🏠 Home › 📚 CSEC Physics › ⚛️ Structure of the Atom

Structure of the Atom: Protons, Neutrons & Electrons

CSEC Physics: Atomic Structure Fundamentals

Essential Understanding: All matter is composed of atoms, the fundamental building blocks of the universe. Understanding atomic structure is crucial for explaining chemical reactions, radioactivity, electricity, and nuclear physics. Master the properties of subatomic particles to predict element behavior.

🔑 Key Skill: Calculating protons, neutrons & electrons

📈 Exam Focus: Atomic number vs. mass number

🎯 Problem Solving: Isotopes & ions calculations

Core Concepts & Subatomic Particles

⊕

Proton

Location: Nucleus (center of atom)

Charge: Positive (+1 elementary charge)

Mass: 1 atomic mass unit (amu)

Key Role: Determines the atomic number (Z) and identity of the element

Symbol: \( p^+ \)

⚫

Neutron

Location: Nucleus (center of atom)

Charge: Neutral (0 charge)

Mass: 1 atomic mass unit (amu) - slightly heavier than proton

Key Role: Contributes to mass number, provides nuclear stability

Symbol: \( n^0 \)

⊖

Electron

Location: Energy levels/shells orbiting nucleus

Charge: Negative (-1 elementary charge)

Mass: ~1/1836 amu (negligible compared to nucleons)

Key Role: Determines chemical properties, bonding, and electricity

Symbol: \( e^- \)

🔬

Atomic Structure

Nucleus: Dense center containing protons & neutrons (~99.97% of mass)

Electron Cloud: Vast region where electrons exist (occupies most of the volume)

Size Comparison: If nucleus = marble, atom = football stadium

Overall Charge: Atoms are neutral (protons = electrons)

Key Atomic Formulas

These relationships are fundamental to atomic structure calculations:

            \[ \text{Atomic Number (Z)} = \text{Number of Protons} = \text{Number of Electrons (in neutral atom)} \]
        

            \[ \text{Mass Number (A)} = \text{Number of Protons} + \text{Number of Neutrons} \]
        

            \[ \text{Number of Neutrons} = \text{Mass Number (A)} - \text{Atomic Number (Z)} \]
        

Interactive Atom Simulator

🔬

Atomic Structure Explorer

Objective: Explore different atomic models and see how changing proton, neutron, and electron counts affects atomic properties.

Protons (Atomic Number): 6

Neutrons: 6

Electrons: 6

Atomic Number (Z)

Protons = Element Identity

Mass Number (A)

Protons + Neutrons

Net Charge

Protons - Electrons

Element Information:

This atom has 6 protons, making it Carbon (C). With 6 neutrons, it's Carbon-12, the most common isotope. It has 6 electrons, making it electrically neutral.

Subatomic Particle Comparison

Analysis: The chart compares key properties of subatomic particles. Note that while protons and neutrons have similar masses, electrons are nearly 2000 times lighter. This explains why atomic mass is essentially the mass of the nucleus.

Property	Proton	Neutron	Electron
Charge	+1.6 × 10⁻¹⁹ C	0 C (neutral)	-1.6 × 10⁻¹⁹ C
Relative Mass	1 amu (1.67 × 10⁻²⁷ kg)	1 amu (1.68 × 10⁻²⁷ kg)	1/1836 amu (9.11 × 10⁻³¹ kg)
Location	Nucleus	Nucleus	Electron shells/orbitals
Role in Atom	Determines element identity	Affects stability & isotopes	Determines chemical properties
Discovered by	Ernest Rutherford (1919)	James Chadwick (1932)	J.J. Thomson (1897)

Element Explorer

Select an element to see its atomic structure:

Carbon (C)

Atomic Number: 6

Mass Number: 12

Protons: 6

Neutrons: 6

Electrons: 6

Electron Configuration: 2,4

Isotopes & Ions

Isotope and Ion Builder

Create different isotopes (same protons, different neutrons) and ions (charged atoms):

Select Element:

Number of Neutrons:

Charge (Ion):

Carbon-12: 6 protons, 6 neutrons, 6 electrons (neutral)

Worked Examples & Past Paper Questions

📝

Example 1: Basic Atomic Structure (CSEC 2019)

Question: An atom of nitrogen has a mass number of 14 and an atomic number of 7. Calculate the number of protons, neutrons, and electrons in this atom.

Identify known values: Atomic number (Z) = 7, Mass number (A) = 14

Protons = Atomic number: Number of protons = 7

Electrons = Protons (neutral atom): Number of electrons = 7

Neutrons = Mass number - Atomic number: 14 - 7 = 7 neutrons

Answer: Protons = 7, Neutrons = 7, Electrons = 7

📝

Example 2: Isotope Calculation (CSEC 2021)

Question: Chlorine has two common isotopes: Cl-35 and Cl-37. Both have 17 protons. Calculate the number of neutrons in each isotope.

Isotope Definition: Isotopes have same protons (same element) but different neutrons.

For Cl-35: Mass number = 35, Protons = 17
Neutrons = 35 - 17 = 18

For Cl-37: Mass number = 37, Protons = 17
Neutrons = 37 - 17 = 20

Answer: Cl-35 has 18 neutrons, Cl-37 has 20 neutrons.

📝

Example 3: Ion Calculation (CSEC 2018)

Question: A sodium ion has a charge of +1 and a mass number of 23. The atomic number of sodium is 11. Calculate the number of protons, neutrons, and electrons in this ion.

Identify known values: Atomic number = 11, Mass number = 23, Charge = +1

Protons = Atomic number: Protons = 11 (unchanged in ions)

Neutrons = Mass number - Protons: 23 - 11 = 12 neutrons

Electrons for +1 ion: Electrons = Protons - Charge = 11 - 1 = 10 electrons

Answer: Protons = 11, Neutrons = 12, Electrons = 10

Key Examination Insights

Common Mistakes

Confusing atomic number with mass number.
Forgetting that ions have different numbers of electrons than protons.
Thinking isotopes have different numbers of protons (they don't!).
Assuming all atoms are neutral (forgetting about ions).

Success Strategies

Always write down: Z = protons = electrons (in neutral atom).
For ions: Electrons = Protons - Charge (for positive ions).
Isotopes have same Z but different A (mass number).
Draw a simple diagram for complex questions.

CSEC Practice Arena

Test Your Understanding

Which particle determines the identity of an element?

Protons

Neutrons

Electrons

Nucleons

Explanation: The number of protons (atomic number) uniquely identifies an element. Changing protons changes the element, while changing neutrons creates isotopes of the same element.

An atom has 12 protons, 12 neutrons, and 10 electrons. What is its charge?

0 (neutral)

-2

Solution: Charge = Protons - Electrons = 12 - 10 = +2. This is a magnesium ion (Mg²⁺).

Two atoms have the same number of protons but different numbers of neutrons. They are:

Ions of the same element

Isotopes of the same element

Atoms of different elements

Molecules

Explanation: Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. Ions have different numbers of electrons.

Where is most of the mass of an atom concentrated?

In the electron cloud

In the nucleus

Evenly distributed

In the energy levels

Explanation: Over 99.9% of an atom's mass is in the nucleus (protons + neutrons). Electrons contribute negligible mass despite occupying most of the volume.

🎯

CSEC Examination Mastery Tip

Remembering Atomic Structure: Use the mnemonic "PEN" to remember the key particles:

P = Protons (Positive, determine element)
E = Electrons (Negative, determine chemical properties)
N = Neutrons (Neutral, determine isotopes)

For calculations, always start with: Atomic Number (Z) = Protons = Electrons (in neutral atom). Then use: Mass Number (A) = Protons + Neutrons.