1.1.1Physical Chemistry
Structure of the Atom — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 1 of 42
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
Atoms are the smallest parts of an element that can take part in chemical reactions.
Nucleus
- Nucleus A tiny, dense centre containing protons and neutrons, which carries an overall positive charge and holds almost all of the atom's mass.
Electrons
- Electrons Negatively charged particles found in shells and orbitals around the nucleus, occupying most of the volume of the atom.
Empty space
- Empty space Most of an atom is empty space, because the nucleus is about one hundred thousand times smaller across than the whole atom.
Neutral atoms
- Neutral atoms An atom has equal numbers of protons and electrons, so the positive and negative charges cancel exactly.
Part 2 of 6Relative Mass and Charge
Relative Mass and Charge
Why relative values
- Why relative values Real masses and charges are far too small for everyday units, so particles are compared with each other on a relative scale.
Proton
- Proton Relative mass 1 and relative charge +1, located in the nucleus.
Neutron
- Neutron Relative mass 1 and no charge, located in the nucleus.
Electron
- Electron Relative mass of about 1/1836 and relative charge -1, so its mass is negligible in most calculations.
| Particle | Relative mass | Relative charge | Position |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 | -1 | Shells outside nucleus |
Part 3 of 6Atomic Number, Mass Number and Ions
1
The atomic number Z is the number of protons in the nucleus, and it defines which element the atom belongs to.
2
The mass number A is the total number of protons plus neutrons in the nucleus of one atom.
3
The number of neutrons is found by subtraction: neutrons = A - Z.
4
In a neutral atom the number of electrons equals Z, but ions form when atoms gain or lose electrons.
5
A positive ion (cation) has fewer electrons than protons, while a negative ion (anion) has more electrons than protons.
Part 4 of 6Nuclear Notation
Format
- Format The element symbol is preceded by the mass number as a superscript and the atomic number as a subscript, both on its left-hand side.
Example
- Example Aluminium-27 is written ²⁷₁₃Al, showing 13 protons, 14 neutrons and, when neutral, 13 electrons.
Charge
- Charge The ionic charge is a superscript after the symbol, for example Al³⁺, and it changes only the electron count.
Part 5 of 6Worked Example
Question State the number of protons, neutrons and electrons in ²⁷Al³⁺ and in ³²S²⁻ (atomic numbers: Al = 13, S = 16).
- For aluminium, protons = 13 and neutrons = 27 - 13 = 14.
- Al³⁺ has lost three electrons, so electrons = 13 - 3 = 10.
- For sulfur, protons = 16 and neutrons = 32 - 16 = 16.
- S²⁻ has gained two electrons, so electrons = 16 + 2 = 18.
Answer
- ²⁷Al³⁺: 13 protons, 14 neutrons, 10 electrons;
- ³²S²⁻: 16 protons, 16 neutrons, 18 electrons.
Part 6 of 6Exam Checklist & Tips
1.1.1 – Key Points for the Exam
- Atomic number (Z) The number of protons in the nucleus of an atom.
- Mass number (A) The total number of protons and neutrons in the nucleus of an atom.
- Ion An atom or group of atoms that has gained or lost electrons and so carries an overall charge.
- Nucleus The dense, positively charged centre of an atom containing protons and neutrons.
Common Mistakes
- Saying the mass number is the number of neutrons; it is protons plus neutrons.
- Changing the number of protons when an ion forms; only the number of electrons changes.
- Giving the electron a relative mass of zero instead of 1/1836.
- Calculating electrons for a cation by adding the charge instead of subtracting it.
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