Ch 2 · 5.1.2 The Nucleus · O Level Physics 5054
Topic 2 of 8 in Nuclear Physics — O Level Physics 5054 — 59 lessons.
Introduction
Carbon atoms in your body, in a diamond and in ancient wood are all carbon, yet some of them are slightly heavier than others, and a few are radioactive. The difference lies in the nucleus. (Syllabus section 5.1.2.)
Key ideas — What is in the nucleus
The nucleus contains two kinds of particle, together called nucleons:
🧑🏫 Teacher's explanation
Think of an atom like a tiny solar system. At the very center, there is a dense, heavy core called the nucleus. Inside this core, you find two types of particles: protons, which carry a positive charge, and neutrons, which are neutral. Together, these two particles are known as 'nucleons' because they live in the nucleus.
📖 Definitions
🔍 Worked example — Easy
An atom of Carbon is represented by the nuclide notation ^12_6C. Calculate the number of protons and neutrons in its nucleus.
🔍 Worked example — Standard
An ion has a charge of +2 and a nucleon number of 24. If the proton number is 12, how many electrons does this ion have?
🔍 Worked example — Exam-style
An isotope of an element has 26 protons and 30 neutrons. Write its nuclide notation ^A_ZX and identify the element using the periodic table (where Z=26 is Iron).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Ions
An atom is neutral: it has the same number of electrons as protons.
🧑🏫 Teacher's explanation
Think of an atom like a balanced scale. In a neutral atom, the number of positive protons exactly balances the number of negative electrons. The nucleus is the heavy, central core containing protons and neutrons, while the electrons orbit around it. Because the nucleus is much heavier than electrons, almost all the mass of an atom is concentrated in that tiny center.
📖 Definitions
🔍 Worked example — Easy
A nuclide is represented by the notation ^23_11Na. Calculate the number of protons, neutrons, and electrons in a neutral atom of this nuclide.
🔍 Worked example — Standard
An ion has a charge of +2 and a mass number of 40. The element is Calcium (Z = 20). Calculate the number of neutrons in this ion.
🔍 Worked example — Exam-style
An atom of an isotope of Carbon gains 3 electrons. If the mass number is 14 and the proton number is 6, determine the final charge of the ion and the number of neutrons in the nucleus.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Proton number and nucleon number
number of neutrons = A - Z
🧑🏫 Teacher's explanation
Think of an atom like a high-security vault. The nucleus is the heavy safe inside, containing protons and neutrons. The number of protons is like the 'ID card' of the vault; it tells you exactly what kind of metal the vault is made of. If you change the number of protons, you change the element itself. The neutrons are like the extra weight or ballast inside the safe; they add mass but don't change the identity of the metal.
📖 Definitions
🔍 Worked example — Easy
An atom of Sodium has a proton number of 11 and a nucleon number of 23. Calculate the number of neutrons in the nucleus.
🔍 Worked example — Standard
A nuclide is represented by the notation ^37_17Cl. Determine the number of protons, neutrons, and electrons in this neutral atom.
🔍 Worked example — Exam-style
An isotope of an element has 26 protons and 30 neutrons. Write its nuclide notation ^A_ZX and identify the element using a periodic table (assume Z=26 is Iron).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Nuclides and nuclide notation
A nuclide is a particular type of nucleus, with a particular number of protons and neutrons. It is written:
🧑🏫 Teacher's explanation
Think of an atom like a high-security vault. The nucleus is the heavy safe in the center, containing the 'identity' of the atom. Inside this safe are two types of particles: protons and neutrons. The number of protons is like a unique ID card; if you change the number of protons, you change the element itself. The neutrons are like extra weight or ballast; they add mass but don't change the identity of the element.
📖 Definitions
🔍 Worked example — Easy
A nuclide is represented by the notation ^37_17Cl. State the number of protons and the number of neutrons in this nucleus.
🔍 Worked example — Standard
An atom of Magnesium has a proton number of 12 and a nucleon number of 24. How many neutrons does it have?
🔍 Worked example — Exam-style
An ion has a charge of +2 and a nucleon number of 40. The element has a proton number of 20. Determine the number of neutrons and the number of electrons in this ion.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Isotopes
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons (so different nucleon numbers).
🧑🏫 Teacher's explanation
Think of an atom like a specific brand of chocolate bar. The 'brand' is determined by the number of protons in the nucleus—this never changes for that element. If you change the number of protons, you change the element entirely. However, you can change the number of neutrons in that bar without changing the brand. These 'different versions' of the same brand are called isotopes. They look and act the same chemically because they have the same number of electrons, but they have different weights because they have different numbers of neutrons.
📖 Definitions
🔍 Worked example — Easy
A nuclide is represented by the notation ^37_17Cl. Calculate the number of protons and neutrons in this atom.
🔍 Worked example — Standard
An isotope of Magnesium has 12 protons and 12 neutrons. Write its nuclide notation ^A_ZMg.
🔍 Worked example — Exam-style
An atom of an unknown element has a nucleon number of 40 and contains 21 neutrons. State the identity of the element by finding its proton number.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Uranium-238 is written ^238_92U. State the number of protons, neutrons and electrons in a neutral atom of it.
🔍 Worked example — Easy
A neutral atom of Sodium is represented by the nuclide notation ^23_11 extNa. State the number of protons, neutrons, and electrons in this atom.
🔍 Worked example — Standard
An ion of Magnesium has a charge of +2 and a nucleon number of 24. The proton number of Magnesium is 12. Determine the number of protons, neutrons, and electrons in this ion.
🔍 Worked example — Exam-style
Two atoms, X and Y, are isotopes of the same element. Atom X has 6 protons and 6 neutrons. Atom Y has 6 protons and 8 neutrons. Write the nuclide notation for both atoms and state the nucleon number for Atom Y.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) Protons and neutrons. (2) The total number of protons and neutrons in the nucleus. (3) 56 - 26 = 30. (4) Atoms of the same element (same number of protons) with different numbers of neutrons. (5) A positive ion (with a charge of +2).