105 articles
Atomic Structure
Protons, neutrons, electrons, orbitals and electron configurations — what atoms are made of and how their electrons are arranged.
How Valence Electrons Determine Chemical Reactivity
Why is caesium explosive in water while neon ignores everything? A beginner's guide to how valence electrons, ionization energy and electron affinity decide how reactive an element is, with trends across the periodic table.
The Neutron: The Uncharged Particle That Holds Nuclei Together
What a neutron is, how James Chadwick discovered it in 1932, why nuclei need neutrons to be stable, how neutron numbers create isotopes, why free neutrons decay, and how neutrons drive nuclear fission, power reactors, date artefacts and probe materials.
The Proton: What It Is and Why It Gives Each Element Its Identity
A guide to the proton: its charge and mass, how Rutherford identified it, why the number of protons defines each element, what holds protons together in the nucleus, quarks inside the proton, protons as hydrogen ions in acid chemistry, and protons in medicine and the Sun.
What Is an Atom? A Clear Guide to the Building Block of Matter
A beginner-friendly guide to atoms: what they are, what they're made of, how protons, neutrons and electrons are arranged, why atoms are mostly empty space, how atoms differ between elements, how they join to make everything around us, and how we know they exist.
The Electron: Discovery, Charge, Mass and Why It Matters
Everything a student needs about the electron: J.J. Thomson's cathode-ray discovery, Millikan's oil-drop measurement of its charge, its tiny mass, where electrons are found in atoms, why they control chemical bonding, electricity and light, and how the electron behaves as both a particle and a wave.
Atomic Mass vs. Mass Number vs. Molar Mass: What's the Difference?
Three terms that sound interchangeable but mean genuinely different things — a whole-number count, a weighted average, and a per-mole quantity — explained with real numbers.
How to Find the Number of Valence Electrons for Any Element
Read any element's valence electrons from its group number. Includes a valence electrons by group chart, a table for the first 20 elements, and the exceptions.
Isotopes vs. Ions vs. Isomers vs. Allotropes: What's the Difference?
Four chemistry terms that all describe a substance changing without becoming a different element or compound — but each one changes something completely different.
How to Find the Number of Protons, Neutrons and Electrons
A simple method for finding protons, neutrons and electrons in any atom, isotope or ion using atomic number, mass number and charge — with worked examples and common mistakes.
What Is an Ion? Cations, Anions and How Ions Form
What ions are, how atoms become cations and anions, how to predict an element's ion charge from the periodic table, and why ions matter in everything from salt to nerve signals.
Antimatter and Positrons: The Mirror Image of Ordinary Matter
What antimatter is, how the positron was predicted and discovered, what happens when matter and antimatter meet, how PET scans use positrons, and why the universe is made almost entirely of matter.
Atomic Emission Spectroscopy: Identifying Elements by Their Light
How atomic emission spectroscopy works, from a simple flame photometer to an argon-plasma ICP-OES: excitation, emission lines, calibration, interferences and where the technique is used.
Inside the Atomic Nucleus: Size, Density and the Strong Force
An intermediate guide to the atomic nucleus: Rutherford's discovery, how big nuclei are and how radius scales with mass number, their astonishing density, the strong nuclear force, binding energy and mass defect, the binding energy per nucleon curve, magic numbers, and why some nuclei are radioactive.
The Shapes of s, p, d and f Orbitals Explained
What atomic orbitals look like and why: spherical s orbitals, dumbbell p orbitals along x, y and z, the five d orbitals including the dz² ring shape, the complex f orbitals, nodes and phases, how orbital size grows with shell number, and why shapes matter for bonding and molecular geometry.
How Is Atomic Radius Measured? Covalent, Metallic and van der Waals Radii
An atom has no hard edge, so how do chemists give it a radius? A clear guide to covalent, metallic, van der Waals and ionic radii, how each is measured, and why the numbers differ.
How Atomic Spectra Tell Us What Stars Are Made Of
How astronomers use atomic spectra to study the universe: Fraunhofer lines, the discovery of helium in the Sun, Cecilia Payne-Gaposchkin's discovery that stars are mostly hydrogen, spectral classes and temperature, redshift and the expanding universe, exoplanet atmospheres and the chemistry of interstellar space.
From Atomic Structure to Bonding: Connecting the Ideas
How electron configurations, valence electrons, electronegativity and atomic size explain why atoms form ionic, covalent or metallic bonds, and how those bonds decide the properties of substances.
The Aufbau Principle: Filling Orbitals from the Bottom Up
What the Aufbau principle says, the order in which subshells fill (1s 2s 2p 3s 3p 4s 3d…), the diagonal rule and the n + l rule for remembering it, why 4s fills before 3d, worked electron configurations, the famous exceptions, and why 4s electrons are lost first when ions form.
Chadwick and the Discovery of the Neutron
The story of James Chadwick's 1932 discovery of the neutron: a decade-long search, Chadwick's years in a German internment camp, the beryllium radiation puzzle, the Joliot-Curies' near miss, the ten days of experiments that settled it, how he calculated the neutron's mass, and how the discovery led to nuclear fission.
Core Electrons vs Valence Electrons
The difference between core and valence electrons: definitions, how to count each for main-group and transition elements, why valence electrons control bonding and reactivity while core electrons shield the nucleus, links to group number, Lewis symbols and ionisation energy jumps, and worked examples.
Electron Configurations of Ions: Cations, Anions and Transition Metals
Worked examples for writing electron configurations of ions: main-group cations and anions, isoelectronic species, why transition metals lose 4s electrons before 3d, Fe²⁺ and Fe³⁺, Cu⁺ and Cu²⁺, Cr³⁺, counting unpaired electrons, and linking configurations to ion stability and colour.
Electron Configuration Practice Questions with Full Answers
Thirty electron configuration practice questions in five graded sets, from writing configurations for light elements to transition metal ions, exceptions, identifying elements from configurations, excited states and exam-style explanations, each with a full worked answer.
Electron Clouds and Probability: Why Orbitals Aren't Orbits
Why modern chemistry describes electrons as clouds of probability rather than particles on tracks: the wavefunction and ψ², dot-density pictures, 90% boundary surfaces, radial probability and the most probable distance, how the cloud picture explains atomic size, bonding and electronegativity, and common misconceptions.
Emission vs Absorption Spectra: How Atoms Reveal Themselves
A comparison of emission and absorption spectra: continuous vs line spectra, bright lines on a dark background vs dark lines in a rainbow, why the lines appear at the same wavelengths, Kirchhoff's laws, Fraunhofer lines in sunlight, the discovery of helium, and uses in chemical analysis and astronomy.
Exceptions to the Octet Rule: Expanded, Incomplete and Odd-Electron Molecules
A list-style guide to the main exceptions to the octet rule, with Lewis structure examples for each: incomplete octets (BF₃, BeCl₂, AlCl₃), odd-electron radicals (NO, NO₂, ClO₂), expanded octets (PCl₅, SF₆, XeF₄, I₃⁻), transition metal compounds, and how to recognise each type in exam questions.
Ground State vs Excited State Electron Configurations
The difference between ground-state and excited-state electron configurations: definitions, how to tell them apart, how atoms become excited (heat, light, electricity), what happens when they relax, worked examples from sodium to carbon, why excited states explain spectra, flame colours and lasers, and common exam traps.
Working Backwards: Finding Isotope Abundances from Average Atomic Mass
Step-by-step worked examples for finding the percentage abundance of isotopes when the relative atomic mass is known: the algebra method with x and (100 − x), fraction method, checking answers, three-isotope problems where one abundance is given, and common mistakes.
Can We See Atoms? Electron Microscopes and Scanning Tunnelling Microscopy
Atoms are far smaller than light waves, so no ordinary microscope can show them. Here is how electron microscopes, scanning tunnelling microscopes and atomic force microscopes finally let us image, and even move, single atoms.
Hund's Rule: Why Electrons Spread Out Before Pairing
Hund's rule of maximum multiplicity explained: electrons occupy degenerate orbitals singly with parallel spins before pairing, why this lowers energy, worked orbital diagrams for carbon, nitrogen, oxygen and iron, unpaired electrons and magnetism, and the link to chromium and copper.
The Hydrogen Emission Spectrum: Lyman, Balmer and Paschen Series
How hydrogen's line spectrum arises from electrons falling between energy levels: the Lyman (ultraviolet), Balmer (visible) and Paschen (infrared) series, why lines converge, reading an energy level diagram, the convergence limit and ionisation energy, and why hydrogen's lines matter in astronomy.