Chapter 1 · Periodic Table, Periodic Properties and Variations of Properties
Shells, Groups, Periods and Valency
Lesson 2 — how a string of numbers like 2, 8, 7 tells you exactly where an element lives.
Work through it on screen · tap every question · print it as revision notes
By the end of this lesson you can
identify the number of occupied shells in an atom;
identify the valence electrons;
find the period of an element;
find the group of a representative element;
define valency;
calculate valency from the electronic configuration;
explain what changes across a period and down a group.
011Concept
What an electronic configuration is
In Lesson 1 you learned to read an element's address off the table. This lesson does the harder, more useful thing: you will work the address out from scratch, with nothing but the electrons.
Every element has an electronic configuration. It looks like a small list of numbers, and it quietly answers five questions at once:
how many shells contain electrons;
how many electrons are present in the outermost shell;
the element's period;
usually, its group;
its valency.
Think of the electronic configuration as an element's address card. Everything in this lesson flows out of it:
Electronic configuration
↓
Number of occupied shells
↓
Period number
Electronic configuration
↓
Outermost-shell electrons
↓
Group and valency
The shells have names
Electrons are arranged in shells around the nucleus. The shells are named:
K, L, M, N, O, P, Q
The shell nearest to the nucleus is the K shell. Picture rings of seating around a stage — the K shell is the front row.
Nucleus → K → L → M → N
An electronic configuration simply shows how the electrons of an atom are distributed among these shells.
Worked out: sodium
Atomic number of sodium = 11, so a neutral sodium atom has 11 electrons. Its electronic configuration is:
Sodium: 2, 8, 1
Reading 2, 8, 1 one shell at a time
Shell
Number of electrons
K
2
L
8
M
1
M shell: 1 electron
┌────────────────────┐
│ │
│ L shell: 8 │
│ ┌────────────┐ │
│ │ K shell: 2 │ │
│ │ Nucleus │ │
│ └────────────┘ │
└────────────────────┘
Sodium therefore has electrons in three shells. Hold on to that number — the next concept turns it into the period.
Three thinking questions — answer aloud before moving on
1. In the configuration 2, 8, 1, how many electrons are present altogether? 2 + 8 + 1 = 11 electrons.
2. Which is the outermost occupied shell? The third shell, M.
3. How many electrons are present in that shell? One electron.
Check your understanding · 1 of 12
In the configuration 2, 8, 1, which is the outermost occupied shell?
Correct answer: C — the M shell. The shells fill in order K, L, M, N…, so the third number in the configuration is the M shell. It holds sodium's single outermost electron.
022Concept
Count the shells — that is the period
A block of flats: the floor you live on is not decided by how many people live in the building, but by how many floors are actually in use below and including yours.
The textbook rule is short enough to memorise today:
The period of an element is determined by the number of occupied electron shells in its atom.
In simpler words:
Number of occupied shells = Period number
Three worked examples
Lithium
Lithium: 2, 1
Number of occupied shells = 2
→ Lithium belongs to Period 2.
Sodium
Sodium: 2, 8, 1
Number of occupied shells = 3
→ Sodium belongs to Period 3.
Potassium
Potassium: 2, 8, 8, 1
Number of occupied shells = 4
→ Potassium belongs to Period 4.
Quick rule
Do not count the electrons. Count the occupied shells — that is, count how many numbers there are in the configuration.
2, 7 → 2 shells → Period 2
2, 8, 2 → 3 shells → Period 3
2, 8, 8, 1 → 4 shells → Period 4
Check your understanding · 2 of 12
An element has the electronic configuration 2, 8, 8, 1. Which period does it belong to?
Correct answer: C — Period 4. There are four numbers in the configuration, so four shells are occupied. (Option D is the trap: 2+8+8+1 = 19 electrons, but electrons are not what you count.)
The misconception to kill now
The period depends on the number of occupied shells, not the total number of electrons. Sodium has three occupied shells, so it belongs to Period 3.
Check your understanding · 3 of 12
A classmate writes: “Magnesium is 2, 8, 2, so it has 12 electrons and belongs to Period 12.” What went wrong?
Correct answer: B. Twelve electrons, yes — but they sit in three shells, and it is the shells that give the period. Magnesium belongs to Period 3. There are only 7 periods in the whole table, so any answer above 7 is instantly wrong.
033Concept
Valence electrons — the outermost ones
Imagine a house with several rooms. People in the inner rooms never meet visitors. The people in the outermost room are the ones who open the door.
Valence electrons
The electrons present in the outermost shell of an atom.
Electrons in the inner shells stay inside. The outermost electrons are nearest to the outside world, so they are the ones mainly responsible for how the atom combines with other atoms.
Inner-shell electrons
↓
Remain inside
Valence electrons
↓
Take part in chemical combination
In your own words
The last number of an electronic configuration is the number of valence electrons. That single number will give you both the group and the valency.
Check your understanding · 4 of 12
How many valence electrons does an element with configuration 2, 8, 6 have?
Correct answer: C — 6. Valence electrons are the electrons in the outermost shell, which is the last number written: 6. (Option D confuses valence electrons with the total of 16 electrons.)
044Concept
Valence electrons give the group
For the representative elements of this chapter, the number of valence electrons tells you the group — with one small adjustment on the right-hand side of the table.
4.1 Groups 1 and 2 — read it straight off
Left-hand side of the table
Valence electrons
Group
1
Group 1
2
Group 2
Sodium: 2, 8, 1 → 1 valence electron → Group 1
Magnesium: 2, 8, 2 → 2 valence electrons → Group 2
4.2 Groups 13 to 18 — add ten
Why the jump? Because the ten transition-element columns (groups 3–12) sit in the middle of the table and have to be stepped over. So for elements with 3 to 8 valence electrons:
This simple rule is used here for the representative elements — groups 1, 2 and 13–18. The central elements of groups 3–12 are transition elements, and this chapter does not use the same simple outermost-electron rule to assign their group numbers.
Preview — one exception worth knowing early
Helium (configuration 2) has only two valence electrons, yet it sits in Group 18 with the noble gases, not in Group 2. Its first shell is complete with two electrons, and a complete outermost shell is what defines the noble gas family. The chapter's +10 rule is written for the elements listed above; treat helium as the one name you simply remember.
4.3 The decoder — try it yourself
Pick an element or type any configuration, then step through it one line at a time.
Check your understanding · 5 of 12
An element has the electronic configuration 2, 8, 5. Which group does it belong to?
Correct answer: C — Group 15. Five valence electrons, and 5 is in the 3-to-8 range, so group = 10 + 5 = 15. (This is phosphorus.)
055Concept
What happens down a group
Moving down a group means moving from top to bottom of a vertical column.
Top
↓
Element
Element
Element
Element
↓
Bottom
1 · The number of shells increases
Each successive element down a group has one more occupied shell than the one above it.
Down a group, the number of occupied shells increases.
2 · The valence electrons stay the same
Lithium: 2, 1 → 1 valence electron
Sodium: 2, 8, 1 → 1 valence electron
Potassium: 2, 8, 8, 1 → 1 valence electron
All three have exactly one electron in the outermost shell.
Down a group, the number of valence electrons remains the same.
Why do elements in the same group have similar properties?
Elements in the same group have the same number of valence electrons. Since chemical properties mainly depend on valence electrons, elements in one group generally have similar chemical properties.
Same group
↓
Same number of valence electrons
↓
Similar outermost electronic configuration
↓
Similar chemical properties
Analogy
Think of students wearing the same school uniform. They may be in different classes or of different heights, but the common uniform shows they belong to the same school. Elements down a group have different numbers of shells, but the same number of valence electrons gives them similar chemical behaviour.
Check your understanding · 6 of 12
Moving down a group, which statement is correct?
Correct answer: B. Going down, each element adds a shell, so the period number changes. The outermost shell keeps the same number of electrons, which is why the whole column behaves alike. Option A describes what happens across a period.
066Concept
What happens across a period
Moving across a period means moving from left to right along a horizontal row.
Left → Element → Element → Element → Right
1 · The number of shells stays the same
The elements of Period 2, left to right
Element
Electronic configuration
Shells
Lithium
2, 1
2
Beryllium
2, 2
2
Boron
2, 3
2
Carbon
2, 4
2
Nitrogen
2, 5
2
Oxygen
2, 6
2
Fluorine
2, 7
2
Neon
2, 8
2
Across a period, the number of occupied shells remains the same.
2 · The valence electrons increase one by one
Li → 1
Be → 2
B → 3
C → 4
N → 5
O → 6
F → 7
Ne → 8
Across a period, the number of valence electrons increases by one from one element to the next.
Across Period 2:
Li Be B C N O F Ne
1 2 3 4 5 6 7 8
──────────────────────────────────→
Valence electrons increase
Walk across Period 2
Tap any bar. The height of the bar is the valency — you will meet that in Concept 7, and this is what it looks like before you learn the rule.
← MetalsBars show valencyNoble gas →
Period 2, from lithium to neon. Every one of these eight elements has exactly two occupied shells. Only the outermost shell is filling up.
Valency of that elementCurrently selected
Across a period vs down a group
The comparison examiners ask for
Property
Across a period
Down a group
Direction
Left to right
Top to bottom
Number of shells
Remains the same
Increases
Valence electrons
Increase one by one
Remain the same
Period number
Same
Changes
Group number
Changes
Same
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 by one each time
Going from lithium (2, 1) to neon (2, 8), the number of occupied shells:
Correct answer: B — it stays at 2. Every element in Period 2 has two occupied shells; that is exactly what makes it Period 2. What increases from 1 to 8 is the number of valence electrons, not shells.
077Concept
Valency — the combining capacity
Valence electrons are what an atom has. Valency is what an atom can do with them.
Valency
The combining capacity of an atom.
In student-friendly language: valency tells us how many electrons an atom can lose, gain or share while combining with another atom. It helps us understand how strongly, or in what proportion, an element can combine.
Valence electrons and valency are not the same
The two terms sound alike, and examiners rely on that.
Up to four, use what you saw; after four, subtract from eight. And a complete outer shell means zero.
8.4 Valency across a period
Across a period, valency first increases from 1 to 4 and then decreases from 4 to 0.
Period 2 in full
Element
Valence electrons
Valency
Lithium
1
1
Beryllium
2
2
Boron
3
3
Carbon
4
4
Nitrogen
5
3
Oxygen
6
2
Fluorine
7
1
Neon
8
0
Valency:
1 → 2 → 3 → 4 → 3 → 2 → 1 → 0
4
/ \
3 3
/ \
2 2
/ \
1 1 → 0
Why does valency first rise and then fall?
For atoms with 1–4 valence electrons, valency equals the number of valence electrons. After four, the atom needs fewer than four electrons to complete its outermost shell, so the smaller number takes over.
Oxygen has 6 valence electrons.
It needs 2 more to reach 8.
Therefore, its valency is 2.
8.5 Valency down a group
Since elements in the same group have the same number of valence electrons, they generally have the same valency.
Correct answer: B — 2. Six valence electrons is more than four, so valency = 8 − 6 = 2. Option A is the classic slip of reporting the valence electrons instead of the valency.
Check your understanding · 9 of 12
Argon has the configuration 2, 8, 8. Its valency is:
Correct answer: C — zero. Argon's outermost shell is complete with eight electrons, so it has no combining capacity. Its group is 18 and its valence electrons number 8 — but its valency is 0.
09WWorked
The four-step method
Whenever an electronic configuration is given, run these four steps in the same order every single time.
STEP 1: Count occupied shells
↓
Find the period
STEP 2: Look at the last number
↓
Find valence electrons
STEP 3: Use valence electrons
↓
Find the group
STEP 4: Calculate combining capacity
↓
Find valency
Example 1 — configuration 2, 8, 3
Find its number of shells, period, valence electrons, group and valency.
Full solution
Number of occupied shells: 3 → Period = 3
Outermost electrons: 3 → Group = 10 + 3 = 13
Valency: valence electrons are 3, which is 4 or fewer, so valency = 3
Final answer: The element belongs to Period 3, Group 13 and has valency 3. (It is aluminium.)
Example 2 — configuration 2, 8, 6
Full solution
Step 1 · Period: three occupied shells → Period = 3
Step 2 · Group: six valence electrons → Group = 10 + 6 = 16
Step 3 · Valency: 8 − 6 = 2
Final answer: The element belongs to Period 3, Group 16 and has valency 2. (It is sulphur.)
Example 3 — configuration 2, 8, 8
Full solution
Step 1 · Period: three occupied shells → Period = 3
Scroll back to the decoder in Concept 4 and type any of these configurations in yourself. Doing the steps by hand first, then checking, is worth far more than watching it happen.
10RRecall
Pause and think
Answer these without scrolling back, then open each strip to check.
1 · What determines the period of an element?
The number of occupied electron shells.
2 · What are valence electrons?
Electrons present in the outermost shell.
3 · What happens to the number of shells down a group?
The number of shells increases.
4 · What happens to valence electrons down a group?
The number of valence electrons remains the same.
5 · What happens to valence electrons across a period?
They increase one by one.
6 · Find the period of an element with configuration 2, 8, 5.
Period 3.
7 · Find its group.
Group 15.
8 · Find its valency.
8 − 5 = 3.
9 · Why do elements in the same group have similar properties?
They have the same number of valence electrons and similar outermost electronic configurations.
10 · What is the valency of a noble gas?
Zero.
Type it out — the three definitions
Recall · 10 of 12
Define valence electrons.
Model answer: The electrons present in the outermost shell of an atom.
Recall · 11 of 12
Define valency.
Model answer: The combining capacity of an atom.
Recall · 12 of 12
How is the period of an element determined?
Model answer: The period of an element is determined by the number of occupied electron shells in its atom.
11✗Traps
Mistakes that cost marks
Read the wrong statement, decide the fix in your head, then tap to confirm.
There are three occupied shells, so the element belongs to Period 3. Count shells, never electrons.
For oxygen, valence electrons = 6 but valency = 8 − 6 = 2. Valence electrons are what the atom has; valency is its combining capacity.
Among representative elements it is Group 17, not Group 7 — remember to add 10 once you are past two valence electrons.
Their combining capacity is zero. A complete outermost shell means the atom has nothing to lose, gain or share.
Across a period the number of shells remains the same. It increases only when moving down a group.
12EExam
Exam notes
Definitions to learn word-for-word
Valence electrons
The electrons present in the outermost shell of an atom.
Valency
The combining capacity of an atom.
Period
A horizontal row in the Periodic Table. The period number equals the number of occupied shells in an atom.
Group
A vertical column in the Periodic Table. Elements in the same group generally have similar outermost electronic configurations.
Important trends
Down a group
Number of shells → Increases
Valence electrons → Remain the same
Valency → Remains the same
Chemical properties → Similar
Across a period
Number of shells → Remains the same
Valence electrons → Increase from 1 to 8
Valency → 1, 2, 3, 4, 3, 2, 1, 0
One full-mark answer
Question: Chlorine has seven valence electrons but a valency of one. Explain.
Write it like this
Chlorine has the electronic configuration 2, 8, 7, so it has seven electrons in its outermost shell. Valency is the combining capacity of an atom, and since chlorine needs only one more electron to complete its outermost shell of eight, its valency is 8 − 7 = 1.
Memory tricks worth keeping
Period
Period = Number of electron paths (shells) occupied
Shell count gives period count.
Group
1 electron → Group 1
2 electrons → Group 2
3 to 8 electrons → Add 10
3 + 10 = Group 13
6 + 10 = Group 16
7 + 10 = Group 17
Valency
1 to 4 → Copy the number
5 to 7 → Subtract from 8
8 → Zero
Up to four, use what you saw; after four, subtract from eight.
13QPractice
Practice questions
Level 1 — easy recall
What are valence electrons?
Answer
The electrons present in the outermost shell of an atom.
Define valency.
Answer
The combining capacity of an atom.
How is the period number determined?
Answer
By the number of occupied electron shells in the atom. Number of occupied shells = period number.
How many valence electrons does chlorine have?
Answer
Seven. Its configuration is 2, 8, 7.
What is the valency of oxygen?
Answer
Two. Oxygen is 2, 6, so valency = 8 − 6 = 2.
What is the valency of neon?
Answer
Zero. Neon (2, 8) has a complete outermost shell.
Level 2 — application
For each electronic configuration, find the period, group and valency.
2, 1Answer
Two shells → Period 2. One valence electron → Group 1. Valency 1. (Lithium.)
2, 8, 2Answer
Three shells → Period 3. Two valence electrons → Group 2. Valency 2. (Magnesium.)
2, 5Answer
Two shells → Period 2. Five valence electrons → Group 15. Valency = 8 − 5 = 3. (Nitrogen.)
2, 8, 7Answer
Three shells → Period 3. Seven valence electrons → Group 17. Valency = 8 − 7 = 1. (Chlorine.)
2, 8, 8Answer
Three shells → Period 3. Eight valence electrons → Group 18. Complete outermost shell → Valency 0. (Argon.)
2, 8, 3Answer
Three shells → Period 3. Three valence electrons → Group 13. Valency 3. (Aluminium.)
Answers to the application practice, all together
Configuration
Period
Group
Valency
2, 1
2
1
1
2, 8, 2
3
2
2
2, 5
2
15
3
2, 8, 7
3
17
1
2, 8, 8
3
18
0
2, 8, 3
3
13
3
Level 3 — explain why
Elements in the same group show similar chemical properties.
Answer
Elements in the same group have the same number of valence electrons and therefore similar outermost electronic configurations. Since chemical properties mainly depend on valence electrons, these elements behave in similar ways.
Sodium and potassium belong to the same group.
Answer
Sodium is 2, 8, 1 and potassium is 2, 8, 8, 1. Both have one electron in the outermost shell, so both belong to Group 1 — although sodium has three shells (Period 3) and potassium four (Period 4).
The elements of Period 3 have the same number of occupied shells.
Answer
The period number equals the number of occupied shells. Every element of Period 3 therefore has three occupied shells; across the period only the outermost shell fills up, one electron at a time.
Chlorine has seven valence electrons but valency one.
Answer
Valency is the combining capacity of an atom. Chlorine needs only one more electron to complete its outermost shell of eight, so its valency is 8 − 7 = 1.
Noble gases have zero valency.
Answer
Their outermost shells are already complete, so they have no tendency to lose, gain or share electrons. Their combining capacity, and therefore their valency, is zero.
ICSE-style questions
An element has atomic number 12 and electronic configuration 2, 8, 2. State (i) its period, (ii) its group, (iii) the number of valence electrons, (iv) its valency.
Answer
(i) Three occupied shells → Period 3. (ii) Two valence electrons → Group 2. (iii) 2 valence electrons. (iv) Valency 2. The element is magnesium, an alkaline earth metal.
An element has three shells and seven electrons in its outermost shell. State (i) its period, (ii) its group, (iii) its valency, (iv) the family to which it belongs.
Answer
The configuration is 2, 8, 7. (i) Period 3. (ii) Group = 10 + 7 = Group 17. (iii) Valency = 8 − 7 = 1. (iv) The halogen family. The element is chlorine.
Explain the following changes: (i) the number of shells while moving down a group, (ii) the number of valence electrons while moving down a group, (iii) the number of valence electrons while moving across a period.
Answer
(i) The number of occupied shells increases by one for each successive element, which is why the period number changes going down. (ii) The number of valence electrons remains the same, which is why all members of a group have similar chemical properties. (iii) The number of valence electrons increases by one from each element to the next, from 1 up to 8, while the number of shells stays fixed.
14✓Close
One-minute revision
ELECTRONIC CONFIGURATION
│
├── Number of occupied shells
│ ↓
│ Period number
│
└── Electrons in outermost shell
↓
Valence electrons
↓
Group number
↓
Valency
Shells: Increase
Valence electrons: Same
Valency: Same
Properties: Similar
The whole lesson as one map
PERIODIC POSITION
│
┌────────────────┴────────────────┐
│ │
Number of shells Outermost electrons
│ │
Period Valence electrons
│
┌─────────────┴─────────────┐
│ │
Group Valency
Mastery check
Next → Lesson 3: Periodicity — why properties repeat in the Periodic Table
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