of neutrons.

Problem 2:

Compare the mass of a neutron with that of an electron.

Solution:

Mass of a neutron (mnm_n) ≈1.675×10−27 kg\approx 1.675 \times 10^{-27}\text{ kg} Mass of an electron (mem_e) ≈9.1×10−31 kg\approx 9.1 \times 10^{-31}\text{ kg}

Ratio: mnme=1.675×10−279.1×10−31≈1840\frac{m_n}{m_e} = \frac{1.675 \times 10^{-27}}{9.1 \times 10^{-31}} \approx 1840

Explanation:

A neutron is approximately 18401840 times heavier than an electron. This explains why the mass of an atom is concentrated in the nucleus and the mass of electrons is often neglected in mass number calculations.

Problem 3:

An isotope of Carbon has 66 protons and 88 neutrons. Write its symbol and determine its mass number.

Solution:

Number of protons (ZZ) = 66 Number of neutrons (nn) = 88

Mass number (AA) = Z+nZ + n A=6+8=14A = 6 + 8 = 14

The symbol for this isotope is 614C^{14}_{6}C.

Explanation:

Isotopes are atoms of the same element (same ZZ) with different numbers of neutrons. Carbon-14 is a heavy isotope of carbon used in radiocarbon dating.