# Classification of Elements - Grade 10 (SEE) Science
## Complete Chapter Notes with All Exercises - NEB Curriculum
---
## 🎯 Chapter Overview
**Welcome to the ultimate guide for "Classification of Elements"!** Think of the periodic table as the ultimate cheat sheet for all of chemistry. This chapter will teach you how to read and understand this amazing tool. These notes are designed to be super easy to follow, with fun tricks and clear explanations, making you a master of the periodic table.
**This chapter is crucial for your SEE exam as it forms the foundation of chemistry.** We'll journey through the history of the periodic table, from early attempts to the Modern Periodic Table we use today. You'll learn how to find any element's "address" (its group and period), understand its properties, and predict how it will behave. **Every single exercise from your textbook is solved here**, ensuring you're fully prepared to tackle any question.
**Weightage:** 7-9 marks in SEE examination
**Time Required:** 15-20 periods
---
## 📚 Learning Objectives
After studying this chapter, you will be able to:
- State Mendeleev's and the Modern Periodic Law.
- Explain the merits and demerits of Mendeleev's Periodic Table.
- Describe the structure of the Modern Periodic Table (groups and periods).
- Find the position (group, period, block) of an element from its electronic configuration.
- Classify elements into s, p, d, and f blocks.
- Understand periodic trends like atomic size and reactivity.
- Differentiate between various types of elements like alkali metals, halogens, and inert gases.
---
## 📜 1. THE STORY OF THE PERIODIC TABLE
For a long time, chemists had a messy collection of elements with no organization. Several scientists tried to bring order to this chaos.

### A. Mendeleev's Periodic Table (The Genius First Draft)
Dmitri Mendeleev, a Russian chemist, was the first to create a truly useful periodic table.
- **Mendeleev's Periodic Law:** *"The physical and chemical properties of elements are the periodic functions of their **atomic masses**."*
- **Merits (What he got right):**
    1.  **Systematic Study:** He arranged all 63 known elements in a systematic way.
    2.  **Predicted New Elements:** He left gaps for elements that hadn't been discovered yet and even predicted their properties! (e.g., Eka-silicon, which later became Germanium).
    3.  **Correction of Atomic Mass:** He corrected the atomic masses of some elements.
- **Demerits (The Problems):**
    1.  **Position of Hydrogen:** Hydrogen behaves like both Group 1 metals and Group 17 non-metals, so he couldn't give it a fixed position.
    2.  **Position of Isotopes:** Isotopes of an element have different atomic masses but similar chemical properties. His table had no place for them.
    3.  **Anomalous Pairs:** He had to place some heavier elements before lighter ones to match properties (e.g., Argon before Potassium).

### B. The Modern Periodic Table (The Final Version)
Henry Moseley, a brilliant young physicist, discovered that **atomic number** (the number of protons) is a more fundamental property of an element than atomic mass.
- **Modern Periodic Law:** *"The physical and chemical properties of elements are the periodic functions of their **atomic numbers**."*
This new law fixed all the problems in Mendeleev's table and gave us the table we use today.



---
## 🗺️ 2. NAVIGATING THE MODERN PERIODIC TABLE
The Modern Periodic Table is like a grid. Understanding its layout is key.
- **Groups:** The 18 vertical columns are called **groups**.
    - **Trick:** All elements in the same group have the **same number of valence electrons** (outermost electrons) and thus have **similar chemical properties**.
- **Periods:** The 7 horizontal rows are called **periods**.
    - **Trick:** All elements in the same period have the **same number of electron shells**.

### How to Find an Element's "Address" (Block, Period, Group)
This is a super important skill! Just follow these steps using the sub-shell electronic configuration.

**Example: Find the position of Sodium (Na), Atomic Number = 11**
**Step 1: Write the electronic configuration.**
Na (11) = $1s^2 \ 2s^2 \ 2p^6 \ 3s^1$

**Step 2: Find the Block, Period, and Group.**
- **Block:** The last electron enters the **'s' sub-shell**. So, it's an **s-block** element.
- **Period:** The highest shell number (the big number in front) is **3**. So, it's in the **3rd Period**.
- **Group:** For s-block elements, the group number is the **number of electrons in the last s sub-shell**. There is **1** electron in 3s. So, it's in **Group 1 (IA)**.

**Pro-Tricks for Finding the Group:**
- For **s-block**: Group No. = No. of valence electrons (1 or 2).
- For **p-block**: Group No. = 10 + No. of valence electrons (e.g., Chlorine has 7 valence electrons, so Group = 10 + 7 = 17).
- For **d-block**: Group No. = No. of electrons in the last 's' sub-shell + No. of electrons in the last 'd' sub-shell.

---
## 🏢 3. THE BLOCKS: NEIGHBORHOODS OF ELEMENTS
Elements are sorted into four main "blocks" or neighborhoods based on their electronic configuration.

1.  **s-Block Elements (The Reactors):**
    - **Groups:** 1 and 2.
    - **Who are they?:** Highly reactive metals.
    - **Group 1 (IA): Alkali Metals.** (e.g., Li, Na, K). They are soft, highly reactive metals that form strong alkalis with water.
    - **Group 2 (IIA): Alkaline Earth Metals.** (e.g., Be, Mg, Ca). They are also reactive metals, commonly found in the Earth's crust.

2.  **p-Block Elements (The Diverse Group):**
    - **Groups:** 13 to 18.
    - **Who are they?:** This block has it all: metals, non-metals, and metalloids.
    - **Group 17 (VIIA): Halogens.** (e.g., F, Cl, Br). They are very reactive non-metals. The name means "salt-formers".
    - **Group 18 (0): Noble/Inert Gases.** (e.g., He, Ne, Ar). They are unreactive because their outermost shell is completely full.

3.  **d-Block Elements (Transition Metals):**
    - **Groups:** 3 to 12.
    - **Who are they?:** The classic metals like Iron (Fe), Copper (Cu), and Gold (Au). They are hard, have high melting points, often form colored compounds, and show variable valency.

4.  **f-Block Elements (Inner Transition Metals):**
    - **Who are they?:** The **Lanthanides** and **Actinides**. They are placed separately at the bottom to keep the main table from being too wide. Most actinides are radioactive.

---
## 📈 4. PERIODIC TRENDS
As you move across or down the periodic table, properties change in a predictable pattern.

- **Atomic Size:**
    - **Down a Group (↓): Increases.** Why? New electron shells are added, making the atom bigger.
    - **Across a Period (→): Decreases.** Why? The number of protons in the nucleus increases, pulling the electron shells in more tightly.
- **Reactivity of Metals:**
    - **Down a Group (↓): Increases.** Why? The outermost electron is farther from the nucleus and easier to lose.
    - **Example:** Potassium (K) is more reactive than Sodium (Na).
- **Reactivity of Non-Metals:**
    - **Down a Group (↓): Decreases.** Why? The atom is larger, so its ability to attract and gain an electron decreases.
    - **Example:** Fluorine (F) is more reactive than Chlorine (Cl).

---
## 📋 5. COMPLETE EXERCISE SOLUTIONS
### **1. Differentiate Between:**
**a. Group and Period**
| Groups | Periods |
|---|---|
| The 18 vertical columns. | The 7 horizontal rows. |
| Elements have the same number of valence electrons. | Elements have the same number of electron shells.|
| Elements have similar chemical properties. | Chemical properties change across the period. |

**b. Mendeleev's Periodic Table and Modern Periodic Table**
| Mendeleev's Periodic Table | Modern Periodic Table |
|---|---|
| Based on increasing **atomic mass**. | Based on increasing **atomic number**. |
| Had defects like the position of isotopes.| These defects are solved. |
| Contained 8 groups and 12 periods.| Contains 18 groups and 7 periods. |

### **2. Give Reasons:**
**a. Argon (Ar) is an inert gas.**
**Answer:** Argon's atomic number is 18, and its electronic configuration is (2, 8, 8). Its outermost shell (valence shell) is completely filled with 8 electrons. This stable configuration makes it chemically unreactive or inert.

**b. The members of Group 1 are called alkali metals.**
**Answer:** Because they are highly reactive metals that react with water to form strong alkalis (water-soluble bases) and produce hydrogen gas. For example: $2Na + 2H_2O \rightarrow 2NaOH + H_2 \uparrow$.

**c. Fluorine is more reactive than Chlorine.**
**Answer:** Both are in Group 17. As we go down the group, atomic size increases. Fluorine is smaller than chlorine, so its nucleus has a stronger pull on incoming electrons. This allows it to gain an electron more easily, making it more reactive.

**d. Lanthanides and Actinides are kept separately in the periodic table.**
**Answer:** They are kept separately at the bottom to maintain the structure of the periodic table and to prevent it from becoming too wide. Their properties are also quite distinct from the d-block elements.

### **3. Answer the following questions:**
**a. State the Modern Periodic Law.**
**Answer:** The Modern Periodic Law states that "the physical and the chemical properties of elements are the periodic functions of their atomic numbers."

**b. Write any two merits of Mendeleev's periodic table.**
**Answer:**
1.  It helped in the systematic study of elements.
2.  Mendeleev left gaps for undiscovered elements and accurately predicted their properties.

**c. An element 'X' has atomic number 17.**
**(i) Write its electronic configuration.**
**(ii) Find its group, period, and block.**
**(iii) Is it a metal or non-metal?**
**Answer:**
(i) **Electronic Configuration:** $1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^5$
(ii) **Block:** p-block (last electron is in 'p'). **Period:** 3 (highest shell is 3). **Group:** 10 + 7 (valence electrons) = **17**.
(iii) It is a **non-metal** (it only needs one electron to complete its octet, so it gains electrons).

---
## ✅ 6. CHAPTER CHECKLIST & EXAM TIPS
**Exam Hotspots 🔥:**
-   **Laws:** Memorize both Mendeleev's and the Modern Periodic Law.
-   **Finding Position:** Practice finding the group, period, and block for elements with atomic numbers 1-20. This is a very common question.
-   **Periodic Trends:** "Give Reason" questions are almost always based on trends in atomic size and reactivity. Understand the "why" behind them.
-   **Definitions:** Be clear on the definitions of alkali metals, halogens, inert gases, and transition metals.

**Your Pre-Exam Checklist:**
- [ ] I can state both periodic laws.
- [ ] I can list two merits and two demerits of Mendeleev's table.
- [ ] I can find the block, group, and period of an element (like Mg, Z=12).
- [ ] I can explain why atomic size decreases across a period.
- [ ] I can explain why potassium is more reactive than sodium.
---
## 🎊 CONCLUSION
You've now unlocked the secrets of the periodic table! It's not just a chart; it's a powerful tool that brings order to the world of chemistry. By understanding its structure and patterns, you can predict the properties and behavior of elements. Keep practicing how to find an element's position and explaining the trends, and you'll be more than ready for your SEE exam.
**Key to Success:** Learn the Laws + Practice Finding Positions + Understand the Trends = A+ in Chemistry!