1.1.1Physical Chemistry
Fundamental Particles — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 1 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Inside the Atom
An atom is mostly empty space around a tiny, dense nucleus.
Nucleus
- Nucleus The nucleus is at the centre of the atom and contains protons and neutrons, which are together called nucleons.
Electrons
- Electrons Electrons occupy orbitals in the space around the nucleus, and this space makes up almost all of the atom’s volume.
Overall charge
- Overall charge A neutral atom has equal numbers of protons and electrons, so the positive and negative charges cancel exactly.
Nuclear charge
- Nuclear charge The nucleus is positive overall because protons are positive and neutrons are neutral.
| Particle | Relative charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | –1 | 1/1836 (negligible) |
Part 2 of 6Atomic and Mass Numbers
Two numbers describe each nuclide.
Atomic and Mass Numbers
Atomic number Z
- Atomic number Z Z is the number of protons in the nucleus, which defines the element and equals the number of electrons in the neutral atom.
Mass number A
- Mass number A A is the total number of protons and neutrons in the nucleus.
Neutron number
- Neutron number The number of neutrons is found from N = A – Z.
Notation
- Notation The nuclide ²⁷₁₃Al shows mass number 27 at the top left and atomic number 13 at the bottom left of the symbol.
Part 3 of 6Ions
1
An ion forms when an atom gains or loses electrons; the number of protons never changes.
2
A positive ion (cation) has lost electrons, so it has fewer electrons than protons.
3
A negative ion (anion) has gained electrons, so it has more electrons than protons.
4
The charge on the ion equals the number of protons minus the number of electrons.
5
When completing a table for an ion, adjust only the electron number and leave the protons and neutrons unchanged.
Part 4 of 6Development of the Model
Plum pudding
- Plum pudding Thomson pictured the atom as a sphere of positive charge with electrons embedded in it, like fruit in a pudding.
Nuclear model
- Nuclear model Later evidence showed a small, dense, positive nucleus surrounded by electrons in the space outside it.
Electron shells
- Electron shells Electrons are arranged in energy levels, and this model explains periodic trends in properties.
Why models change
- Why models change Scientists revise models when new experimental evidence cannot be explained by the earlier version.
Part 5 of 6Worked Example
Question State the numbers of protons, neutrons and electrons in ²⁷₁₃Al³⁺ and in ³⁵₁₇Cl⁻.
- ²⁷₁₃Al³⁺: protons = 13 (atomic number); neutrons = 27 – 13 = 14.
- The 3+ charge means three electrons have been lost, so electrons = 13 – 3 = 10.
- ³⁵₁₇Cl⁻: protons = 17; neutrons = 35 – 17 = 18.
- The 1– charge means one electron was gained, so electrons = 17 + 1 = 18.
Answer
- Al³⁺ has 13 protons, 14 neutrons and 10 electrons;
- Cl⁻ has 17 protons, 18 neutrons and 18 electrons.
Part 6 of 6Exam Checklist & Tips
1.1.1 – Key Points for the Exam
- Proton A positive particle in the nucleus with relative mass 1 and relative charge +1.
- Neutron A neutral particle in the nucleus with relative mass 1.
- Atomic number The number of protons in the nucleus of an atom.
- Mass number The total number of protons and neutrons in the nucleus of an atom.
Common Mistakes
- Changing the number of protons when working out an ion; only electrons are gained or lost.
- Giving the electron a mass of zero instead of a very small relative mass of 1/1836.
- Writing the mass number and atomic number the wrong way round in nuclide notation.
- Saying that the nucleus contains electrons.
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