1.2.2Additional lessons
Subatomic Particles — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 98
Part 1 of 5The Three Particles
Each atom is built from protons, neutrons and electrons.
Proton
- Found in the nucleus, with relative charge +1 and relative mass 1.
Neutron
- Found in the nucleus, with no charge and a relative mass of 1.
Electron
- Found outside the nucleus, with relative charge −1 and a relative mass of about 1/1836, which is negligible in comparison.
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 | −1 | Outside nucleus |
Part 2 of 5Atomic and Mass Numbers
Two numbers define a nuclide.
Atomic number Z
- The number of protons in the nucleus, which identifies the element.
Mass number A
- The total number of protons and neutrons in the nucleus.
Neutron number
- Number of neutrons = A − Z.
Notation
- A nuclide is written with A as a superscript and Z as a subscript to the left of the symbol, for example ¹²₆C.
Part 3 of 5Ions
Gaining or losing electrons changes the charge but not the nucleus.
1
A positive ion, a cation, forms when an atom loses one or more electrons, so protons outnumber electrons.
2
A negative ion, an anion, forms when an atom gains electrons, so electrons outnumber protons.
3
The number of electrons is found by taking the atomic number and subtracting the ion's charge.
4
The number of protons and neutrons in an ion is the same as in the neutral atom, as the nucleus is unchanged.
Part 4 of 5Worked Example
Question State the number of protons, neutrons and electrons in ²⁷₁₃Al³⁺.
- Protons = atomic number = 13.
- Neutrons = 27 − 13 = 14.
- Electrons = 13 − 3 = 10, because the 3+ charge means three electrons have been lost.
Answer 13 protons, 14 neutrons, 10 electrons
Part 5 of 5Exam Checklist & Tips
1.2.2 – Key Points for the Exam
- Atomic number The number of protons in an atom of an element.
- Mass number The sum of protons and neutrons in the nucleus of an atom.
- Cation A positively charged ion formed by losing electrons.
- Anion A negatively charged ion formed by gaining electrons.
Common Mistakes
- Calculating electrons for an ion from the mass number instead of the atomic number.
- Saying the electron has a relative mass of 1; it is about 1/1836.
- Forgetting that a charge of 3+ means three fewer electrons than protons.
- Changing the number of protons when forming an ion.
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