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B2.2 · Analyse element data to identify periodic trends

Learn to analyse element data to identify periodic trends through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Matter, Chemical Trends, and Chemical Bonding

Ontario Grade 11 Chemistry | B2.2: Analyse element data to identify periodic trends

Some elements have atoms that are larger than others. Some atoms also need more energy than others to lose an electron. These differences are not random: when elements are arranged in the periodic table, their properties show patterns. In this lesson, you will use element data to identify those patterns. A periodic trend is a general change in a property as you move across a period or down a group. A period is a horizontal row in the periodic table. A group is a vertical column. Trends describe broad patterns; they do not mean that every neighbouring element has a value that changes by exactly the same amount.

What you will learn

Read and compare element data, including units.
Describe general trends in atomic radius and first ionization energy across a period and down a group.
Use evidence from a data set to identify a periodic trend while noticing that individual values may not follow a perfectly smooth pattern.

1. Read the table before looking for a trend

You already know that elements are arranged in the periodic table by atomic number. Atomic number is the number of protons in an atom’s nucleus. A neutral atom also has electrons, which occupy regions around the nucleus. These ideas help us discuss why properties can change across the table, but the first task is to read the data accurately.

A data set may list a property and its unit beside each element. Compare elements in the same period to study a left-to-right pattern. Compare elements in the same group to study a top-to-bottom pattern. Do not compare values without checking that they refer to the same property and use the same unit.

Atomic radius is a measure of the size of an atom. Atomic radius values are often given in picometres, written as pm. A picometre is a very small unit of length. First ionization energy is the energy needed to remove one electron from a neutral atom in the gas state. It is commonly reported in kilojoules per mole, written as kJ/mol. For this lesson, use it as a measured property to compare elements.

A period runs horizontally; a group runs vertically.
Check the property and unit before comparing numbers.
A trend is a general pattern, not a promise that every step is identical.

2. Recognize patterns in atomic radius

A familiar observable effect is that objects can have different sizes. Atoms are too small to compare by sight, so scientists use measurements and models instead. Atomic radius gives a consistent way to compare atom size.

Across a period from left to right, atomic radius generally decreases. In a simple atomic model, the atoms gain protons as atomic number increases, and electrons are added to the same main energy level. The stronger positive charge in the nucleus attracts the electrons more strongly overall. This helps explain why the atom’s radius tends to get smaller across the period.

Down a group, atomic radius generally increases. Atoms lower in a group have electrons in additional main energy levels, farther from the nucleus. This makes the atoms larger overall. Use this explanation as a course-level model for the observed pattern; the data are what you analyse to identify the trend.

Across a period: atomic radius generally decreases.
Down a group: atomic radius generally increases.
Use the direction of change in the data to support a trend claim.

3. Recognize patterns in first ionization energy

First ionization energy measures how much energy is needed to remove one electron from a neutral atom in the gas state. It is not the same as the atom’s size. A higher value means more energy is needed for that removal.

Across a period, first ionization energy generally increases. As the positive charge of the nucleus increases across a period, the attraction between the nucleus and the outer electrons generally becomes stronger. More energy is therefore usually needed to remove an electron.

Down a group, first ionization energy generally decreases. The outer electrons are farther from the nucleus in larger atoms, so they are generally less strongly attracted. Less energy is then needed to remove one. Actual values can include small reversals in the overall pattern. When that happens, describe the general trend and report the data accurately rather than pretending every value follows a perfect sequence.

Electronegativity is another periodic property. It describes how strongly an atom attracts shared electrons in a chemical bond. At this level, use a periodic table or course reference data to compare values. Across a period, electronegativity generally increases; down a group, it generally decreases. The main skill in B2.2 is to support trend statements by analysing element data.

Across a period: first ionization energy generally increases.
Down a group: first ionization energy generally decreases.
Across a period: electronegativity generally increases; down a group: it generally decreases.
Use “generally” when a data set contains exceptions to a smooth pattern.

4. Make a trend claim from evidence

A strong trend statement names the property, the direction of movement, and the direction of change. It also gives evidence from values. For example, state that a property generally decreases across a period, then cite the starting and ending values and their units.

If values rise and fall slightly, look at the overall pattern rather than selecting only one pair of elements. Check whether the elements are in the same period or group. Keep units attached to numerical values. If you calculate a difference, subtract values with matching units and keep the unit in the result.

A useful analysis separates observation from explanation. First say what the data do. Then use the simple atomic model to suggest why the trend occurs. Do not let an explanation replace the data: a trend claim should still be supported by the values you were given.

Name the property and the direction of comparison.
Quote numerical evidence with units.
Separate the observed data pattern from the model used to explain it.

Approximate Period 2 data for trend analysis

Element	Atomic radius (pm)	First ionization energy (kJ/mol)
Lithium	152	520
Beryllium	112	900
Boron	85	801
Carbon	77	1086
Nitrogen	75	1402
Oxygen	73	1314
Fluorine	72	1681

Worked example

Compare radius and ionization energy across a period

The table gives approximate values for selected elements in Period 2. Use the data to describe the general change in atomic radius and first ionization energy from lithium to fluorine. Include evidence and units.

Confirm the comparison
All listed elements are in Period 2, so comparing them in order examines a left-to-right pattern. Radius is measured in pm, while first ionization energy is measured in kJ/mol.
Read the radius values
The radius falls from 152 pm for lithium to 72 pm for fluorine. The intermediate values also mostly fall, so the overall pattern is a decrease across the period.
$152\ \mathrm{pm} \rightarrow 72\ \mathrm{pm}$
Read the energy values
First ionization energy rises overall from 520 kJ/mol for lithium to 1681 kJ/mol for fluorine. There are small reversals between some neighbours, so the careful conclusion is that the general trend is an increase, not that every step increases.
$520\ \mathrm{kJ/mol} \rightarrow 1681\ \mathrm{kJ/mol}$
Connect the model to the evidence
The radius data show that atoms generally become smaller across this period. The energy data show that removing an electron generally requires more energy. In the course-level model, increasing nuclear charge across the period helps account for both overall patterns.

Answer: Across Period 2 from lithium to fluorine, atomic radius generally decreases, from 152 pm to 72 pm. First ionization energy generally increases, from 520 kJ/mol to 1681 kJ/mol. The intermediate values show small reversals in ionization energy, so the conclusion describes the overall trend rather than every neighbouring pair.

Check: The values being compared have matching units within each property. The conclusion names the direction and includes evidence for both trends.

Common mistakes and how to avoid them

Saying that every value must change smoothly because there is a periodic trend.

Correction: A trend is an overall pattern. Describe small reversals accurately and use “generally” when appropriate.

Mixing up periods and groups when describing direction.

Correction: A period is a horizontal row, so across a period means left to right. A group is a vertical column, so down a group means top to bottom.

Stating a trend without evidence or units.

Correction: Name the property, state its overall direction, and quote relevant values with their units.

Treating atomic radius and first ionization energy as the same property.

Correction: Radius describes atom size. First ionization energy describes the energy needed to remove one electron from a neutral gaseous atom.

Lesson summary

Periodic trends are general changes in element properties across periods or down groups.
Atomic radius generally decreases across a period and increases down a group.
First ionization energy generally increases across a period and decreases down a group.
Electronegativity generally increases across a period and decreases down a group.
Support trend claims with data, units, and careful wording.

Check your understanding

Question 1

An element data set shows atomic radius values generally getting larger from the top to the bottom of a group. Which conclusion matches the trend?

Atomic radius generally increases down a group.
Atomic radius generally decreases down a group.
Atomic radius generally increases across a period.
Atomic radius has no pattern down a group.

Show answer and explanation

Atomic radius generally increases down a group.

The comparison is down a group, and the values get larger, so atomic radius generally increases in that direction.

Question 2

Across a period, a data set shows first ionization energy increasing overall, with one small decrease between neighbouring elements. What is the best description?

The overall trend is an increase, with a small reversal in the data.
The trend is a decrease because one value falls.
There is no trend because the values are not perfectly smooth.
The values prove that ionization energy is the same as atomic radius.

Show answer and explanation

The overall trend is an increase, with a small reversal in the data.

A periodic trend is an overall pattern. A small reversal does not erase the general increase, but it should be acknowledged.

Question 3

Which information is needed to make a useful trend claim from a table?

The property, the direction of comparison, and values with units.
Only the largest value in the entire table.
The element names without the property or units.
A claim that every value must change by the same amount.

Show answer and explanation

The property, the direction of comparison, and values with units.

A clear claim identifies what is being compared and supports the direction of change with correctly labelled data.

Key terms

Atomic number: The number of protons in the nucleus of an atom.
Period: A horizontal row in the periodic table.
Group: A vertical column in the periodic table.
Periodic trend: A general change in an element property as you move across a period or down a group.
Atomic radius: A measure used to compare the size of atoms.
First ionization energy: The energy needed to remove one electron from a neutral atom in the gas state.
Electronegativity: A measure of how strongly an atom attracts shared electrons in a chemical bond.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation B2.2. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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