Chapter 1 · Periodic Table, Periodic Properties and Variations of Properties
Periodicity
Lesson 3 — why the same kinds of properties keep coming back.
Work through it on screen · tap every question · print it as revision notes
By the end of this lesson you can
define periodic properties;
define periodicity;
explain the cause of periodicity;
explain why elements in the same group have similar properties;
use the halogens as a worked example of a chemical family;
name the main periodic properties studied in this chapter.
00★Recap
Where we left off
Lesson 2 left you holding three facts. This lesson turns them into one answer.
Elements in the same group have the same number of valence electrons.
Elements across a period show a gradual change in valence electrons.
Chemical properties depend mainly on the electrons in the outermost shell.
So here is the question the whole chapter has been building towards:
Why do elements with similar properties appear again and again in the Periodic Table?
The one-word answer is periodicity. The chain behind that word looks like this:
Electronic configurations repeat in a pattern
↓
Similar outermost-shell arrangements reappear
↓
Similar properties reappear
↓
This repetition is called PERIODICITY
011Concept
What is a periodic property?
Some properties do not just change randomly from element to element. They come back — reliably, at regular intervals — as the atomic number climbs.
Those are the properties the chapter calls periodic properties.
In your own words
A periodic property is a property that follows a regular pattern and reappears after an interval in the Periodic Table.
Periodic properties
Properties that reappear at regular intervals or show regular variation when elements are arranged in increasing order of atomic number are called periodic properties.
Everyday analogy: the weekly timetable
Monday is a school day. It is not a school day again tomorrow — but seven days later, Monday is back.
Monday → Tuesday → Wednesday → ... → Sunday → Monday
↑ ↑
Similar position in the repeating weekly cycle
Monday does not appear every day. It returns after a regular interval. Element properties behave the same way: they change gradually from one element to the next, and similar properties reappear when a similar outer electronic arrangement returns.
Check your understanding · 1 of 9
Which statement best describes a periodic property?
Correct answer: B. The two words that must appear in your definition are regular intervals and increasing order of atomic number. A property that never changed would not be periodic at all — it would just be constant.
022Concept
What is periodicity?
A periodic property is a thing. Periodicity is the happening. Examiners love the difference.
Periodicity — exam definition
The repetition of similar properties of elements at regular intervals, when elements are arranged in increasing order of atomic number, is called periodicity.
Periodicity — the short form in the exam notes
The phenomenon of the repetition of similar properties at regular intervals is called periodicity.
Do not confuse the two terms
Two words, one letter apart in your head, several marks apart on paper
Term
Meaning
Periodic property
A property that shows regular repetition or variation
Periodicity
The phenomenon of properties repeating at regular intervals
Property that repeats → Periodic property
Process or phenomenon of repetition → Periodicity
Grammar trick
Words ending in –ity almost always name a phenomenon: gravity, humidity, elasticity, periodicity. Atomic size is a property. Its coming-back is periodicity.
Sort these — periodicity or a periodic property?
One tap each. Answers lock in.
Atomic size
The phenomenon of similar properties reappearing at regular intervals
Electronegativity
Caused by the recurrence of similar electronic configurations
Metallic character
Check your understanding · 2 of 9
Periodicity is best described as:
Correct answer: B. Periodicity is a phenomenon, not a property. Say "the repetition of similar properties at regular intervals" and the mark is yours.
033Concept
What causes periodicity?
This is the single most important sentence in the lesson. If you learn one thing today, learn this.
Cause of periodicity
Periodicity is caused by the recurrence of similar electronic configurations, particularly the same number of electrons in the outermost shell.
Why should the outermost shell matter so much? Because chemical properties are decided by which electrons an atom can lose, gain or share — and those are the outermost ones. When the same outer arrangement comes back, the same chemical behaviour comes back with it.
Same number of outermost electrons
↓
Similar outer electronic configuration
↓
Similar way of losing, gaining or sharing electrons
↓
Similar chemical properties
Example: Group 1
Three different-sized atoms, one identical outer shell
Element
Electronic configuration
Valence electrons
Lithium
2, 1
1
Sodium
2, 8, 1
1
Potassium
2, 8, 8, 1
1
The total number of electrons is different. The number of shells is different. But each atom has exactly one electron in its outermost shell.
Lithium: 2, 1
┐
Sodium: 2, 8, 1│ Same outermost arrangement:
│ one valence electron
Potassium: 2, 8, 8, 1
┘
So they are placed in the same group and show similar chemical properties.
A house analogy
House 1: 2 rooms
House 2: 3 rooms
House 3: 4 rooms
Three houses of different sizes. But suppose each one has exactly one person standing in the outermost room. That person is the first to meet any visitor, so from the doorstep all three houses behave the same way.
Atoms are the same. They may have different numbers of shells, but their chemical behaviour is controlled mainly by whoever is standing in the outer shell.
Work it out yourself
Pick a pair, then step through the reasoning one line at a time.
Thinking check
Lithium is 2, 1. Sodium is 2, 8, 1. Sodium has eight more electrons. Why do they still behave alike?
Answer
Both have one electron in their outermost shell. Chemical properties depend mainly on the outermost-shell electrons, so the eight extra electrons buried in sodium's inner shell make little difference to how it reacts.
Check your understanding · 3 of 9
According to the chapter, what causes periodicity?
Correct answer: A. Neutrons sit in the nucleus and take no part in ordinary chemical reactions. It is the outermost electrons that repeat, and their repetition is what makes properties repeat.
044Concept
A group is a family
Elements in the same group generally have the same number of valence electrons — so they have similar outer electronic configurations, so they behave alike.
Same group
↓
Same number of valence electrons
↓
Similar outer electronic configuration
↓
Similar chemical properties
That is why a group is often called a family of elements. Just as members of a family share certain features, elements of a chemical family share important chemical properties.
Check your understanding · 4 of 9
Which pair is most likely to show similar chemical properties?
Correct answer: B — lithium and sodium. Both have one electron in their outermost shell and belong to Group 1. The other pairs are neighbours across a period, not members of the same group.
The halogens — Group 17 as the worked example
The chapter uses Group 17, the halogen family, to show what "similar group properties" actually looks like. Halogens have seven electrons in their outermost shell.
Fluorine: 2, 7
Chlorine: 2, 8, 7
Both have 7 valence electrons → both are halogens
1 · They are coloured non-metals
The halogens are non-metallic elements and have characteristic colours.
2 · They form negatively charged ions
Each halogen needs just one electron to complete its outermost shell, so it forms an ion carrying a single negative charge.
Halogen atom + 1 electron → Halide ion with –1 charge
Cl + e⁻ → Cl⁻
3 · They are very reactive
Since only one electron is missing, the shell is almost complete and the atom grabs that electron eagerly. That is why halogens are generally found in the combined state and not free by themselves.
4 · Their solubility follows a similar pattern
Halogens are only slightly soluble in water, but more soluble in organic solvents such as carbon disulphide, chloroform and alcohol.
5 · Melting and boiling points increase down the group
Top of Group 17
↓
Melting point increases
Boiling point increases
↓
Bottom of Group 17
Notice that this is a regular variation inside the family, not a repetition — periodicity covers both.
6 · They are good oxidising agents
The chapter identifies halogens as good oxidising agents.
Preview — later in the chapter
"Oxidising agent" is properly explained when you meet electron affinity further on. For now, hold on to the link: a halogen is desperate to gain an electron, and a substance that takes electrons from others is an oxidising agent.
Halogen family summary
GROUP 17: HALOGENS
│
├── Seven valence electrons
├── Coloured non-metals
├── Form ions with –1 charge
├── Highly reactive
├── Usually found combined
├── Slightly soluble in water
├── More soluble in certain organic solvents
├── Melting and boiling points increase down the group
└── Good oxidising agents
Check your understanding · 5 of 9
A halogen atom completes its outermost shell by:
Correct answer: B. Seven outer electrons means one short of eight. Gaining that single electron gives a halide ion carrying a –1 charge, for example Cl + e⁻ → Cl⁻.
Concept check
Fluorine has configuration 2, 7; chlorine has configuration 2, 8, 7.
1 · How many valence electrons does each have?
Seven valence electrons each.
2 · Which group do they belong to?
Group 17.
3 · Why do they have similar chemical properties?
They have similar outer electronic configurations — the same number of electrons in the outermost shell.
055Concept
Similar does not mean identical
This is where careless answers lose marks. Fluorine and chlorine are not the same element wearing different names.
They differ in:
atomic number;
number of shells;
atomic size;
melting point;
boiling point.
What they share is important chemical behaviour, because they have the same number of valence electrons.
The correct wording
Elements in the same group have similar properties, not exactly identical properties.
Analogy: the school uniform
Two students wear the same uniform. Their heights, ages, voices and personalities are all different. The uniform tells you they belong to the same school — it does not make them the same person.
Valence electrons are the uniform. They connect the elements of a group. Everything else may change gradually as you go down.
Marking tip
If a question says "explain why group members have similar properties", the word similar must survive into your answer. Writing "identical" turns a correct explanation into a wrong statement.
066Concept
Down the columns, across the rows
Periodicity is two things at once: properties repeating down the groups, and properties varying gradually across the periods. The grid below shows both at the same time.
Tap a group number, a period label, or any single element. The small figure in the corner of each tile is its valence-electron count.
Neither line is random. Valence electrons climb steadily; valency rises to 4 and then falls back — the up-and-down pattern you met in Lesson 2. Both are examples of regular variation across a period.
Why does the pattern start again?
At the end of a period the outermost shell is complete. The next period has to begin a new shell, and that new shell starts with one electron in it.
Lithium: 2, 1 → Group 1
Sodium: 2, 8, 1 → Group 1
The full configurations are not identical — sodium has an extra filled shell. But the outermost part is the same, and that is the part chemistry cares about. So Group 1 properties reappear.
Period 2 begins: Li → one valence electron
Period 3 begins: Na → one valence electron
↓
Group 1 properties reappear
F: 2, 7 Cl: 2, 8, 7
↓ ↓
Both have seven valence electrons
↓
Both belong to Group 17
↓
Similar halogen properties
Sort these — across a period, or down a group?
One tap each. Answers lock in.
The number of occupied shells increases
The number of valence electrons increases one by one
The number of valence electrons stays the same
The number of occupied shells stays the same
Similar chemical properties are retained
Properties change gradually from one element to the next
Check your understanding · 6 of 9
As you move down a group, which of these stays the same?
Correct answer: C. Down a group the shells increase and the atom grows, but the outermost shell keeps the same number of electrons — which is exactly why the chemical family holds together.
077Concept
The six periodic properties
The chapter names six properties that behave periodically. Every remaining lesson in this chapter is one of them.
Atomic size or atomic radius
Metallic character
Non-metallic character
Ionisation potential or ionisation energy
Electron affinity
Electronegativity
Each of them shows regular variation in two directions:
Across a period → Left to right
Down a group → Top to bottom
Preview — Lesson 4 onward
The one-line meanings below are previews, not this lesson's syllabus. Each property is defined properly, with its full trend and reasoning, in a later lesson. For now you only need to be able to name the six.
Tap any property for a one-line preview.
How large the atom is — measured from the nucleus to the outermost shell. Lesson 4 covers it in full.
How readily an element behaves like a metal — chiefly, how easily it gives up electrons. Defined properly later in the chapter.
The opposite tendency — how readily an element behaves like a non-metal. Defined properly later in the chapter.
Also called ionisation energy — connected with removing an electron from an atom. Defined properly later in the chapter.
Connected with an atom's tendency to accept an electron — the property behind the halogens being good oxidising agents. Defined properly later in the chapter.
Connected with an atom's tendency to attract shared electrons in a bond. Defined properly later in the chapter.
Close the lesson in your head. Answer each one aloud before you open the strip.
1 · Why do chemical properties depend mainly on valence electrons?
Because valence electrons take part in the loss, gain or sharing of electrons during chemical combination.
2 · Why are lithium and sodium placed in the same group?
Both contain one electron in their outermost shell.
3 · Why are fluorine and chlorine both called halogens?
Both belong to Group 17, have seven valence electrons and show similar chemical properties.
4 · Does having similar properties mean two elements are identical?
No. They have similar chemical properties, but many physical properties vary down the group.
5 · Why does a pattern begin again after the completion of a period?
The next period begins with a new outer shell. When an outer configuration similar to an earlier element appears, similar properties also reappear.
6 · What is the difference between periodicity and a periodic property?
Periodicity is the phenomenon of repetition. A periodic property is a particular property that shows a regular pattern.
7 · How many valence electrons do halogens possess?
Seven.
8 · What charge is carried by a halide ion?
A single negative charge, –1.
9 · Name any three periodic properties.
Any three of: atomic size, metallic character, non-metallic character, ionisation potential, electron affinity, electronegativity.
Complete the flowchart
Same number of __________ electrons
↓
Similar outer electronic configuration
↓
Similar __________ properties
Answer
Same number of valence electrons
↓
Similar outer electronic configuration
↓
Similar chemical properties
Type it out
Recall · 7 of 9
Define periodicity in one sentence.
Model answer: The repetition of similar properties of elements at regular intervals, when elements are arranged in increasing order of atomic number, is called periodicity.
Recall · 8 of 9
Why do elements belonging to the same group have similar chemical properties?
Model answer: Elements belonging to the same group have the same number of electrons in their outermost shells and therefore have similar outer electronic configurations. Since chemical properties depend mainly on the outermost-shell electrons, these elements show similar chemical properties.
Recall · 9 of 9
State the cause of periodicity.
Model answer: Periodicity is caused by the recurrence of similar electronic configurations, particularly the same number of electrons in the outermost shell.
09✗Traps
Mistakes that cost marks
Five sentences that sound reasonable and are wrong. Tap each one to see the correction.
Properties change from one element to another. Similar properties reappear only when similar outer electronic configurations return.
They have similar chemical properties, but their physical properties and the strength of their chemical behaviour may change down the group.
Periodicity is caused by the recurrence of similar electronic configurations, especially the same number of electrons in the outermost shell.
Across a period the number of valence electrons increases one by one. It is elements in the same group that share the same number of valence electrons.
The modern table has periods of different lengths. The real idea is that similar properties reappear whenever similar outer electronic configurations recur.
10EExam
Exam notes
Definitions to learn word-for-word
Periodic properties
Properties that reappear at regular intervals or show regular variation when elements are arranged in increasing order of atomic number are called periodic properties.
Periodicity
The phenomenon of the repetition of similar properties at regular intervals is called periodicity.
Cause of periodicity
Periodicity is caused by the recurrence of similar electronic configurations, particularly the same number of electrons in the outermost shell.
Two “give reason” answers, written out in full
Question: Why do elements belonging to the same group have similar chemical properties?
Write it like this
Elements belonging to the same group have the same number of electrons in their outermost shells and therefore have similar outer electronic configurations. Since chemical properties depend mainly on the outermost-shell electrons, these elements show similar chemical properties.
Question: Why do lithium and sodium show similar chemical properties?
Write it like this
Lithium has electronic configuration 2, 1, while sodium has electronic configuration 2, 8, 1. Both have one valence electron. Therefore, they have similar outer electronic configurations and show similar chemical properties.
ICSE-style question 1
What is periodicity? State its cause.
Model answer
Periodicity is the recurrence of similar properties of elements at regular intervals when elements are arranged in increasing order of atomic number. It is caused by the recurrence of similar electronic configurations, especially the same number of electrons in the outermost shell.
ICSE-style question 2
Give a reason: elements of Group 17 show similar chemical properties.
Model answer
Elements of Group 17 have seven electrons in their outermost shells. Since chemical properties depend mainly on the number of valence electrons, they show similar chemical properties.
ICSE-style question 3 — true or false?
One tap each. Corrections for the false ones are in the strip below.
Elements in the same period have the same number of valence electrons.
Elements in the same group have similar outer electronic configurations.
Periodicity is caused mainly by recurring outer electronic configurations.
Halogens usually form ions carrying a positive charge.
Physical properties may change gradually down a group.
Corrections for the false statements
1. False. Across a period, valence electrons increase one by one.
2. True.
3. True.
4. False. Halogens form ions carrying a single negative charge.
5. True.
Memory tricks worth keeping
Periodic means "coming back in a pattern."
OUTER SAME → BEHAVIOUR SIMILAR
Same group
Same outer electrons
Similar properties
"Same outside, similar chemical side."
A M N I E E
Atomic size
Metallic character
Non-metallic character
Ionisation potential
Electron affinity
Electronegativity
"A Merry New Idea Excites Everyone."
11QPractice
Practice questions
Level 1 — straight recall
Define periodicity.
Answer
The repetition of similar properties of elements at regular intervals, when elements are arranged in increasing order of atomic number, is called periodicity.
What is a periodic property?
Answer
A property that reappears at regular intervals, or shows regular variation, when elements are arranged in increasing order of atomic number.
State the main cause of periodicity.
Answer
The recurrence of similar electronic configurations, particularly the same number of electrons in the outermost shell.
How many valence electrons do halogens possess?
Answer
Seven.
What charge is carried by a halide ion?
Answer
A single negative charge (–1).
Name any three periodic properties.
Answer
Atomic size, metallic character and ionisation potential. (Non-metallic character, electron affinity and electronegativity are equally acceptable.)
Level 2 — understanding
Explain why elements in the same group have similar chemical properties.
Answer
They have the same number of electrons in their outermost shells and therefore similar outer electronic configurations. Since chemical properties depend mainly on outermost-shell electrons, these elements show similar chemical properties.
Explain why lithium and sodium belong to the same group.
Answer
Lithium is 2, 1 and sodium is 2, 8, 1. Both have one electron in the outermost shell, so both are placed in Group 1.
Explain why fluorine and chlorine show similar chemical behaviour.
Answer
Fluorine is 2, 7 and chlorine is 2, 8, 7. Both have seven valence electrons, so both belong to Group 17 and behave as halogens — each needs one electron to complete its outermost shell.
Distinguish between periodicity and periodic properties.
Answer
Periodicity is the phenomenon of similar properties repeating at regular intervals. A periodic property is a particular property that shows this regular pattern or variation.
Why does a new pattern begin after the completion of a period?
Answer
At the end of a period the outermost shell is complete. The next period begins a new outer shell containing one valence electron, so an outer configuration similar to an earlier element reappears — and with it, similar properties.
Why are the properties of elements in a group similar but not completely identical?
Answer
The valence electrons are the same, which fixes the main chemical behaviour. But the number of shells, atomic size, melting point and boiling point all change gradually down the group, so the elements are similar rather than identical.
Level 3 — application
Element X has configuration 2, 6. Element Y has configuration 2, 8, 6.
How many valence electrons does each possess?
Are X and Y likely to belong to the same group?
Are they likely to have similar chemical properties?
Give a reason for your answer.
Answer
a. Both possess six valence electrons.
b. Yes.
c. Yes.
d. They have similar outer electronic configurations and therefore show similar chemical properties.
Three elements have these configurations — A: 2, 1 · B: 2, 2 · C: 2, 8, 1. Which two will show the most similar chemical properties? Give a reason.
Answer
A and C will show the most similar properties, because both have one electron in their outermost shell.
Two elements have configurations P: 2, 7 and Q: 2, 8, 7. State their group, their family name, their valency and the type of ion they form.
Answer
Group: 17.
Family: halogens.
Valency: 1.
Ion: they form negatively charged ions carrying a single negative charge.
12✓Wrap
One page, everything
PERIODICITY
│
├── Meaning:
│ Similar properties reappear at regular intervals
│
├── Cause:
│ Similar outer electronic configurations recur
│
├── Same group:
│ Same valence electrons
│ ↓
│ Similar chemical properties
│
├── Across a period:
│ Valence electrons increase gradually
│ Properties vary regularly
│
└── Important periodic properties:
Atomic size
Metallic character
Non-metallic character
Ionisation potential
Electron affinity
Electronegativity
Mind map
PERIODICITY
│
┌────────────────┴────────────────┐
│ │
What is it? Why occurs?
│ │
Similar properties repeat Similar outer configurations
at regular intervals recur
│
Same valence electrons
│
Similar chemical properties
│
┌────────────────────────────┴─────────────┐
│ │
Same group Across a period
│ │
Similar chemical properties Gradual variation
Mastery check
Next → Lesson 4: Atomic Size — why atoms become larger or smaller in the Periodic Table
MyStudyStack≈ 60–70 min