Atomic Structure Basics

Every atom is made up of three fundamental particles that you need to know inside out. Protons have a relative mass of 1 and a charge of +1, neutrons also have a relative mass of 1 but are neutral (charge of 0), and electrons are tiny with a relative mass of 1/1836 and a charge of -1.

The atomic number tells you how many protons are in the nucleus - this never changes for a given element. The mass number is the total of protons and neutrons (electrons are too light to count). When dealing with ions, simply add or subtract electrons based on the charge.

Key Tip: Remember that protons define what element you're dealing with - change the number of protons, and you've got a completely different element!

Historical Models of the Atom

Scientists didn't always understand atomic structure - it took centuries of brilliant minds to figure it out! John Dalton (1803) imagined atoms as solid spheres, which was groundbreaking but missed the fact that atoms contain smaller particles.

JJ Thompson (1904) discovered electrons and proposed the plum pudding model - electrons embedded in a positive "pudding." Rutherford (1911) proved there's a dense nucleus through his famous gold foil experiment. Niels Bohr (1913) suggested electrons orbit in fixed paths, explaining why atoms don't collapse.

Finally, Schrödinger (1926) gave us the modern quantum model, where electrons exist in probability clouds called orbitals rather than fixed orbits. This model actually works for all atoms and is what we use today.

Remember: Each model built on the previous one's strengths whilst fixing its weaknesses - that's how science progresses!

Isotopes and Relative Mass

Isotopes are atoms of the same element with different numbers of neutrons - they're like identical twins with different weights! They have the same atomic number but different mass numbers, which means they behave almost identically in chemical reactions.

The extra neutrons don't affect chemical properties much, but they do change physical properties like melting points and densities. This is why hydrogen and deuterium (heavy hydrogen) have slightly different boiling points.

Relative atomic mass is the weighted average mass of all an element's isotopes compared to 1/12th of a carbon-12 atom. Relative isotopic mass refers to individual isotopes. These definitions might seem abstract, but they're essential for understanding why atomic masses aren't whole numbers.

Quick Check: If you see a non-whole atomic mass on the periodic table, you know that element has multiple isotopes!

Mass Spectrometry - The Basics

Mass spectrometry is like a sophisticated weighing scale that can identify and measure individual atoms and molecules. It tells you exactly what isotopes are present and in what percentages - incredibly useful for chemists!

The process starts by bombarding your sample with high-energy electrons, which knocks electrons off atoms to create positive ions. These ions get accelerated through an electric field, then separated based on their mass-to-charge ratio $m/z$.

Since most ions have a +1 charge, the m/z ratio essentially just tells you the mass. A computer detects these separated ions and creates a mass spectrum - a graph showing what masses are present and how abundant they are.

Real-world Application: Mass spectrometry is used everywhere from detecting drugs in athletes to identifying unknown compounds in forensic investigations!

How Mass Spectrometry Works

There are two main methods for separating ions in mass spectrometry. The magnetic deflection method uses a magnetic field to bend the path of ions - heavier ions get deflected less than lighter ones, allowing separation.

Time of flight spectrometry is more elegant - all ions get the same kinetic energy, so lighter ions move faster than heavier ones. By measuring how long it takes ions to travel a fixed distance, you can calculate their masses with incredible precision.

The final mass spectrum is straightforward to read: the x-axis shows relative isotopic mass, and the y-axis shows the percentage abundance of each isotope. The height of each peak tells you how common that particular isotope is.

Pro Tip: The tallest peak represents the most abundant isotope - this is often called the base peak!

Calculating Relative Atomic Mass

You'll definitely need to master these calculations for your exams! The formula is: Ar = Σ(relative isotopic mass × abundance) ÷ 100. This weighted average accounts for how common each isotope is.

For missing isotope masses, substitute 'x' for the unknown value, expand the brackets, multiply by 100, then solve the linear equation. It's just algebra with a chemistry twist!

When calculating percentage abundances, call one isotope 'x' and the other '100-x', then follow the same process. Your final percentages must always add up to 100% - if they don't, you've made an error somewhere.

Exam Success: Always double-check your percentages add to 100% and that your final atomic mass makes sense compared to the individual isotope masses!

Relative Molecular and Formula Mass

Calculating relative molecular mass and relative formula mass is refreshingly straightforward after all those isotope calculations! Simply add up the relative atomic masses of all atoms in the compound.

Use relative molecular mass for simple molecules (like H₂O or CO₂) and relative formula mass for compounds with giant structures like ionic compounds. The calculation method is identical - just different terminology.

Start by writing out all elements present, look up their relative atomic masses from the periodic table, multiply by how many of each atom you have, then add everything together. That's your final answer!

Time-Saver: Make a systematic list of elements and their quantities before calculating - it prevents silly mistakes and saves time in exams!