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Structure of the Atom - Charged Particles in Matter

Grade 9CBSE

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

🔑Concepts

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Matter is composed of atoms, which were initially thought to be indivisible. However, the discovery of charged particles like electrons (e−e^-) and protons (p+p^+) proved that atoms have an internal structure.

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The first indication of subatomic particles came from the study of static electricity and the discovery of 'Canal Rays' by E. Goldstein in 18861886.

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Canal rays are positively charged radiations which eventually led to the discovery of the proton. A proton has a charge of +1+1 and a mass of approximately 1 unit1 \text{ unit}.

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J.J. Thomson identified the electron, a negatively charged particle. The mass of an electron is considered negligible, being about 12000\frac{1}{2000} times the mass of a hydrogen atom, and its charge is −1-1.

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Under normal conditions, an atom is electrically neutral because the number of protons (p+p^+) is equal to the number of electrons (e−e^-), meaning the total positive charge balances the total negative charge.

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The absolute charge of an electron/proton is 1.6×10−19 C1.6 \times 10^{-19} \text{ C}, where C\text{C} stands for Coulombs.

📐Formulae

Mass of Proton (mp)≈1.67×10−27 kg\text{Mass of Proton } (m_p) \approx 1.67 \times 10^{-27} \text{ kg}

Mass of Electron (me)≈9.11×10−31 kg\text{Mass of Electron } (m_e) \approx 9.11 \times 10^{-31} \text{ kg}

Charge of Electron (e)=−1.6×10−19 C\text{Charge of Electron } (e) = -1.6 \times 10^{-19} \text{ C}

Net Charge of Atom=(np×qp)+(ne×qe)=0\text{Net Charge of Atom} = (n_p \times q_p) + (n_e \times q_e) = 0

Mass Ratio: mpme≈1840\text{Mass Ratio: } \frac{m_p}{m_e} \approx 1840

💡Examples

Problem 1:

If an atom has 1111 protons and 1111 electrons, what is the net charge on the atom?

Solution:

The net charge is 00.

Explanation:

The total positive charge from 1111 protons is +11+11 units and the total negative charge from 1111 electrons is −11-11 units. The net charge is (+11)+(−11)=0(+11) + (-11) = 0. This makes the atom electrically neutral.

Problem 2:

Calculate the approximate mass of an atom that contains 22 protons and 22 neutrons, given the mass of an electron is negligible and mass of p≈n≈1 up \approx n \approx 1 \text{ u}.

Solution:

Mass ≈4 u\approx 4 \text{ u}.

Explanation:

Atomic mass is the sum of the masses of protons and neutrons. Mass=(2×1 u)+(2×1 u)=4 u\text{Mass} = (2 \times 1 \text{ u}) + (2 \times 1 \text{ u}) = 4 \text{ u}. The electrons are not counted as their mass is approximately 11840 u\frac{1}{1840} \text{ u} each.

Problem 3:

What are canal rays and what is their charge?

Solution:

Canal rays are beams of positively charged ions.

Explanation:

Discovered by E. Goldstein, canal rays are composed of positively charged particles (p+p^+) that travel in the opposite direction to cathode rays in a gas discharge tube.

Charged Particles in Matter - Revision Notes & Key Formulas | CBSE Class 9 Science