Periodic Table and Chemical Bonding
Periodic Table and Chemical Bonding
The periodic table arranges elements so that similar ones fall in the same column. NTPC asks how Mendeleev and Moseley arranged elements, the names of groups, trends across a period and down a group, and whether a compound is ionic or covalent.
1. History and Layout
| Item | Fact |
|---|---|
| Döbereiner | Triads (such as Li, Na, K) |
| Newlands | Law of octaves |
| Mendeleev (1869) | Arranged elements by increasing atomic mass; left gaps for undiscovered elements |
| Moseley (1913) | Modern periodic law: properties depend on atomic number |
| Modern table | 18 groups (columns) and 7 periods (rows); 118 elements |
| Group 1 | Alkali metals (Li, Na, K) |
| Group 2 | Alkaline earth metals (Be, Mg, Ca) |
| Group 17 | Halogens (F, Cl, Br, I) |
| Group 18 | Noble gases (He, Ne, Ar), with full outer shells |
| Groups 3 to 12 | Transition metals |
| Liquids at room temperature | Mercury (metal) and bromine (non-metal) |
2. Periodic Trends
| Property | Across a period (left to right) | Down a group |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Electronegativity | Increases | Decreases |
| Ionisation energy | Increases | Decreases |
Most electronegative element: fluorine; noble gases are the least reactive because their outer shells are full
3. Chemical Bonding
| Bond | How it forms | Properties and examples |
|---|---|---|
| Ionic (electrovalent) | Transfer of electrons from metal to non-metal | High melting point, conduct when molten or dissolved; NaCl, MgO |
| Covalent | Sharing of electrons between non-metals | Low melting point, poor conductors; H₂O, CH₄, CO₂ |
| Single, double, triple covalent | 1, 2 or 3 shared pairs | Cl₂ single, O₂ double, N₂ triple |
| Valency | Electrons lost, gained or shared to complete the octet | Na 1, Mg 2, Al 3, C 4, N 3, O 2 |
✗ Mendeleev arranged elements by atomic number | ✓ Mendeleev used atomic mass; Moseley's modern law uses atomic number
✗ Metallic character increases across a period | ✓ Metallic character decreases across a period and increases down a group
हिंदी नोट: मेंडलीफ़ ने तत्वों को परमाणु द्रव्यमान के बढ़ते क्रम में और मोसले ने परमाणु संख्या के आधार पर व्यवस्थित किया। आधुनिक आवर्त सारणी में 18 समूह और 7 आवर्त हैं। आवर्त में बाएँ से दाएँ जाने पर धात्विक गुण घटता है और समूह में नीचे जाने पर बढ़ता है।
Exam Pointer: Verified NTPC patterns: the basis of Mendeleev's arrangement (June 2025 Graduate CBT-1) and a statement on the trend in metallic character (June 2025 Graduate CBT-1). Bonding questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Mendeleev, Moseley and the layout
[PYQ: NTPC Graduate CBT-1 5-Jun-2025 Shift-2]
EXAM LEVEL
Q. On what basis does the modern periodic law arrange elements, and who gave it?
On increasing atomic number, given by Henry Moseley.
Answer: Atomic number; Moseley
EXAMATLAS LEVEL
Q. How many groups and periods does the modern periodic table have? Name the group of halogens and the group of noble gases.
It has 18 groups and 7 periods. Halogens are in group 17 and noble gases in group 18.
Answer: 18 and 7; 17 and 18
Pattern 2: Periodic trends
[PYQ: NTPC Graduate CBT-1 18-Jun-2025 Shift-3]
EXAM LEVEL
Q. How does atomic size change as we go down a group?
It increases, because a new shell is added at each step.
Answer: Increases
EXAMATLAS LEVEL
Q. Which statement is correct: (a) metallic character increases across a period, (b) electronegativity increases down a group, (c) atomic size decreases across a period, (d) ionisation energy decreases across a period?
Across a period the nuclear charge rises and pulls electrons closer, so atomic size decreases. Metallic character falls across a period, electronegativity falls down a group and ionisation energy rises across a period.
Answer: (c)
Pattern 3: Ionic and covalent compounds
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. What type of bond holds sodium chloride together?
An ionic bond, formed when sodium gives an electron to chlorine.
Answer: Ionic bond
EXAMATLAS LEVEL
Q. Classify as ionic or covalent: MgO, CH₄, KCl, CO₂. How many shared electron pairs are in a nitrogen molecule?
MgO and KCl are metal with non-metal, so ionic. CH₄ and CO₂ are between non-metals, so covalent. N₂ has a triple bond, so three shared pairs.
Answer: Ionic: MgO, KCl; covalent: CH₄, CO₂; three
Pattern 4: Groups, valency and special elements
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Which two elements are liquid at room temperature?
Mercury, a metal, and bromine, a non-metal.
Answer: Mercury and bromine
EXAMATLAS LEVEL
Q. An element has configuration 2, 8, 2. Find its group, period and valency, and name it.
Two outer electrons put it in group 2; three shells put it in period 3; it loses 2 electrons, so valency 2. It is magnesium.
Answer: Group 2, period 3, valency 2; magnesium
60-Second Revision
- Mendeleev: atomic mass; Moseley: atomic number; 18 groups, 7 periods.
- Group 1 alkali, 2 alkaline earth, 17 halogens, 18 noble gases.
- Across a period: size and metallic character fall; electronegativity rises.
- Ionic: transfer (NaCl); covalent: sharing (H₂O, CH₄); N₂ triple bond.