SCI8 Q2W5D4: The Revised/Modern Periodic Law

By the end of this lesson, you will be able to:

1. Describe Moseley’s discovery of atomic number and how it revised Mendeleev’s law.
2. Differentiate between the old mass-based table and the new number-based table.
3. Explain how the modern Periodic Law forms the foundation of today’s Periodic Table.

• Modern Periodic Law – properties of elements are periodic functions of their atomic numbers.
• Atomic Number – the number of protons in an atom’s nucleus, the true basis of order.
• Henry Moseley – the English physicist who discovered atomic number in 1913.
• Periodic Table (Modern) – arrangement of elements by increasing atomic number.
• Noble Gases – a group of unreactive elements discovered after Mendeleev, fitted perfectly into the modern table.

Sorting by the Right Key

Think back: triads grouped elements in threes; octaves repeated every eighth element; Mendeleev arranged by atomic mass but left gaps and made predictions.

Activity: Imagine sorting a class by height vs. age. Height might misplace students; age restores the natural order.

Question: What “better property” solved Mendeleev’s mismatches?

Show Answer

Atomic number (proton count), discovered by Moseley in 1913, which fixed conflicts like Tellurium and Iodine without breaking periodicity.

1) The unfinished puzzle of Mendeleev’s table

Mendeleev’s 1869 table ordered elements by atomic mass and grouped similar ones together. It allowed gaps for undiscovered elements, but conflicts remained—for example, the Tellurium–Iodine placement by mass contradicted their chemical families.

Checkpoint: Why “break” mass order to keep Iodine with halogens?

Show Answer

Properties define families better than mass alone. Preserving halogen behavior maintained periodicity despite numerical exceptions.

---

2) Enter Henry Moseley and atomic number

In 1913, Moseley used X-ray spectroscopy to show each element has a unique frequency related to its proton count. This identified atomic number as the fundamental identifier.

Moseley’s relation:

$\nu =K\times (Z-b)2^{\mathrm{}}$

Here, $\nu$ is X‑ray frequency, $Z$ the atomic number, and $K$, $b$ constants.

Checkpoint: What did this prove about element identity?

Show Answer

Each element is defined by its number of protons (atomic number), not by its mass.

---

3) The modern periodic law

Recasting the principle after Moseley:

$\mathrm{Properties}=f(\mathrm{atomic} \mathrm{number})$

Arranging by atomic number aligned numbers and properties, resolving mass-based anomalies like Te (52) and I (53).

Checkpoint: Why does atomic number work better?

Show Answer

It is fundamental and discrete (proton count), while mass varies with isotopes and measurement.

---

4) Noble gases find their place

Late-19th-century discoveries of He, Ne, Ar, Kr, Xe, Rn revealed a family of inert elements. The number-based table placed them naturally in Group 18.

Checkpoint: Why are they inert?

Show Answer

Their outer electron shells are full, minimizing the tendency to gain or lose electrons.

---

5) Electron configuration explains periodicity

Ordering by atomic number reflects how electrons occupy shells and subshells. Similar outer configurations produce similar chemistry (for example, alkali metals with one valence electron).

Shell capacity rule:

$\mathrm{Capacity}=2n^2$

Checkpoint: For $n=3$, how many electrons?

Show Answer

$2\times 3^2=18$

---

6) Real-life importance

Number-based ordering guides medicine (radioisotopes), technology (silicon electronics), energy (uranium fuel), and lighting (argon in bulbs). Predictable placement improves safety and design.

Mini-Reflection: Name one home product relying on an element’s placement by atomic number.

Show Answer

Examples: neon signage (Ne), lithium batteries (Li), argon-filled bulbs (Ar), silicon chips (Si).

---

7) Advantages of the modern law

• Resolves mass-order conflicts (e.g., Te/I).
• Accommodates noble gases as Group 18.
• Connects periodicity to electron structure.
• Supports discovery and placement of superheavy elements.

Checkpoint: Which advantage most persuaded scientists?

Show Answer

Correcting mismatches like Iodine/Tellurium because both numbers and properties finally agreed.

---

8) A growing table

The table now extends to Z=118. New elements are synthesized and placed by number, with expected trends inferred from group and period neighbors.

Checkpoint: Why call it a “map” of chemistry?

Show Answer

It shows locations of known elements and predicts where new ones will belong.

---

9) Transition recap

• Original: properties repeat with mass.
• Revised: properties repeat with atomic number.
• Explanation: electron configurations drive trends.

---

10) Mini-summaries

• Mass-based ordering worked broadly but had conflicts.
• Number-based ordering fixed conflicts and added depth.
• Modern trends support real-world applications.

Worked Example 1

Why Te comes before I in the modern table and how the issue was resolved.

Show Answer

Mendeleev used properties over mass; Moseley proved ordering by Z: Te=52, I=53; both number and chemistry now align.

Worked Example 2

State the modern form of the law.

Show Answer

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

Worked Example 3

Using $2n^2$ find capacity of the second shell.

Show Answer

For n=2: $2\times 2^2=8$ electrons.

Worked Example 4

Why Na (Z=11) and K (Z=19) share a group.

Show Answer

Both have one valence electron; similar reactivity and compound formation.

Worked Example 5

Placement of noble gases in the modern table.

Show Answer

Group 18 with full outer shells explains their inertness and consistent placement by atomic number.

Now You Try (5)

1. State the ordering key of the modern table.
2. Name the corrected mismatch solved by Z.
3. Compute shell capacity for n=4 using $2n^2$.
4. Explain why Group 18 is inert.
5. Describe how number-based order predicts superheavy element traits.

Show Answer

• Atomic number.
• Tellurium–Iodine.
• $2\times 4^2=32$.
• Full outer shells minimize reactivity.
• Placement by Z implies comparable outer configurations and trends, guiding predictions.

1. Who discovered atomic number as the true basis of the periodic table?
   Show Answer

   Henry Moseley.
2. State the modern form of the Periodic Law.
   Show Answer

   The properties of elements are periodic functions of their atomic numbers.
3. What are the atomic numbers of Tellurium and Iodine?
   Show Answer

   Te: 52; I: 53.
4. How many electrons can the third shell hold? Use $2n^2$.
   Show Answer

   $2\times 3^2=18$ electrons.
5. Why did noble gases fit perfectly into the modern table?
   Show Answer

   Atomic numbers and full outer shells place them consistently in Group 18.
6. Which is more fundamental: atomic mass or atomic number? Why?
   Show Answer

   Atomic number, because it uniquely identifies an element by proton count.
7. What principle explains why Sodium and Potassium share a group?
   Show Answer

   Similar outer electron configurations (one valence electron).
8. How many electrons can the first shell hold?
   Show Answer

   $2\times 1^2=2$ electrons.
9. What problem did Moseley’s discovery solve?
   Show Answer

   It corrected mismatches caused by mass-based ordering, aligning with chemical families.
10. What does the modern table predict about elements beyond Uranium?
    Show Answer

    Logical placement by atomic number and trends allows inferred properties even for synthetic elements.

1. MCQ: Who discovered atomic number as the basis of the table? A) Mendeleev B) Newlands C) Moseley D) Döbereiner
   Show Answer

   C) Henry Moseley.
2. True/False: The modern law is based on atomic mass.
   Show Answer

   False. It is based on atomic number.
3. MCQ: Which mismatch was corrected by using atomic number? A) Na/K B) Te/I C) F/Cl D) C/O
   Show Answer

   B) Tellurium and Iodine.
4. Short Answer: Atomic number of Oxygen?
   Show Answer

   8.
5. True/False: Noble gases were included in Mendeleev’s original table.
   Show Answer

   False. They were discovered later.
6. MCQ: Electrons in the 2nd shell: A) 2 B) 4 C) 8 D) 18
   Show Answer

   C) 8.
7. Short Answer: Why are noble gases unreactive?
   Show Answer

   Full valence shells.
8. True/False: Atomic number equals neutrons.
   Show Answer

   False. It equals protons.
9. MCQ: Element with atomic number 14? A) C B) Si C) N D) Mg
   Show Answer

   B) Silicon.
10. Short Answer: Electron configuration of Sodium (Z=11)?
    Show Answer

    2,8,1.
11. True/False: The modern law allows prediction of synthetic elements.
    Show Answer

    True.
12. MCQ: Who proposed octaves? A) Newlands B) Moseley C) Mendeleev D) Lavoisier
    Show Answer

    A) Newlands.
13. Short Answer: Formula for shell capacity?
    Show Answer

    $2n^2$.
14. True/False: Each element is defined by its proton number.
    Show Answer

    True.
15. MCQ: What limitation of Mendeleev’s table did the modern table correct? A) Missing noble gases B) Mass mismatches C) Lack of explanation D) All of the above
    Show Answer

    D) All of the above.

1. Moseley’s X‑ray Experiment Simulation
   Show Answer

   X‑ray frequency increases with proton number; each element has a unique frequency, proving atomic number defines identity.
2. Periodic Table as a Map
   Show Answer

   Group 18 houses noble gases; Mendeleev had no dedicated column, but number-based order accommodates them naturally.
3. Predict the Future
   Show Answer

   Element 119 would start a new period in Group 1 with extreme reactivity, consistent with group trends.
4. Historical Reflection
   Show Answer

   Pride in validation of periodicity, acceptance that ordering required refinement by number.
5. Electron Configuration Challenge
   Show Answer

   Na: 2,8,1 (reactive); Cl: 2,8,7 (reactive); Ar: 2,8,8 (inert). Configurations explain properties.

Notebook Task (3–2–1):

• 3 things I learned about the Modern Periodic Law.
• 2 questions I still have about atomic number or electron configuration.
• 1 connection between the modern periodic table and a technology or product I use daily.