What is the basic structure of an atom in atomic physics?

In atomic physics, the basic structure of an atom consists of a nucleus made up of protons and neutrons, surrounded by electrons that orbit in electron shells. The protons have a positive charge, neutrons are neutral, and electrons have a negative charge. This structure is fundamental to understanding atomic behavior and interactions in science, particularly within the IB MYP curriculum.

How do the number of protons, neutrons, and electrons determine an element's identity?

The number of protons in an atom's nucleus, known as the atomic number, determines the element's identity. For example, carbon has six protons. Neutrons contribute to the atomic mass but do not affect the element's identity. Electrons, equal in number to protons in a neutral atom, determine the atom's chemical properties and reactivity. Understanding these concepts is crucial in the study of atomic structure in the IB MYP science curriculum.

What is the significance of electron shells in atomic structure?

Electron shells are significant because they determine how atoms interact and bond with each other. Electrons are arranged in shells around the nucleus, with each shell having a specific capacity. The arrangement of electrons in these shells influences an atom's chemical properties and its ability to form bonds. This concept is essential in atomic physics and is a key topic in the IB MYP science curriculum.

How does the concept of isotopes relate to atomic structure?

Isotopes are variations of the same element that have the same number of protons but different numbers of neutrons. This difference in neutron number results in different atomic masses for the isotopes. Isotopes are important in atomic physics because they can have different physical properties and stability, which are studied in various scientific applications. Understanding isotopes is a critical part of the atomic structure topic in the IB MYP science curriculum.

Why is understanding atomic structure important in science?

Understanding atomic structure is crucial because it forms the foundation for explaining the behavior of matter and the interactions between different elements. It helps in predicting chemical reactions, understanding material properties, and exploring new scientific phenomena. In the IB MYP science curriculum, atomic structure is a fundamental topic that supports further learning in chemistry, physics, and other scientific disciplines.

Why is "Placing electrons in the nucleus." a common mistake in Atomic Structure?

Why it happens: Students draw all three particles together for compactness. How to avoid it: Protons and neutrons sit in the nucleus. Electrons orbit in shells AROUND it.

Why is "Treating isotopes as different elements." a common mistake in Atomic Structure?

Why it happens: Different mass numbers feel like different substances. How to avoid it: Isotopes have the SAME atomic number → SAME element. They just have different numbers of neutrons.

Why is "Thinking the atom is densely packed with matter." a common mistake in Atomic Structure?

Why it happens: Solid materials feel solid. How to avoid it: Most of the atom is empty space. Rutherford's experiment showed alpha particles mostly pass straight through with no obstruction.

Atomic physicsIB MYP Sciences (Years 1-5)

Atomic Structure

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Atomic Structure — IB MYP Sciences: protons, neutrons, electrons, isotopes and the atomic model

Everything you can touch is built from atoms. This MYP Sciences note covers the three subatomic particles, atomic and mass numbers, isotopes, and how our model of the atom evolved from a featureless ball to a tiny nucleus surrounded by electrons.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Describe the structure of an atom and the properties of protons, neutrons and electrons.
MYP Sciences A — Define atomic number, mass number and isotope; calculate the number of each subatomic particle.
MYP Sciences A — Outline how the model of the atom has changed over time.
MYP Sciences D — Evaluate Rutherford's scattering experiment as a turning point in atomic theory.

Protons, neutrons and electrons

The atom has a tiny dense nucleus of protons and neutrons, surrounded by electrons.

An atom is made of three kinds of subatomic particle:

Particle	Relative charge	Relative mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	≈ 1/1836 (negligible)	Shells around nucleus

The nucleus is incredibly small but contains nearly all the mass. If a hydrogen atom were scaled up so its nucleus was a football, the nearest electron would be ~10 km away — almost all of an atom is empty space.

A neutral atom has the same number of protons and electrons, so the charges cancel. If an atom loses or gains electrons, it becomes an ion with net charge.

Proton: +1 charge, mass 1 u, in nucleus.
Neutron: 0 charge, mass 1 u, in nucleus.
Electron: −1 charge, tiny mass, in shells.
Atom is mostly empty space.

Atomic number, mass number and isotopes

Z = protons; A = protons + neutrons. Isotopes share Z but differ in A.

Every element has a unique atomic number (Z) — the number of protons in its nucleus. Carbon always has Z = 6; oxygen always has Z = 8; gold always has Z = 79.

The mass number (A) is the total of protons and neutrons. Standard notation:

$^{A}_{Z}\!X$

For example, $^{12}_{\;6}\text{C}$ has A = 12 and Z = 6 → 6 protons and 12 - 6 = 6 neutrons.

Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A). Carbon-12 has 6 neutrons; carbon-14 has 8 neutrons. Same chemistry (same electron arrangement), different mass.

Many elements exist as a mixture of isotopes. The relative atomic mass on the Periodic Table is a weighted average across all naturally-occurring isotopes.

Schematic atom: tiny nucleus of protons + neutrons; electrons orbit in distinct shells. Not to scale — the nucleus is ~10⁵ times smaller than the atom.

Z (atomic number) = number of protons.
A (mass number) = protons + neutrons.
Number of neutrons = A − Z.
Isotopes: same Z, different A.

How the model of the atom evolved

Each model fixed the problems of the one before.

Our picture of the atom has been completely overturned several times:

1803 — Dalton's solid ball. John Dalton imagined atoms as tiny, indivisible solid balls — one type per element. Fitted what was known about chemical compounds but said nothing about internal structure.

1897 — Thomson's "plum pudding". J.J. Thomson discovered the electron. He proposed atoms were positive 'dough' studded with negative electrons like raisins in a pudding.

1909 — Rutherford's gold-foil experiment. Ernest Rutherford fired alpha particles at a thin gold foil. Most went straight through, but a tiny fraction bounced back at large angles — impossible if Thomson was right. Conclusion: most of the atom is empty space, with a tiny dense positive nucleus at the centre.

1913 — Bohr's electron shells. Niels Bohr fixed a problem in Rutherford's model: classical physics said orbiting electrons should spiral in. Bohr proposed electrons occupy fixed energy levels (shells) and can only jump between them by absorbing or emitting specific quantities of energy.

1932 — Chadwick's neutron. James Chadwick discovered the neutron, completing the modern picture of a nucleus of protons and neutrons.

Each step was made possible by NEW experimental evidence — a perfect MYP criterion D talking point.

Dalton (1803): solid indivisible ball.
Thomson (1897): plum pudding (positive dough + electron raisins).
Rutherford (1909): tiny positive nucleus + mostly empty space.
Bohr (1913): electrons in fixed energy shells.
Chadwick (1932): neutron discovered.

How it’s examined

Criterion A tests subatomic particle properties, calculations of p/n/e from A and Z, and the history of the atom. Criterion D often asks for evaluation of an experiment (typically Rutherford's gold-foil).

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Atomic Structure

Step-by-step solutions to past-paper-style questions on atomic structure, written exactly the way a tutor would explain them at the board.

1Find protons, neutrons and electrons
Getting started• A and Z
Question
An atom is written ²³₁₁Na. Find the number of protons, neutrons and electrons.
Step-by-step solution
1. Step 1
  Z = 11 (atomic number), so 11 protons.
2. Step 2
  Neutrons = A − Z = 23 − 11 = 12.
3. Step 3
  Neutral atom → electrons = protons = 11.
Answer
11 protons, 12 neutrons, 11 electrons.
2Identifying isotopes
Building confidence• isotopes
Question
Two atoms are ¹²₆C and ¹⁴₆C. What are they called, and how do they differ?
Step-by-step solution
1. Step 1
  Both have Z = 6, so they are both CARBON — same element.
2. Step 2
  Mass numbers differ (12 vs 14) → different numbers of neutrons (6 vs 8).
3. Step 3
  These are called ISOTOPES of carbon.
Answer
Isotopes of carbon — same atomic number (6 protons) but different mass numbers; ¹²C has 6 neutrons, ¹⁴C has 8 neutrons.
3How did Rutherford's experiment change the model?
Building confidence• history
Question
What did Rutherford's gold-foil experiment show, and how did it change the model of the atom?
Step-by-step solution
1. Step 1
  Most alpha particles passed straight through the foil, but a small fraction were deflected at large angles.
2. Step 2
  This was impossible if Thomson's plum-pudding model were right — there should have been NO large-angle bouncing.
3. Step 3
  Rutherford concluded the atom must have a tiny dense POSITIVE NUCLEUS at its centre, with mostly empty space around it.
Answer
Most alpha particles passed straight through, but a small fraction bounced back. Conclusion: atoms are mostly empty space with a tiny dense positive nucleus.
Examiner tip
MYP criterion D often asks for an evaluation of how new evidence forces models to change.

Key Definitions and Keywords — Atomic Structure

Definitions to memorise and the exact keywords mark schemes credit for atomic structure answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Atom
Examiner keyword
The smallest particle of an element that retains its chemical identity. Consists of a nucleus surrounded by electrons.
Proton
Positively charged particle in the nucleus. Charge +1, mass 1 u.
Neutron
Uncharged particle in the nucleus. Charge 0, mass 1 u.
Electron
Negatively charged particle in shells around the nucleus. Charge −1, very small mass.
Atomic number (Z)
Examiner keyword
Number of protons in the nucleus. Defines the element.
Mass number (A)
Examiner keyword
Total number of protons and neutrons in the nucleus.
Isotope
Examiner keyword
Atoms of the same element (same Z) with different mass numbers (different numbers of neutrons).

Common Mistakes and Misconceptions — Atomic Structure

The traps other students keep falling into on atomic structure questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Placing electrons in the nucleus.
Why it happens
Students draw all three particles together for compactness.
How to avoid it
Protons and neutrons sit in the nucleus. Electrons orbit in shells AROUND it.
✕Treating isotopes as different elements.
Why it happens
Different mass numbers feel like different substances.
How to avoid it
Isotopes have the SAME atomic number → SAME element. They just have different numbers of neutrons.
✕Thinking the atom is densely packed with matter.
Why it happens
Solid materials feel solid.
How to avoid it
Most of the atom is empty space. Rutherford's experiment showed alpha particles mostly pass straight through with no obstruction.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Atomic Structure — frequently asked questions

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Atomic Structure

Short Study Notes in the form of Slides

Short Study Notes — Atomic Structure

Detailed Notes

What you’ll learn

How it’s examined

1Find protons, neutrons and electrons

2Identifying isotopes

3How did Rutherford's experiment change the model?

Atom

Proton

Neutron

Electron

Atomic number (Z)

Mass number (A)

Isotope

✕Placing electrons in the nucleus.

✕Treating isotopes as different elements.

✕Thinking the atom is densely packed with matter.

Practice questions

Past paper style quiz

Assess your understanding

Atomic Structure — frequently asked questions