Introduction to Periodic Trends
The periodic table is one of the most powerful tools in chemistry — not just a list of elements, but a map of patterns. When elements are arranged by increasing atomic number, their physical and chemical properties repeat in a predictable way. These repeating patterns are called periodic trends.
Understanding trends means you can predict the behaviour of an element just by knowing its position in the table — even before you've ever worked with it in a lab!
A regular, repeating pattern in the physical or chemical properties of elements when they are arranged in order of increasing atomic number in the periodic table.
Think of the periodic table like a city map arranged by street numbers. Once you know the pattern of the neighbourhood — where the shops, parks, and schools tend to be — you can predict what's on a new street just by its number, without having visited it.
MYP Key Concept — Change; Related Concept — Patterns:
In MYP Sciences, we study how things change and look for patterns in those changes. Periodic trends are a perfect example: as atomic number changes, properties change in a predictable, patterned way. Recognising this pattern is exactly what allowed the chemist Dmitri Mendeleev to predict the existence of elements that hadn't even been discovered yet!
Nature of Science — Prediction: When Mendeleev published his periodic table in 1869, he left gaps for elements he predicted must exist based on the patterns he saw. He even forecast their approximate atomic mass and properties. When those elements (like gallium and germanium) were later discovered, they matched his predictions closely — a powerful demonstration that science uses patterns to make testable predictions about the unknown.
The main trends we study are:
- Atomic radius (size of atoms)
- Ionisation energy (energy to remove an electron)
- Electronegativity (ability to attract electrons in a bond)
- Metallic character (how metal-like an element behaves)
- Reactivity (how readily an element reacts)
Linking Trends to Electron Configuration
Before exploring each trend in detail, it helps to understand the two underlying causes that explain almost every periodic trend. Keep these in mind throughout this topic — they will come up again and again.
- Nuclear charge: The number of protons in the nucleus. More protons = stronger pull on electrons.
- Electron shielding: Inner electron shells reduce the effective pull of the nucleus on outer electrons. Each inner shell acts like a 'buffer' between the nucleus and the outermost electrons.
The effective nuclear charge () is a way of thinking about the 'net' pull experienced by the outermost electrons. As a simplified model:
This is a simplified model — in reality, electrons in the same shell also partially shield each other, and different shells shield to different degrees. But this approximation is a useful way to think about why outer electrons feel a stronger or weaker pull from the nucleus depending on the element's position in the table.
Imagine you're at a concert (you're an outer electron) trying to hear the stage (the nucleus). Across a period, the band gets louder (more protons) while you stay in the same row. Down a group, you get moved further back with more rows of people (inner shells) between you and the stage — even if the band gets louder, you hear less of it.
Summary of explanations:
| Trend | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases (↑ nuclear charge, same shell) | Increases (more shells, more shielding) |
| Ionisation energy | Increases (electrons harder to remove) | Decreases (electrons easier to remove) |
| Electronegativity | Increases (stronger pull on bond electrons) | Decreases (weaker pull on bond electrons) |
| Metallic character | Decreases | Increases |
In your exam, always explain trends using nuclear charge and shielding — don't just state the trend. This is what earns top marks in MYP Criterion A.
11 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.