Atomic Structure

Ever wondered what makes up everything around you? Atoms are the building blocks of matter, and understanding their structure is your gateway to mastering chemistry.

An atom consists of a nucleus at the centre containing protons (positively charged) and neutrons (neutral charge). These particles together are called nucleons, held together by the strong nuclear force - much stronger than the forces between protons and electrons.

Electrons (negatively charged) orbit the nucleus in outer shells. Despite being crucial for chemical reactions, electrons are incredibly light compared to protons and neutrons - about 1,800 times lighter!

Quick Tip: Think of an atom like a solar system - the nucleus is the sun, and electrons are planets orbiting around it.

Atomic Numbers and Mass Numbers

The atomic number is like an element's fingerprint - it tells you exactly which element you're dealing with. Change this number, and you've got a completely different element. The atomic number equals the number of protons in an atom.

The mass number tells you how many nucleons $protons + neutrons$ are in the nucleus. Since electrons are so light, they don't contribute significantly to an atom's mass.

Looking at the data table, you'll see that protons and neutrons have nearly identical masses, whilst electrons are thousands of times lighter. This is why we can ignore electron mass when calculating atomic mass.

Exam Focus: Remember that atomic number = protons, and mass number = protons + neutrons. You'll use this constantly!

Isotopes and Relative Mass

Isotopes are atoms of the same element with identical numbers of protons but different numbers of neutrons. They behave the same chemically but have different physical properties like melting points. Some larger isotopes, like Carbon-14, are unstable and radioactive.

Relative atomic mass compares an atom's mass to one-twelfth of a Carbon-12 atom's mass. Carbon-12 is our reference standard because it makes up 98.93% of all carbon isotopes.

To calculate relative atomic mass from isotope data, use: (Isotope 1 × % abundance) + (Isotope 2 × % abundance) ÷ 100. For magnesium's isotopes: (24×79%) + (25×10%) + (26×11%) ÷ 100.

Calculator Ready: These calculations appear frequently in exams, so practice with different isotope mixtures until it becomes automatic.

Time of Flight Mass Spectrometry - The Process

Mass spectrometry is like a high-tech weighing scale that can identify atoms and molecules by their mass. It works through four main stages that separate particles by how fast they travel.

Ionisation happens in two ways. Electron impact fires high-energy electrons at samples, knocking off electrons to create positive ions $X → X⁺ + e⁻$. Electrospray ionisation is gentler, used for large molecules like proteins - the sample gains a proton instead $X + H⁺ → XH⁺$.

Acceleration gives all ions the same kinetic energy using an electric field. Since KE = ½mv², lighter ions move faster than heavier ones with the same energy. Detection occurs when ions hit a negatively charged plate, creating a current - more ions mean bigger current.

Real World: This technique is used everywhere from drug testing to identifying explosives at airports!

Mass Spectrometry - Key Concepts

The beauty of mass spectrometry lies in its ability to identify unknown substances and determine their abundance. You'll need to understand both ionisation methods for your exams.

Electron impact works by bombarding samples with electrons from an electron gun. This aggressive method suits elements and small compounds, creating positive ions by removing electrons completely.

Electrospray ionisation takes a gentler approach for fragile, large molecules. The sample dissolves in a volatile solvent and gets sprayed through a charged needle, picking up protons to become positive ions.

After acceleration, all ions have identical kinetic energy. Using KE = ½mv², you can see why lighter ions travel faster - they need higher velocity to match the kinetic energy of heavier ions.

Exam Strategy: Practice identifying which ionisation method suits different types of compounds - it's a common exam question!

Ion Drift and Detection

In the ion drift stage, ions travel through a vacuum tube at different speeds based on their mass. The vacuum prevents unwanted collisions with air particles that would mess up your results.

Detection is brilliantly simple - positive ions hit a detector plate and grab electrons. This creates electron flow (current) as electrons move to replace the missing ones. More ions hitting the detector means greater current, showing higher abundance.

The time of flight calculations use the relationship between kinetic energy $KE = ½mv²$ and velocity $v = d/t$. Since all ions have the same kinetic energy, lighter ions must travel faster to reach the detector first.

Maths Connection: These calculations combine physics equations you already know - kinetic energy and velocity formulas working together.

Electron Configuration

Forget GCSE electron shells - A-level uses energy levels divided into sub-levels (s, p, d, f). Think of it like a hotel with floors (energy levels) and different room types $sub-levels$ holding specific numbers of guests (electrons).

Sub-level capacity is crucial: s holds 2 electrons, p holds 6, d holds 10, and f holds 14. Electrons live in orbitals - regions of space around the nucleus.

Writing electron configuration shows where electrons live. Oxygen (8 electrons) is 1s²2s²2p⁴. Since oxygen's outermost electrons occupy p-orbitals, it's a p-block element - matching its position on the periodic table.

Memory Trick: Remember "2, 6, 10, 14" for s, p, d, f capacities - it's the foundation for all electron configurations.

Noble Gas Shorthand and Electron Spin

Noble gas shorthand saves time by using the nearest noble gas as a starting point. Instead of writing magnesium as 1s²2s²2p⁶3s², you can write $Ne$3s² - much quicker for larger atoms!

Electron spin follows important rules. Electrons in the same orbital must have opposite spins for stability. When filling orbitals, electrons prefer to occupy empty orbitals first before pairing up - like people preferring empty bus seats before sharing.

The energy level diagram shows how orbitals fill in order. Notice that 4s fills before 3d - this unusual order catches many students out in exams.

Exam Trap: Remember 4s fills before 3d, but when writing electron configurations for ions, remove 4s electrons first!

Ionisation Energy Basics

Ionisation energy is the energy needed to remove an electron from a gaseous atom. Think of it as the "stickiness" between the nucleus and electrons - stronger attraction means higher ionisation energy.

Successive ionisation energies increase dramatically. Removing the first electron from sodium takes 496 kJ/mol, but the second needs 4563 kJ/mol! This happens because removing electrons increases the positive charge, creating stronger attraction to remaining electrons.

Three factors affect ionisation energy: atomic radius (bigger atoms hold electrons more weakly), nuclear charge $more protons = stronger attraction$, and shielding (inner electrons repel outer ones).

Pattern Recognition: Massive jumps in successive ionisation energies reveal electron shell structures - perfect for identifying group numbers!

Ionisation Energy Trends

Ionisation energy decreases down groups because atoms get larger and gain more electron shells. The outer electrons are further from the nucleus and experience more shielding from inner electrons, making them easier to remove.

Ionisation energy increases across periods as nuclear charge increases while electrons stay at roughly the same distance. More protons mean stronger attraction to outer electrons.

Some interesting exceptions exist: magnesium has lower first ionisation energy than neon because magnesium's outer electron is in a higher energy 3s orbital. Sulfur has lower ionisation energy than phosphorus due to electron repulsion - sulfur's outer electron shares an orbital, making it easier to remove.

Trend Breakers: Learn the exceptions in periods 2 and 3 - they're exam favourites and show you understand electron-electron repulsion!