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Chapter 1 · Periodic Table, Periodic Properties and Variations of Properties

The Modern Periodic Table

Lesson 1 — how 118 elements were tamed into 18 columns and 7 rows.

Work through it on screen · tap every question · print it as revision notes

By the end of this lesson you can

  1. explain why elements had to be classified at all;
  2. describe what Döbereiner, Newlands, Mendeleev and Moseley each contributed;
  3. state the modern periodic law word-for-word;
  4. tell a group apart from a period without hesitating;
  5. name the important families — alkali metals, halogens, noble gases and the rest;
  6. read an element's "address" off the table.
011Concept

Why were elements classified?

Walk into a library where the books have been shelved at random. A physics text sits beside a cookbook; the dictionary hides behind a novel. Nothing is missing — but nothing can be found either.

The librarian's fix is not to buy fewer books. It is to shelve them by kind:

Science │ History │ Literature │ Geography

Chemists hit exactly this wall. Elements are pure substances made of only one kind of atom, and as more and more of them were discovered, studying each one separately became impossible. But scientists noticed something useful: some elements behave in strikingly similar ways. So they did what the librarian did.

Collect elements ↓ Compare their properties ↓ Place similar elements together ↓ Find patterns in their behaviour

That process is the classification of elements, and its finished product is the Periodic Table.

Periodic Table

A tabular arrangement of elements in groups and periods, showing regular trends in their properties.

In your own words

The Periodic Table arranges elements so that elements with similar properties sit together, and changes in properties can be studied easily.

Think about it

Your cupboard at home is already a periodic table: shirts with shirts, trousers with trousers, winter clothes together. You never sort clothes randomly, because grouping similar things makes them easier to find and easier to understand.

Check your understanding · 1 of 7

A classmate says: “Elements were classified only to make the Periodic Table look neat.” Is that right?

Correct answer: B. Neatness is a side effect. Classification was needed to study elements systematically, to place similar elements together, and to reveal regular patterns in their properties.
Three questions to answer aloud before moving on

1. Why would studying 100 separate elements be difficult?
Each would have to be memorised individually, with no way to predict its behaviour.

2. How does grouping help?
Learn one member of a family well and you already know roughly how the others behave.

3. What must be compared before grouping elements?
Their physical and chemical properties.

022Concept

The historical journey

The modern table was not built in one attempt. Four scientists, each fixing what the last one could not.

Döbereiner → Newlands → Mendeleev → Moseley → Modern Periodic Table

Tap each name to open the full story.

Döbereiner placed elements in groups of three. Such a group was called a triad, and the three elements in it had similar properties.

Element 1 Element 2 Element 3 ← all three behave alike

Limitation: the system could not classify all the known elements — not every element could be fitted into a suitable group of three.

Memory hook

Triad → Trio → Three.

Newlands arranged elements in increasing order of their atomic masses and noticed that every eighth element resembled the first — just as the eighth note of a musical scale repeats the first.

When elements are arranged in increasing order of atomic mass, every eighth element has properties similar to the first element.
1 2 3 4 5 6 7 8 ↑ ↑ first similar again

Limitation: the pattern could not explain elements beyond calcium.

Memory hook

Newlands → new notes → octaves.

Dmitri Mendeleev arranged the 63 known elements in order of increasing atomic mass, into eight vertical columns called groups and horizontal rows called periods.

Mendeleev's Periodic Law

The properties of elements are a periodic function of their atomic masses.

What does “periodic function” mean? That when elements are arranged in a particular order, similar properties reappear after regular intervals — exactly like the days of the week:

Monday → Tuesday → … → Sunday → Monday again

Monday returns after a fixed interval. Certain chemical properties do the same.

Henry Moseley showed that atomic number, not atomic mass, is the more suitable basis for arranging elements. This single correction produced the table we use today.

Modern Periodic Law

The physical and chemical properties of elements are periodic functions of their atomic numbers.

Read as a student would say it: arrange the elements in increasing order of atomic number, and similar physical and chemical properties repeat at regular intervals.

Increasing atomic number ↓ Elements arranged in order ↓ Properties change gradually ↓ Similar properties reappear
The M–M trap

Both start with M, and examiners know it.
Mendeleev used Mass. Moseley made the Modern correction — atomic nuMber.

The two laws side by side
ScientistBasis of arrangementMain idea
MendeleevAtomic massProperties are periodic functions of atomic masses
Moseley / modern lawAtomic numberProperties are periodic functions of atomic numbers
Check your understanding · 2 of 7

Newlands arranged elements according to:

Correct answer: B — increasing atomic mass. Atomic number arrived much later, with Moseley. Döbereiner, Newlands and Mendeleev all worked with atomic mass.
Check your understanding · 3 of 7

Newlands' Law of Octaves broke down after which element?

Correct answer: C — calcium. Up to calcium the eighth-element pattern held; beyond it, the octaves stopped working.
Mendeleev used atomic mass. The modern table uses atomic number, following Moseley's work.
033Concept

Inside the modern table

Two numbers hold this whole chapter together: 18 groups and 7 periods.

18 vertical columns = GROUPS 7 horizontal rows = PERIODS

Tap a group number along the top, a period number down the side, or any single element.

The whole table at a glance.
Colours mark the families. Tap a column, a row, or an element to read its address.
Group 1 · alkali metals Group 2 · alkaline earth metals Groups 3–12 · transition elements Groups 13–17 · representative Group 18 · noble gases Lanthanides & actinides

3.1 Groups — the vertical families

Picture a school building. Each vertical column of rooms is one group, and everyone in that column belongs to the same family. Elements in the same group usually show similar chemical properties, because their outermost electron arrangements are similar. (Exactly why, comes in Lesson 2.)

The named families — tap a row to find it on the table above
GroupName to learn
1Alkali metals
2Alkaline earth metals
3–12Transition elements
13Boron family
14Carbon family
15Nitrogen family
16Oxygen family, or chalcogens
17Halogens
18Noble gases, inert gases or zero-group elements
If you learn only three

Group 1 → alkali metals. Group 17 → halogens. Group 18 → noble gases. These three carry most of the marks in this chapter.

Representative elements

Elements of groups 1, 2, 13, 14, 15, 16, 17 and 18 are called representative, normal or typical elements. In other words: the two columns on the left and the six on the right — everything except the middle block.

Groups: 1 2 │ 3 — 12 │ 13 14 15 16 17 18 └────┘ └──────┘ └──────────────┘ representative transition representative

Transition elements

Elements of groups 3 to 12 are the transition elements. They occupy the central portion of the table.

Lanthanides and actinides

The two rows parked separately at the bottom are the lanthanides and actinides. They belong to group 3 of periods 6 and 7, but are shown below the main table to keep it compact.

3.2 Periods — the horizontal rows

Think of rows of seats in a cinema. Row 1 is period 1, row 2 is period 2, and so on down to row 7. Moving left to right means moving across a period; moving top to bottom means moving down a group.

How long each period is
PeriodDescriptionNumber of elements
1Shortest period2
2Short period8
3Short period8
4Long period18
5Long period18
6Longest period32
7Longest, incomplete period32 positions
Non-negotiable

7 periods. 18 groups. Groups go down. Periods pass across.

Sort these — group or period?

One tap each. Answers lock in.

A vertical column of the Periodic Table
A horizontal row of the Periodic Table
There are 18 of these
There are 7 of these
Members share similar chemical properties
Contains 32 elements at its longest
Halogens all belong to the same one
Check your understanding · 4 of 7

Transition elements are found in which groups?

Correct answer: B — groups 3 to 12, the central block. Everything to the left (1, 2) and right (13–18) of it is representative.
044Concept

Every element has an address

Just as a house has a street and a house number, an element has a period and a group.

Element address = Group number + Period number

The electronic configuration gives you both, free of charge:

The two counts

Number of occupied shells → period number.
Number of outermost electrons → group number (for elements with 1 or 2 outer electrons).

Pick an element, then step through it.

Coming in Lesson 2

For elements with 3 to 8 electrons in the outermost shell, the group number is outermost electrons + 10 — which is why chlorine (2, 8, 7) sits in group 17 and not group 7. You will derive this properly next lesson; for now just recognise it.

Check your understanding · 5 of 7

An element has the electronic configuration 2, 8, 8, 2. Where does it sit?

Correct answer: B. Four occupied shells → period 4. Two outermost electrons → group 2. It is an alkaline earth metal (calcium).
05RRecall

Pause and think

Close the lesson in your head. Answer these without scrolling back, then open each strip to check.

1 · What problem did classification solve?
It made the study of many elements systematic and placed similar elements together.
2 · What was a Döbereiner triad?
A group of three elements with similar properties.
3 · Why did Newlands use the word “octaves”?
He observed that every eighth element resembled the first in its properties, like the eighth note of a musical octave.
4 · What was the basis of Mendeleev's arrangement?
Increasing atomic mass.
5 · What is the basis of the modern Periodic Table?
Increasing atomic number.
6 · How many groups and periods are there?
18 groups and 7 periods.
7 · Are groups horizontal or vertical?
Groups are vertical columns. Periods are the horizontal rows.
8 · In which groups are transition elements found?
Groups 3 to 12.
9 · What are Group 17 elements called?
Halogens.
10 · What are Group 18 elements called?
Noble gases, inert gases or zero-group elements.

Type it out — the two laws

Recall · 6 of 7

State the modern periodic law in one sentence.

Model answer: The physical and chemical properties of elements are periodic functions of their atomic numbers.
Recall · 7 of 7

State Mendeleev's periodic law.

Model answer: The properties of elements are periodic functions of their atomic masses.
06✗Traps

Mistakes that cost marks

Read the wrong statement, decide the fix in your head, then tap to confirm.

Groups are vertical columns.
Periods are horizontal rows.
Mendeleev used atomic mass.
It is based on atomic number.
Group 17 contains halogens.
Group 18 contains noble gases.
Transition elements are in groups 3–12.
07EExam

Exam notes

Definitions to learn word-for-word

Periodic Table

A tabular arrangement of elements in groups and periods, showing regular trends in their properties.

Mendeleev's Periodic Law

The properties of elements are periodic functions of their atomic masses.

Modern Periodic Law

The physical and chemical properties of elements are periodic functions of their atomic numbers.

Group

A vertical column in the Periodic Table.

Period

A horizontal row in the Periodic Table.

One full-mark comparison answer

Question: Differentiate between Mendeleev's periodic law and the modern periodic law.

Write it like this

Mendeleev's periodic law states that the properties of elements are periodic functions of their atomic masses, whereas the modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.

Memory tricks worth keeping

Group = Goes down Period = Passes across
D → N → M → M Döbereiner · Newlands · Mendeleev · Moseley "Do Not Miss Meals"
Mendeleev used Mass. Moseley used atomic nuMber.
08QPractice

Practice questions

Level 1 — straight recall

  1. How many groups are present in the modern Periodic Table?
    Answer
    18.
  2. How many periods are present?
    Answer
    7.
  3. Name the scientist who proposed the Law of Octaves.
    Answer
    Newlands.
  4. Name the scientist whose arrangement was based on atomic mass.
    Answer
    Mendeleev.
  5. What are Group 17 elements called?
    Answer
    Halogens.
  6. What are Group 18 elements called?
    Answer
    Noble gases, also called inert gases or zero-group elements.

Level 2 — understanding

  1. Why was classification of elements necessary?
    Answer
    As the number of known elements grew, studying each one separately became difficult. Classification placed similar elements together, made study systematic, and revealed regular patterns in properties.
  2. Why is Newlands' arrangement called the Law of Octaves?
    Answer
    Because when elements were arranged in increasing order of atomic mass, every eighth element had properties similar to the first — like the eighth note of a musical octave repeating the first.
  3. Explain the difference between a group and a period.
    Answer
    A group is a vertical column of the Periodic Table; there are 18, and their elements have similar chemical properties. A period is a horizontal row; there are 7, and properties change gradually across it.
  4. Why are lanthanides and actinides shown separately at the bottom?
    Answer
    They belong to group 3 of periods 6 and 7, but are placed separately below the main table to keep it compact.
  5. What are representative elements?
    Answer
    The elements of groups 1, 2, 13, 14, 15, 16, 17 and 18. They are also called normal or typical elements.
  6. Why was atomic number accepted as the basis of the modern arrangement?
    Answer
    Moseley showed that atomic number, rather than atomic mass, is the more suitable basis: the physical and chemical properties of elements are periodic functions of their atomic numbers.

Level 3 — ICSE style

  1. State the modern periodic law.
    Answer
    The physical and chemical properties of elements are periodic functions of their atomic numbers.
  2. State Mendeleev's periodic law.
    Answer
    The properties of elements are periodic functions of their atomic masses.
  3. Name the following:
    1. Vertical columns of the Periodic Table
    2. Horizontal rows of the Periodic Table
    3. Elements in groups 3–12
    4. Elements in group 1
    5. Elements in group 2
    Answer
    a. Groups  ·  b. Periods  ·  c. Transition elements  ·  d. Alkali metals  ·  e. Alkaline earth metals.
  4. An element is located in group 17. To which family does it belong?
    Answer
    The halogen family.
  5. Correct the statement: “Moseley arranged the elements in increasing order of atomic mass.”
    Answer
    Moseley showed that elements should be arranged in increasing order of atomic number. It was Mendeleev who used atomic mass.
  6. Give one similarity and one difference between Newlands' and Mendeleev's arrangements.
    Answer
    Similarity: both arranged elements in increasing order of atomic mass.
    Difference: Newlands' Law of Octaves said every eighth element repeated the first and failed beyond calcium, while Mendeleev organised 63 elements into eight groups and several periods and stated that properties are a periodic function of atomic mass.
09✓Close

One-minute revision

Need for classification ↓ Similar elements placed together ↓ Döbereiner : triads (groups of three) ↓ Newlands : every eighth element · atomic mass ↓ Mendeleev : periodic function of atomic mass · 63 elements ↓ Moseley : atomic number is the true basis ↓ MODERN PERIODIC TABLE → 18 groups + 7 periods

Mastery check

Next → Lesson 2: Groups, Periods, Electronic Configuration, Shells and Valency