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Comparing Mass and Charge

Electrons vs. Protons vs. Neutrons

CSEC Physics: Subatomic Particles

Essential Understanding: Understanding the fundamental properties of subatomic particles—mass and charge—is essential for grasping how atoms work. The electron, proton, and neutron have dramatically different properties that determine atomic structure, chemical behavior, and nuclear phenomena.

🔵 Key Particle: Electron
🔴 Key Particle: Proton
⚪ Key Particle: Neutron

The Three Fundamental Subatomic Particles

Every atom in the universe is composed of just three types of particles. Despite their tiny size, these particles have vastly different properties that determine everything about matter—from the elements we see on the periodic table to the light emitted by stars.

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The Electron (e⁻)

Discovered by: J.J. Thomson (1897)

Location: Orbiting the nucleus in energy shells

Key Properties:

  • Charge: -1 (negative)
  • Relative Mass: 1/1836
  • Absolute Mass: 9.109 × 10⁻³¹ kg
  • Absolute Charge: -1.602 × 10⁻¹⁹ C

Role: Determines chemical properties and electrical conductivity

☀️

The Proton (p⁺)

Discovered by: Ernest Rutherford (1919)

Location: Inside the nucleus

Key Properties:

  • Charge: +1 (positive)
  • Relative Mass: 1
  • Absolute Mass: 1.673 × 10⁻²⁷ kg
  • Absolute Charge: +1.602 × 10⁻¹⁹ C

Role: Determines atomic number and element identity

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The Neutron (n⁰)

Discovered by: James Chadwick (1932)

Location: Inside the nucleus

Key Properties:

  • Charge: 0 (neutral)
  • Relative Mass: 1
  • Absolute Mass: 1.675 × 10⁻²⁷ kg
  • Absolute Charge: 0 C

Role: Stabilizes nucleus, enables isotopes

Visual Comparison of Particle Sizes and Masses

Relative Sizes and Masses of Subatomic Particles Actual Size Comparison Electron r ≈ 10⁻¹⁵ m Proton r ≈ 10⁻¹⁵ m Neutron r ≈ 10⁻¹⁵ m Relative Mass Comparison Electron 1/1836 Proton = 1 Proton ≈ 1.67 × 10⁻²⁷ kg Neutron = 1 Neutron ≈ 1.67 × 10⁻²⁷ kg Electric Charge Comparison -1 0 +1 e⁻ Charge = -1 n⁰ Charge = 0 p⁺ Charge = +1 ⚠️ Electron is ~1836× lighter than proton/neutron, but has equal magnitude charge

Complete Properties Table

Standard Notation for Subatomic Particles

Particle Symbol Relative Mass Relative Charge Absolute Mass (kg) Absolute Charge (C) Location
Electron e⁻ or ₋₁⁰e 1/1836 ≈ 0.0005 -1 9.109 × 10⁻³¹ -1.602 × 10⁻¹⁹ Orbitals/Shells
Proton p or p⁺ or ₁¹H 1 +1 1.673 × 10⁻²⁷ +1.602 × 10⁻¹⁹ Nucleus
Neutron n or n⁰ 1 0 1.675 × 10⁻²⁷ 0 Nucleus

Interactive Particle Mass Comparison

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Particle Mass Visualizer

Objective: Explore the dramatic difference in mass between electrons and protons/neutrons. Notice how many electrons would be needed to equal the mass of a single proton!

1 Proton = ? Electrons

Key Insight: Even though electrons and protons have charges of equal magnitude, electrons are approximately 1836 times less massive!

Chart: Mass and Charge Comparison

Analysis: This chart clearly shows that protons and neutrons have approximately equal mass (relative mass = 1), while electrons are roughly 1/1836th the mass of a proton. However, all three particles have charges that are integer multiples of the elementary charge.

Understanding Relative vs. Absolute Values

Why Do We Use “Relative” Values?

Working with actual masses in kilograms is inconvenient because the numbers are extremely small. Scientists use a clever system where the proton mass is defined as 1, making comparisons much easier:

Relative Mass Formula

\[ \text{Relative Mass} = \frac{\text{Actual Mass of Particle}}{\text{Actual Mass of Proton}} \]

For electrons: \( \frac{9.109 \times 10^{-31}}{1.673 \times 10^{-27}} = \frac{1}{1836} \)

Absolute Values in Coulombs

The elementary charge (e) is one of the fundamental constants in physics. Both the electron and proton have charges of this magnitude, but with opposite signs:

Elementary Charge

\[ e = 1.602 \times 10^{-19} \, \text{Coulombs (C)} \]

Electron charge: \( q_e = -e = -1.602 \times 10^{-19} \, \text{C} \)

Proton charge: \( q_p = +e = +1.602 \times 10^{-19} \, \text{C} \)

Neutron charge: \( q_n = 0 \, \text{C} \)

Why This Matters: Atomic Structure

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Mass Distribution in Atoms

The Discovery: Despite electrons being present in equal numbers to protons in neutral atoms, the nucleus contains virtually all the mass.

Example – Carbon-12:

  • 6 protons + 6 neutrons = 12 nucleons
  • 6 electrons orbiting
  • But: ~99.97% of mass is in the nucleus!

Reason: Electron mass is 1/1836 of proton mass. Even with equal numbers, electrons contribute negligibly to atomic mass.

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Charge Balance in Atoms

Neutral Atoms: For an atom to be electrically neutral, the total positive charge must equal total negative charge.

The Rule:

\[ \text{Number of protons} = \text{Number of electrons} \]

Example – Sodium (Na):

  • Atomic number: 11
  • 11 protons in nucleus
  • 11 electrons in shells
  • Total charge: 11(+1) + 11(-1) = 0

Interactive Charge Balance Simulation

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Build an Atom: Charge Balance

Objective: Understand how protons and electrons determine atomic charge. Add or remove particles and observe the effect!

Protons

6

Electrons

6

Net Charge

0

Neutral Atom

Element: Carbon (C)

Worked Examples

1
Example 1: Comparing Masses

Question: If a proton has a mass of 1.673 × 10⁻²⁷ kg, calculate the mass of 5000 electrons.

Solution:

  • Mass of 1 electron = \( 1.673 \times 10^{-27} \div 1836 = 9.11 \times 10^{-31} \, \text{kg} \)
  • Mass of 5000 electrons = \( 5000 \times 9.11 \times 10^{-31} \, \text{kg} \)
  • Mass of 5000 electrons = \( 4.555 \times 10^{-27} \, \text{kg} \)
  • Compare to 1 proton: \( 4.555 \times 10^{-27} \div 1.673 \times 10^{-27} = 2.72 \)
  • Answer: 5000 electrons have about 2.72× the mass of a single proton
2
Example 2: Calculating Charge

Question: A magnesium ion has 12 protons and 10 electrons. Calculate its net charge.

Solution:

  • Total positive charge = 12 × (+1.602 × 10⁻¹⁹ C) = +1.9224 × 10⁻¹⁸ C
  • Total negative charge = 10 × (-1.602 × 10⁻¹⁹ C) = -1.602 × 10⁻¹⁸ C
  • Net charge = (+1.9224 × 10⁻¹⁸) + (-1.602 × 10⁻¹⁸) C
  • Net charge = +0.3204 × 10⁻¹⁸ C = +3.204 × 10⁻¹⁹ C
  • Alternatively: Net charge = (12 – 10) × (+1.602 × 10⁻¹⁹ C) = +2e
  • Answer: The ion has a charge of +2 (or +3.204 × 10⁻¹⁹ C)
3
Example 3: Why Neutrons Matter

Question: Explain why adding neutrons to an atom doesn’t change its charge but does change its mass.

Solution:

  • Neutrons have zero charge, so adding them doesn’t affect the net charge
  • Neutrons have mass approximately equal to protons (relative mass = 1)
  • Therefore, more neutrons = more mass but no change in charge
  • This is why isotopes have same chemical properties (same electron configuration) but different atomic masses
  • Example: Carbon-12 (6p, 6n) vs Carbon-14 (6p, 8n) — both neutral, different masses

Key Takeaways for CSEC

Essential Points for Your Examination

Mass Relationships

  • Proton relative mass = 1
  • Neutron relative mass = 1
  • Electron relative mass = 1/1836
  • Nucleus contains 99.97%+ of atomic mass

Charge Relationships

  • Proton charge = +1
  • Electron charge = -1
  • Neutron charge = 0
  • Neutral atom: protons = electrons
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CSEC Examination Mastery Tip

Memorization Strategy: CSEC examiners often ask you to state the relative mass and charge of particles. Use this memory aid:

“Protons are Positive (mass = 1)

Neutrons are Neutral (mass = 1)

Electrons are Extremely light (mass = 1/1836, charge = -1)

Common Trap: Students often confuse which particle has which mass. Remember: electrons are the light ones (~2000× lighter than nucleons)!

CSEC Practice Arena

Test Your Understanding

1
What is the relative mass of an electron compared to a proton?
1
1/1836
1836
0
Explanation: Electrons have a relative mass of approximately 1/1836 compared to protons. This means a proton is about 1836 times more massive than an electron.
2
Which particle has no electric charge?
Proton
Electron
Neutron
Alpha particle
Explanation: The neutron has zero electric charge. This is why it’s called a “neutron”—it’s electrically neutral. Protons are positive (+1) and electrons are negative (-1).
3
A neutral atom has 15 protons. How many electrons does it have?
7
30
15
Cannot be determined
Explanation: In a neutral atom, the number of positive charges (protons) must equal the number of negative charges (electrons). Therefore, 15 protons = 15 electrons.
4
What percentage of an atom’s mass is found in the nucleus?
About 10%
About 50%
About 99.97%
About 75%
Explanation: The nucleus contains virtually all of an atom’s mass (approximately 99.97% or more) because protons and neutrons are about 1836 times more massive than electrons.
5
The absolute charge of an electron is -1.602 × 10⁻¹⁹ C. What is the absolute charge of a proton?
-1.602 × 10⁻¹⁹ C
+1.602 × 10⁻¹⁹ C
0 C
-1 C
Explanation: Protons and electrons have charges of equal magnitude but opposite sign. The proton charge is +1.602 × 10⁻¹⁹ C, while the electron is -1.602 × 10⁻¹⁹ C.

Past Paper Questions

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CSEC Past Paper Practice

Question 1 (CSEC Physics 2022)

(a) Complete the table below for the three main subatomic particles:

Particle Relative Mass Relative Charge
Proton ? ?
Neutron ? ?
Electron ? ?

Sample Answer:

Proton 1 +1
Neutron 1 0
Electron 1/1836 -1

Question 2 (CSEC Physics 2021)

(a) Explain why the nucleus of an atom contains most of its mass even though the nucleus is so small.

(b) An atom of aluminum has 13 protons and 14 neutrons.

(i) What is the atomic number of aluminum?

(ii) How many electrons does a neutral atom of aluminum have?

(iii) Calculate the total charge on the nucleus of an aluminum atom.

Sample Answer:

(a) The nucleus contains protons and neutrons, each with relative mass of 1. Electrons have relative mass of only 1/1836. Therefore, the nucleus (containing only protons and neutrons) contains virtually all the mass.

(b) (i) Atomic number = 13 (number of protons)

(ii) 13 electrons (for neutral atom)

(iii) Total charge = 13 × (+1.602 × 10⁻¹⁹ C) = +2.0826 × 10⁻¹⁸ C

Question 3 (CSEC Physics 2020)

(a) State the relative charge of (i) a proton, (ii) a neutron, (iii) an electron.

(b) Explain why an atom is normally electrically neutral.

Sample Answer:

(a) (i) Proton: +1, (ii) Neutron: 0, (iii) Electron: -1

(b) An atom is normally electrically neutral because it has equal numbers of protons (positive charge) and electrons (negative charge). The total positive charge equals the total negative charge, resulting in net zero charge.

Summary: The Key Particle Properties

Understanding the properties of subatomic particles is fundamental to CSEC Physics. Remember these key relationships:

Electron

  • Mass: 1/1836 (very light!)
  • Charge: -1 (negative)
  • Location: Orbiting nucleus
  • Role: Chemistry & bonding

Proton

  • Mass: 1 (standard)
  • Charge: +1 (positive)
  • Location: In nucleus
  • Role: Defines the element

Neutron

  • Mass: 1 (same as proton)
  • Charge: 0 (neutral)
  • Location: In nucleus
  • Role: Stabilizes nucleus
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