What are periodic trends in the periodic table?

Periodic trends refer to patterns observed in the periodic table that show how certain properties of elements change across periods and down groups. These trends include atomic radius, ionization energy, electron affinity, and electronegativity. Understanding these trends helps predict the behavior of elements in chemical reactions, which is a key concept in the IB MYP Science curriculum.

How does atomic radius change across a period and down a group?

The atomic radius generally decreases across a period from left to right due to the increase in nuclear charge, which pulls electrons closer to the nucleus. Conversely, the atomic radius increases down a group as additional electron shells are added, making the atoms larger. This trend is important for students studying the periodic table in the IB MYP Science program.

Why does ionization energy increase across a period?

Ionization energy increases across a period because the nuclear charge increases, which means electrons are held more tightly by the nucleus. As a result, more energy is required to remove an electron. This trend is crucial for understanding how elements interact and bond, a key topic in the IB MYP Science curriculum.

What is the trend in electronegativity across the periodic table?

Electronegativity tends to increase across a period from left to right and decreases down a group. This is because atoms with a higher nuclear charge attract electrons more strongly. Understanding electronegativity is essential for predicting how atoms will bond and form molecules, a concept emphasized in the IB MYP Science curriculum.

How does electron affinity vary in the periodic table?

Electron affinity generally becomes more negative across a period, indicating a greater tendency to gain electrons, while it becomes less negative down a group. This trend is due to the increasing nuclear charge across a period and the increasing atomic size down a group. Grasping this concept is important for students learning about chemical reactivity in the IB MYP Science curriculum.

Why is "Saying Group 1 reactivity DECREASES going down." a common mistake in The Periodic Trends ?

Why it happens: Confusing with Group 7. How to avoid it: Group 1 reactivity INCREASES down. Group 7 reactivity DECREASES down. Easy to remember: opposite ends of the table behave oppositely.

Why is "Saying atoms get larger across a period because they have more protons." a common mistake in The Periodic Trends ?

Why it happens: More protons = more mass and you assume more size. How to avoid it: More protons = STRONGER pull on the same shell of electrons → atom SHRINKS. Mass and size are different.

Why is "Predicting noble gas reactivity changes down the group." a common mistake in The Periodic Trends ?

Why it happens: Other groups have trends. How to avoid it: All noble gases have full outer shells. They are essentially unreactive — the trend is 'no change'.

Periodic tableIB MYP Sciences (Years 1-5)

The Periodic Trends

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The Periodic Trends — IB MYP Sciences: how properties change across periods and down groups

Position in the Periodic Table tells you about an element's behaviour. This MYP Sciences note covers the three big trends — atomic radius, reactivity and metallic character — and the electron-configuration reason why they exist.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Describe how atomic radius changes across a period and down a group.
MYP Sciences A — Explain the reactivity trend in Group 1 (alkali metals) and Group 7 (halogens).
MYP Sciences A — Identify a metal or non-metal from its position.
MYP Sciences D — Use trends to PREDICT properties of less-familiar elements.

Atomic radius

Atoms get smaller across a period and larger down a group.

Across a period (left to right), the number of protons in the nucleus INCREASES while electrons go into the SAME shell. The increasing nuclear charge pulls the existing electrons in more strongly → atomic radius DECREASES. So sodium (Na) is larger than chlorine (Cl) even though they're in the same period.

Down a group, each element has ONE MORE filled electron shell than the one above. Extra shells make the atom much bigger → atomic radius INCREASES. Lithium is small; caesium is huge.

These patterns matter because:

Bigger atoms have outer electrons FURTHER from the nucleus → easier to lose → more reactive (for metals).
Smaller atoms attract electrons MORE strongly → more reactive (for non-metals).

Atomic radius decreases left-to-right across a period (stronger nuclear pull) and increases top-to-bottom down a group (more shells).

Across a period: more protons, same shells → radius decreases.
Down a group: more shells → radius increases.
Affects reactivity: distance of outer electron from nucleus matters.

Reactivity trends down Groups 1, 7 and 0

Group 1 gets more reactive; Group 7 less; Group 0 stays unreactive.

Group 1 — alkali metals. All have ONE outer electron. They react by LOSING this electron. The further from the nucleus it is, the easier to lose → reactivity INCREASES down the group.

Lithium: vigorous fizz with water.
Sodium: rapid reaction with cold water; floats and dissolves.
Potassium: ignites the hydrogen produced (lilac flame).
Rubidium / caesium: explosive reaction.

Group 7 — halogens. All have SEVEN outer electrons. They react by GAINING one. Closer to the nucleus = stronger pull on the incoming electron = MORE reactive. So fluorine (closest) is most reactive; iodine (furthest down) is least reactive.

Fluorine: extremely reactive, attacks glass.
Chlorine: gas, kills bacteria.
Bromine: liquid at room temp, less reactive than chlorine.
Iodine: solid, even less reactive.

A neat experiment: chlorine water added to potassium bromide solution → chlorine 'displaces' bromine (Cl₂ + 2KBr → 2KCl + Br₂) because chlorine is more reactive. This is a displacement reaction.

Group 0/18 — noble gases. All have FULL outer shells → very low tendency to react. He, Ne, Ar, Kr stay essentially inert. (Only the heavier ones — Xe, Rn — form a few compounds in extreme conditions.)

Group 1 reactivity INCREASES down — outer electron easier to lose.
Group 7 reactivity DECREASES down — harder to attract an incoming electron.
Group 0 stays unreactive throughout.
Halogen displacement: more reactive halogen displaces less reactive from its salt.

Metallic character

Metals on the left, non-metals on the right, with metalloids on the stairstep border.

Going LEFT to RIGHT across a period, elements gradually change from metallic to non-metallic. Period 3 illustrates this beautifully:

Element	Type	Notes
Na	metal	shiny, reactive, conducts
Mg	metal	shiny, less reactive than Na
Al	metal	shiny, lighter
Si	metalloid	semi-conducting properties
P	non-metal	red and white forms exist
S	non-metal	yellow solid
Cl	non-metal	green gas
Ar	noble gas	unreactive

The 'zigzag' on the right of the Periodic Table separates metals (left) from non-metals (right). Elements ON the zigzag (B, Si, Ge, As, Sb, Te) are metalloids — they have intermediate properties used in semiconductors.

Going DOWN a group, metallic character generally INCREASES. Lead is more metallic than carbon despite both being Group 14.

This explains why elements like germanium and silicon, sitting right between metals and non-metals on the table, are vital in modern electronics.

Across period (L→R): metallic → non-metallic.
Down a group: metallic character increases.
Metalloids (B, Si, Ge, As, Sb, Te) sit on the zigzag.

How it’s examined

Criterion A tests the trends and explanations using electron configuration. Criterion D often asks for predictions — e.g. 'predict the behaviour of an unfamiliar element using its position'.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — The Periodic Trends

Step-by-step solutions to past-paper-style questions on the periodic trends , written exactly the way a tutor would explain them at the board.

1Atomic radius trend
Getting started• atomic radius
Question
Which is larger: a lithium atom or a caesium atom? Explain.
Step-by-step solution
1. Step 1
  Both are Group 1 alkali metals. Caesium is lower down → more electron shells.
2. Step 2
  More shells means the outermost electron is FURTHER from the nucleus.
3. Step 3
  So caesium has the larger atomic radius.
Answer
Caesium is larger — it has more electron shells than lithium.
2Group 1 reactivity
Building confidence• reactivity
Question
Predict and explain whether rubidium (Rb) or sodium (Na) reacts more vigorously with cold water.
Step-by-step solution
1. Step 1
  Both are Group 1, reacting by losing their single outer electron.
2. Step 2
  Rubidium is BELOW sodium → more electron shells → outer electron is further from the nucleus.
3. Step 3
  Further outer electron = weaker hold by nucleus = easier to lose = MORE reactive.
Answer
Rubidium reacts more vigorously — its outer electron is further from the nucleus and so more easily lost.
3Halogen displacement
Stretch• halogens
Question
Predict whether chlorine displaces bromine from potassium bromide solution. Write a word equation.
Step-by-step solution
1. Step 1
  Chlorine is ABOVE bromine in Group 7, so more reactive.
2. Step 2
  The more reactive halogen displaces the less reactive one from its salt.
3. Step 3
  Equation: chlorine + potassium bromide → potassium chloride + bromine. Solution turns yellow/orange (bromine).
Answer
Yes, chlorine displaces bromine: chlorine + potassium bromide → potassium chloride + bromine.

Key Definitions and Keywords — The Periodic Trends

Definitions to memorise and the exact keywords mark schemes credit for the periodic trends answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Atomic radius
Examiner keyword
The size of an atom, measured as the distance from nucleus to outer electrons.
Periodic trend
Examiner keyword
A pattern in element properties as you move across a period or down a group.
Group 1 reactivity trend
Examiner keyword
Reactivity INCREASES down the group — outer electron is further from the nucleus, easier to lose.
Group 7 reactivity trend
Examiner keyword
Reactivity DECREASES down the group — harder to attract an extra electron from greater distance.
Metalloid
An element with intermediate metallic/non-metallic properties (e.g. silicon, germanium). Sit on the zigzag.
Halogen displacement
A more reactive halogen replaces a less reactive halogen in its salt solution.

Common Mistakes and Misconceptions — The Periodic Trends

The traps other students keep falling into on the periodic trends questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Saying Group 1 reactivity DECREASES going down.
Why it happens
Confusing with Group 7.
How to avoid it
Group 1 reactivity INCREASES down. Group 7 reactivity DECREASES down. Easy to remember: opposite ends of the table behave oppositely.
✕Saying atoms get larger across a period because they have more protons.
Why it happens
More protons = more mass and you assume more size.
How to avoid it
More protons = STRONGER pull on the same shell of electrons → atom SHRINKS. Mass and size are different.
✕Predicting noble gas reactivity changes down the group.
Why it happens
Other groups have trends.
How to avoid it
All noble gases have full outer shells. They are essentially unreactive — the trend is 'no change'.

Practice questions

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The Periodic Trends

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Detailed Notes

What you’ll learn

How it’s examined

1Atomic radius trend

2Group 1 reactivity

3Halogen displacement

Atomic radius

Periodic trend

Group 1 reactivity trend

Group 7 reactivity trend

Metalloid

Halogen displacement

✕Saying Group 1 reactivity DECREASES going down.

✕Saying atoms get larger across a period because they have more protons.

✕Predicting noble gas reactivity changes down the group.

Practice questions

Past paper style quiz

Assess your understanding

The Periodic Trends — frequently asked questions