(b) particle and its corresponding antiparticle have same mass; electron…

The idea

A particle and its antiparticle have exactly the same mass but opposite values of charge and other quantum numbers such as baryon number and lepton number.

For example: The electron and positron both have mass 9.11 × 10⁻³¹ kg, but the electron has charge −1.60 × 10⁻¹⁹ C while the positron has charge +1.60 × 10⁻¹⁹ C. Similarly, the proton and antiproton have the same mass (1.67 × 10⁻²⁷ kg) but opposite charge: proton +1.60 × 10⁻¹⁹ C, antiproton −1.60 × 10⁻¹⁹ C. In MCQs, you may be asked to compare masses or charges, or to identify which property is opposite and which is the same.

What earns the marks

• Particle and antiparticle have identical mass.
• Electron and positron have opposite charge.
• Proton and antiproton have opposite charge.
• Other quantum numbers (e.g. baryon number, lepton number) are also opposite.
• Charge is conserved in particle–antiparticle creation and annihilation.

Common mistakes and how to put them right

• Thinking antiparticles have negative mass.

  Put it right: Mass is always positive and identical for particle and antiparticle.

• Assuming all antiparticles are negatively charged.

  Put it right: The antiproton is negative, but the antineutron is neutral and the positron is positive.

• Believing the positron has the same charge as a proton.

  Put it right: The positron has charge +1.60 × 10⁻¹⁹ C, same magnitude as the electron but opposite sign, while the proton also has +1.60 × 10⁻¹⁹ C but a much larger mass.

• Forgetting that lepton number and baryon number are opposite for antiparticles, not just charge.

Exam tips

• Compare masses first: they are equal for particle and antiparticle.
• Check the sign of charge: it is opposite for particle and antiparticle.
• Use conservation of charge to check if a proposed reaction is possible.