Ever wondered what atoms are actually made of and how... Show more
Chemistry168 views·Updated May 23, 2026·8 pagesAQA A Level Chemistry: Understanding Atomic Structure (1.1)
Posy Chapman@osyhapman_okhwsrybqi
1 / 8
1
of 8
Fundamental Particles and Atomic History
Your understanding of atoms has come a long way since John Dalton's solid sphere model in 1803. Scientists like J.J. Thomson, Ernest Rutherford, Niels Bohr, and Erwin Schrödinger each added pieces to the puzzle, leading to our current quantum model.
Every atom contains three fundamental particles with specific properties you need to memorise. Protons have a relative mass of 1 and charge of +1, neutrons have a relative mass of 1 and no charge, whilst electrons have a tiny relative mass of 1/1836 and charge of -1.
The nucleus sits at the atom's centre, containing all protons and neutrons (called nucleons). Since electrons are so light, virtually all the atom's mass concentrates in this tiny nucleus.
Quick tip: Remember that electrons are about 1,800 times lighter than protons and neutrons - they barely contribute to the atom's mass!
2
of 8
Mass Numbers and Isotopes
Working out particle numbers is straightforward once you know the rules. The atomic number equals the number of protons, whilst the mass number equals protons plus neutrons combined. In neutral atoms, electrons equal protons, but in ions, you subtract the charge from the atomic number.
Isotopes are atoms of the same element with different numbers of neutrons. For example, carbon-12, carbon-13, and carbon-14 all have 6 protons but different neutrons. These isotopes behave identically in chemical reactions because they have the same electron arrangement.
Scientists use mass spectrometers to study isotopes and calculate relative atomic mass. This average mass accounts for all isotopes and their abundance in nature.
Exam focus: You'll often need to calculate particle numbers from mass and atomic numbers - practice this skill until it becomes automatic!
3
of 8
Mass Spectrometry
Time of flight (TOF) mass spectrometry works by giving all ions the same kinetic energy, then measuring how fast they travel. Since kinetic energy equals ½mv², lighter ions zoom through faster than heavier ones.
The process involves five key steps: vaporising the sample, ionisation, acceleration in an electric field, separation in the flight tube, and detection. The detector records flight times and converts them into a mass spectrum showing relative abundance against mass-to-charge ratio.
Scientists can use two ionisation methods. Electron impact fires high-energy electrons at the sample, whilst electrospray ionisation uses charged droplets from a volatile solvent. Electron impact can fragment larger molecules, so electrospray works better for delicate compounds.
Real-world connection: Mass spectrometry helps identify unknown substances in everything from forensic investigations to pharmaceutical development!
4
of 8
Understanding Ionisation Methods
Electron impact ionisation works by bombarding vaporised samples with high-energy electrons at low pressure. This knocks off outer electrons, creating positive ions. However, this method can fragment larger organic molecules due to its high energy.
Electrospray ionisation offers a gentler approach. The sample dissolves in a polar, volatile solvent and gets pumped through a narrow capillary tube. A high voltage creates charged droplets, and as the solvent evaporates, gaseous ions form.
After ionisation, an electric field accelerates all ions to the same kinetic energy. In the flight tube, lighter ions travel faster and reach the detector first, allowing separation by mass.
Key insight: Choose electron impact for small molecules and elements, but use electrospray for larger, fragile organic compounds to prevent fragmentation!
5
of 8
Detection and Analysis
The detector records ion flight times and measures abundance through the electric current produced when positive ions arrive. Faster, lighter ions create signals first, followed by slower, heavier ones.
Data analysis converts flight times into a mass spectrum - a graph plotting relative abundance against mass-to-charge ratio . This spectrum acts like a fingerprint, helping identify unknown substances and determine molecular masses.
Understanding key terms helps: polar molecules have positive and negative ends, volatile solvents evaporate easily, and capillary tubes are incredibly thin tubes that move liquids against gravity. Abundance simply means the percentage of each isotope present.
Practical tip: When calculating relative atomic mass from abundance data, remember all percentages must add up to 100% - use this to find unknown values!
6
of 8
Electron Configuration and Orbitals
Electrons don't just randomly orbit the nucleus - they occupy specific shells and sub-shells with particular shapes and energies. The first shell contains one s orbital, the second has one s and three p orbitals, and the third adds five d orbitals.
Each orbital holds maximum two electrons spinning in opposite directions. Hund's rule states that electrons prefer sitting alone in orbitals before pairing up, like people preferring empty bus seats.
There's a crucial exception: chromium and copper don't follow the expected pattern. Chromium has the configuration [Ar] 4s¹ 3d⁵ rather than 4s² 3d⁴, whilst copper has [Ar] 4s¹ 3d¹⁰ instead of 4s² 3d⁹. When forming ions, electrons are removed from the highest energy level first (4s before 3d).
Memory trick: Remember "4s before 3d" for both filling and emptying - it's counterintuitive but absolutely essential for A-level success!
7
of 8
Ionisation Energies
Ionisation energy measures the energy needed to remove electrons from gaseous atoms. The first ionisation energy removes one electron, the second removes another, and so on. Each successive ionisation energy increases because you're removing electrons from increasingly positive ions.
Three factors affect ionisation energy: atomic radius (larger atoms hold electrons less tightly), nuclear charge (more protons pull electrons stronger), and shielding (inner electrons repel outer ones).
Across periods, ionisation energy generally increases due to stronger nuclear attraction. However, there are two important dips: Group 2 to 3 (removing p electrons is easier than s electrons) and Group 5 to 6 (paired electrons in orbitals repel each other).
Pattern recognition: Group 1 elements have low ionisation energies (easy to lose electrons), whilst Group 0 elements have high values (very stable configurations)!
8
of 8
Energy Levels and Orbital Shapes
Understanding orbital shapes helps visualise electron arrangements. S orbitals are spherical, p orbitals are dumbbell-shaped, and d orbitals have more complex geometries. Energy increases as you move further from the nucleus.
Shielding occurs when inner electron shells repel outer electrons, reducing the nuclear attraction they feel. This explains why atoms get larger down groups despite having more protons.
Remember that 4s orbitals have lower energy than 3d orbitals when filling, but 4s electrons are removed first when forming ions. This seemingly contradictory behaviour is crucial for understanding transition metal chemistry.
Visual learning: Drawing orbital diagrams and electron configurations repeatedly will make these concepts stick - practice with different elements until it becomes second nature!
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Chemistry168 views·Updated May 23, 2026·8 pagesAQA A Level Chemistry: Understanding Atomic Structure (1.1)
Posy Chapman@osyhapman_okhwsrybqi
Ever wondered what atoms are actually made of and how we figured it out? Understanding atomic structure is like solving a massive puzzle that scientists have been working on for over 200 years, and it's absolutely crucial for your chemistry... Show more
1
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Fundamental Particles and Atomic History
Your understanding of atoms has come a long way since John Dalton's solid sphere model in 1803. Scientists like J.J. Thomson, Ernest Rutherford, Niels Bohr, and Erwin Schrödinger each added pieces to the puzzle, leading to our current quantum model.
Every atom contains three fundamental particles with specific properties you need to memorise. Protons have a relative mass of 1 and charge of +1, neutrons have a relative mass of 1 and no charge, whilst electrons have a tiny relative mass of 1/1836 and charge of -1.
The nucleus sits at the atom's centre, containing all protons and neutrons (called nucleons). Since electrons are so light, virtually all the atom's mass concentrates in this tiny nucleus.
Quick tip: Remember that electrons are about 1,800 times lighter than protons and neutrons - they barely contribute to the atom's mass!
2
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Mass Numbers and Isotopes
Working out particle numbers is straightforward once you know the rules. The atomic number equals the number of protons, whilst the mass number equals protons plus neutrons combined. In neutral atoms, electrons equal protons, but in ions, you subtract the charge from the atomic number.
Isotopes are atoms of the same element with different numbers of neutrons. For example, carbon-12, carbon-13, and carbon-14 all have 6 protons but different neutrons. These isotopes behave identically in chemical reactions because they have the same electron arrangement.
Scientists use mass spectrometers to study isotopes and calculate relative atomic mass. This average mass accounts for all isotopes and their abundance in nature.
Exam focus: You'll often need to calculate particle numbers from mass and atomic numbers - practice this skill until it becomes automatic!
3
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Mass Spectrometry
Time of flight (TOF) mass spectrometry works by giving all ions the same kinetic energy, then measuring how fast they travel. Since kinetic energy equals ½mv², lighter ions zoom through faster than heavier ones.
The process involves five key steps: vaporising the sample, ionisation, acceleration in an electric field, separation in the flight tube, and detection. The detector records flight times and converts them into a mass spectrum showing relative abundance against mass-to-charge ratio.
Scientists can use two ionisation methods. Electron impact fires high-energy electrons at the sample, whilst electrospray ionisation uses charged droplets from a volatile solvent. Electron impact can fragment larger molecules, so electrospray works better for delicate compounds.
Real-world connection: Mass spectrometry helps identify unknown substances in everything from forensic investigations to pharmaceutical development!
4
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Understanding Ionisation Methods
Electron impact ionisation works by bombarding vaporised samples with high-energy electrons at low pressure. This knocks off outer electrons, creating positive ions. However, this method can fragment larger organic molecules due to its high energy.
Electrospray ionisation offers a gentler approach. The sample dissolves in a polar, volatile solvent and gets pumped through a narrow capillary tube. A high voltage creates charged droplets, and as the solvent evaporates, gaseous ions form.
After ionisation, an electric field accelerates all ions to the same kinetic energy. In the flight tube, lighter ions travel faster and reach the detector first, allowing separation by mass.
Key insight: Choose electron impact for small molecules and elements, but use electrospray for larger, fragile organic compounds to prevent fragmentation!
5
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Detection and Analysis
The detector records ion flight times and measures abundance through the electric current produced when positive ions arrive. Faster, lighter ions create signals first, followed by slower, heavier ones.
Data analysis converts flight times into a mass spectrum - a graph plotting relative abundance against mass-to-charge ratio . This spectrum acts like a fingerprint, helping identify unknown substances and determine molecular masses.
Understanding key terms helps: polar molecules have positive and negative ends, volatile solvents evaporate easily, and capillary tubes are incredibly thin tubes that move liquids against gravity. Abundance simply means the percentage of each isotope present.
Practical tip: When calculating relative atomic mass from abundance data, remember all percentages must add up to 100% - use this to find unknown values!
6
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
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Electron Configuration and Orbitals
Electrons don't just randomly orbit the nucleus - they occupy specific shells and sub-shells with particular shapes and energies. The first shell contains one s orbital, the second has one s and three p orbitals, and the third adds five d orbitals.
Each orbital holds maximum two electrons spinning in opposite directions. Hund's rule states that electrons prefer sitting alone in orbitals before pairing up, like people preferring empty bus seats.
There's a crucial exception: chromium and copper don't follow the expected pattern. Chromium has the configuration [Ar] 4s¹ 3d⁵ rather than 4s² 3d⁴, whilst copper has [Ar] 4s¹ 3d¹⁰ instead of 4s² 3d⁹. When forming ions, electrons are removed from the highest energy level first (4s before 3d).
Memory trick: Remember "4s before 3d" for both filling and emptying - it's counterintuitive but absolutely essential for A-level success!
7
of 8Sign up to see the content. It's free!
- Access to all documents
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Ionisation Energies
Ionisation energy measures the energy needed to remove electrons from gaseous atoms. The first ionisation energy removes one electron, the second removes another, and so on. Each successive ionisation energy increases because you're removing electrons from increasingly positive ions.
Three factors affect ionisation energy: atomic radius (larger atoms hold electrons less tightly), nuclear charge (more protons pull electrons stronger), and shielding (inner electrons repel outer ones).
Across periods, ionisation energy generally increases due to stronger nuclear attraction. However, there are two important dips: Group 2 to 3 (removing p electrons is easier than s electrons) and Group 5 to 6 (paired electrons in orbitals repel each other).
Pattern recognition: Group 1 elements have low ionisation energies (easy to lose electrons), whilst Group 0 elements have high values (very stable configurations)!
8
of 8Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Energy Levels and Orbital Shapes
Understanding orbital shapes helps visualise electron arrangements. S orbitals are spherical, p orbitals are dumbbell-shaped, and d orbitals have more complex geometries. Energy increases as you move further from the nucleus.
Shielding occurs when inner electron shells repel outer electrons, reducing the nuclear attraction they feel. This explains why atoms get larger down groups despite having more protons.
Remember that 4s orbitals have lower energy than 3d orbitals when filling, but 4s electrons are removed first when forming ions. This seemingly contradictory behaviour is crucial for understanding transition metal chemistry.
Visual learning: Drawing orbital diagrams and electron configurations repeatedly will make these concepts stick - practice with different elements until it becomes second nature!
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
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