HUMAN DIGESTIVE SYSTEM – NEET EXAM KEY NOTES BIOLOGY FOR CBSE, ICSE, STATE STUDENTS

 

1. Human Digestive System Overview

PARTS OF HUMAN DIGESTIVE SYSTEM

• Digestive tract (Alimentary canal): Mouth → Pharynx → Oesophagus → Stomach → Small Intestine → Large Intestine → Rectum → Anus

• Associated glands: Salivary glands, Liver, Pancreas

2. Parts and Functions

a. Mouth & Buccal Cavity

• Digestion begins in the mouth.

• Saliva → secreted by 3 pairs of salivary glands (parotid, submandibular, sublingual).

• Saliva composition: water, salts, mucus, lysozyme, salivary amylase (ptyalin).

• Enzyme action:

  • Starch → Maltose by salivary amylase (pH ≈ 6.8).

• Teeth: 32 permanent (2-1-2-3 per quadrant, i.e., 2123 dental formula).

• Tongue: taste and manipulation of food.

b. Pharynx & Oesophagus

• Pharynx: connects mouth to oesophagus; common passage for food and air.

• Epiglottis: prevents food entry into windpipe.

• Oesophagus: muscular tube → peristaltic movement pushes food into stomach.

• No enzyme secretion here.

c. Stomach

• Regions: cardiac, fundic, pyloric.

• Gastric glands secrete:

  • HCl → converts pepsinogen → pepsin, kills microbes, maintains acidic pH (~1.8).

  • Pepsinogen → active pepsin (proteins → peptones & proteoses).

  • Mucus → protects stomach lining.

  • Intrinsic factor → helps in Vitamin B₁₂ absorption.

• Chyme: semi-digested food leaving stomach.

d. Small Intestine

• Longest part (~6.5 m).

• Divided into duodenum, jejunum, ileum.

• Major digestion and absorption occur here.

Digestive juices:

1. Bile (from liver)

   • No enzymes.

   • Contains bile salts, bile pigments, cholesterol.

   • Emulsifies fats (breaks into small droplets).

2. Pancreatic juice (from pancreas)

   • Trypsinogen → Trypsin (by enterokinase): proteins → peptides

   • Chymotrypsinogen → Chymotrypsin: proteins → peptides

   • Carboxypeptidase: peptides → amino acids

   • Amylase: starch → maltose

   • Lipase: fats → fatty acids + glycerol

3. Intestinal juice (Succus entericus)

   • Maltase: maltose → glucose

   • Lactase: lactose → glucose + galactose

   • Sucrase: sucrose → glucose + fructose

   • Peptidase: peptides → amino acids

   • Lipase: fats → fatty acids + glycerol

   • Nucleosidase: nucleotides → sugar + base

e. Absorption of Digested Products

Nutrient	Site of Absorption	Method

Glucose, amino acids

Small intestine (villi)

Active transport

Fatty acids, glycerol

Small intestine (lacteals)

Diffusion

Water, minerals

Large intestine

Diffusion

Alcohol, some drugs

Stomach

Simple diffusion

f. Large Intestine

• Parts: caecum, colon, rectum.

• Functions:

  Absorption of water, minerals, vitamins.
• Formation of feces.
• Houses symbiotic bacteria (e.g., E. coli produces vitamin K and B-complex).

g. Disorders

Disease	Cause	Symptoms
Jaundice	Liver disorder (bile pigments in blood)	Yellowing of skin & eyes
Vomiting	Reflex action (medulla oblongata)	Expulsion of gastric contents
Diarrhea	Intestinal infection	Watery stools, dehydration
Constipation	Reduced bowel movement	Hard stools
Indigestion	Improper enzyme secretion	Abdominal discomfort

3. Enzymes Summary Table (Most Important)

Enzyme	Source	Substrate	Product
Salivary amylase	Saliva	Starch	Maltose
Pepsin	Stomach	Proteins	Peptones
Lipase (gastric)	Stomach	Fats	Fatty acids + Glycerol
Trypsin	Pancreas	Proteins/Peptides	Peptides/Amino acids
Amylase (pancreatic)	Pancreas	Starch	Maltose
Lipase (pancreatic)	Pancreas	Fats	Fatty acids + Glycerol
Maltase	Intestine	Maltose	Glucose
Sucrase	Intestine	Sucrose	Glucose + Fructose
Lactase	Intestine	Lactose	Glucose + Galactose
Peptidase	Intestine	Peptides	Amino acids

What is Naegleria fowleri-caused Amoebic meningitis (also called Primary Amoebic Meningoencephalitis, PAM)?

Amoebic meningitis

• This is a very rare but extremely serious brain infection caused by the free-living amoeba Naegleria fowleri. 

• The amoeba lives in warm fresh water (like lakes, ponds, poorly-treated swimming pools) and typically infects a person when contaminated water enters the nose, then the amoeba travels along the olfactory nerve into the brain. 

• Once inside the brain, it causes severe inflammation (meningoencephalitis) and very rapid brain tissue destruction. The disease progresses extremely fast. 

• While often called “amoebic meningitis”, technically it is meningoencephalitis (inflammation of both the brain substance + membranes) rather than classic bacterial meningitis.

causative organism

brain inflammation caused by the organism

Causes & risk factors

1. The major causative agent: Naegleria fowleri. 
2. Mode of infection: Water containing the amoeba enters the nose (for example when swimming, diving into fresh warm water, or during nasal rinsing with non-sterile water). It is not typically transmitted by drinking the water. 
3. Conditions that favour the amoeba: Warm, stagnant fresh water; poorly chlorinated pools; warm climates raising water temperature. 
4. In the recent Indian context: The state of Kerala has reported a surge in cases — for instance ~80 cases and 21 deaths in 2025. 

Symptoms & clinical progression

• Early symptoms (within a few days after exposure): Headache, fever, nausea, vomiting. 

• Progressive/advanced symptoms: Stiff neck, confusion, seizures, hallucinations, coma. Rapid deterioration is common. 

• Because the disease progresses so quickly and is so rare, diagnosis often occurs late, making management very difficult. 

• Mortality is extremely high historically (often cited > 90%), although early and aggressive treatment may improve chances. 

Prevention & control

Because treatment options are limited and outcomes often poor, prevention is key. Some important measures:

• Avoid swimming or diving in warm stagnant fresh water bodies (ponds, lakes, rivers) especially when water is warm and flows slowly. 

• If entering such water, avoid letting water go up the nose — e.g., keep head above water, use nose clips if possible. 

• Use only boiled, cooled, or sterile/distilled water for nasal rinsing (e.g., neti pot), ablutions, or similar practices; do not use untreated tap or stored water. 

• Ensure swimming pools, hot-tubs, and other recreational water bodies are well-maintained and properly chlorinated/disinfected. 

• Public-health level: Monitoring water quality, surveying warm fresh-water bodies for the amoeba, ensuring well-water, storage tanks, and supply systems are safe. 

• Awareness: Because the disease is rare, many aren’t aware of it — public education about risk environments, symptoms, and prompt medical care is critical. 

Treatment & recent developments

• There is no standard, guaranteed treatment; historically, data are limited. 

• Some centres have used a combination of drugs including amphotericin B and more recently off-label use of Miltefosine in suspected cases, with some success noted in Kerala. 

• Early detection and aggressive management are critical for any chance of survival.

Recent incident in Kerala, India

• The state of Kerala has had a surge of cases: For 2025, ~80 cases and 21 deaths were reported in the state. 

• Experts link this rise to warmer temperatures (which favour the amoeba), more intensive testing (so more detection), and perhaps higher exposure in warm water bodies. 

• The public health response includes: setting up more diagnostic labs, issuing technical guidelines for early detection / treatment / prevention, and emphasising chlorination / water safety.

CSIR NET LIFESCIENCE FREQUENTLY ASKED QUESTIONS WITH ANSWERS (2 MARKS)

 

1. What is the function of DNA polymerase I in E. coli replication?

It removes RNA primers using 5′→3′ exonuclease activity and fills gaps with DNA.

2. Which step of translation requires GTP hydrolysis by EF-Tu in prokaryotes?

Binding of aminoacyl-tRNA to the A site of the ribosome

3. In the lac operon, what happens when glucose and lactose are both present?

Catabolite repression prevents lac operon transcription (low cAMP → CAP inactive).

4. What does a Lineweaver–Burk plot show for competitive inhibition?

Lines intersect on the Y-axis; $V_{max}$ unchanged, $K_m$ increases.

5. Which checkpoint ensures all chromosomes are properly attached to spindle fibers?

The spindle assembly checkpoint (metaphase–anaphase transition).

6. What is the second messenger produced by phospholipase C activity?

IP₃ and DAG (from PIP₂ hydrolysis).

7. Where does the Calvin cycle occur in C₃ plants?

In the stroma of chloroplasts.

8. What is the function of primers in PCR?

They provide free 3′-OH groups for DNA polymerase to initiate synthesis.

9. What type of chromatin is transcriptionally active?

Euchromatin.

10. What is the main function of peroxisomes?

Breakdown of hydrogen peroxide and long-chain fatty acids (β-oxidation).

11. What does a recombination frequency of 50% indicate?

Independent assortment; genes are unlinked or on different chromosomes.

12. Which antibody is present in highest concentration in blood plasma?

IgG.

13. What is the function of restriction endonucleases?

They cut DNA at specific palindromic sequences.

14. What is the role of the organizer region in amphibian embryos?

It induces dorsal axis formation (Spemann’s organizer).

15. What type of selection maintains stable polymorphism?

Balancing (heterozygote advantage) selection.

16. What is the Shannon diversity index used to measure?

Species diversity considering richness and evenness.

17. What triggers stomatal opening?

Blue light activates H⁺-ATPase, leading to K⁺ uptake and turgor increase in guard cells.

18. What hormone regulates water reabsorption in kidneys?

Antidiuretic hormone (ADH / vasopressin).

19. What does a p-value < 0.05 indicate?

The result is statistically significant at the 5% level.

20. What is the purpose of SDS in SDS-PAGE?

It denatures proteins and gives them uniform negative charge proportional to size.

NEET-Oriented Biology Q&A (Class XI) CBSE, STATE, ICSE

  

UNIT 1: The Living World

Q1. What is taxonomy?
A. Science of naming, describing, and classifying organisms.

Q2. Who introduced the binomial nomenclature system?
A. Carolus Linnaeus.

Q3. Define species.
A. A group of similar organisms capable of interbreeding to produce fertile offspring.

Q4. Arrange taxonomic hierarchy in ascending order.
A. Species → Genus → Family → Order → Class → Phylum → Kingdom.

Q5. What is a type specimen?
A. The specimen selected to represent a species in classification.

UNIT 2: Biological Classification

Q1. Who proposed the five-kingdom classification?
A. R.H. Whittaker (1969).

Q2. Name the five kingdoms.
A. Monera, Protista, Fungi, Plantae, Animalia.

Q3. Give examples of monerans.
A. Bacteria, cyanobacteria (blue-green algae), mycoplasma.

Q4. What are archaebacteria?
A. Primitive bacteria that live in extreme conditions — e.g., Methanogens, Halophiles, Thermoacidophiles.

Q5. Which kingdom has organisms with both autotrophic and heterotrophic modes of nutrition?
A. Protista.

UNIT 3: Plant Kingdom

Q1. What is the dominant phase in bryophytes?
A. Gametophyte.

Q2. What is the plant body of algae called?
A. Thallus.

Q3. Name one example each of red, brown, and green algae.
A. Polysiphonia, Fucus, Ulva.

Q4. Which plant group is called “amphibians of the plant kingdom”?
A. Bryophytes.

Q5. What are heterosporous pteridophytes?
A. Those producing two types of spores — microspores and megaspores (e.g., Selaginella).

UNIT 4: Animal Kingdom

Q1. What is the level of organization in Porifera?
A. Cellular level.

Q2. Name diploblastic animals.
A. Cnidaria (Hydra, Jellyfish).

Q3. What is metamerism?
A. Segmentation of body into repetitive units (e.g., Earthworm).

Q4. Which phylum includes both open and closed circulatory systems?
A. Annelida.

Q5. What is the distinguishing feature of Chordates?
A. Presence of notochord, dorsal nerve cord, and pharyngeal gill slits (at some stage).

UNIT 5: Morphology of Flowering Plants

Q1. What is a taproot?
A. Primary root that grows directly from the radicle.

Q2. What are adventitious roots?
A. Roots that arise from parts other than the radicle (e.g., in grasses, banyan).

Q3. Name the modification of stem for food storage.
A. Tuber (Potato).

Q4. Define inflorescence.
A. Arrangement of flowers on the floral axis.

Q5. What is the function of the pericarp?
A. Protects the seed; may be fused with the seed coat (as in maize).

UNIT 6: Anatomy of Flowering Plants

Q1. What is the outermost layer of the plant body?
A. Epidermis.

Q2. What is xylem composed of?
A. Tracheids, vessels, xylem fibres, xylem parenchyma.

Q3. Which meristem causes increase in length?
A. Apical meristem.

Q4. What is periderm?
A. Protective tissue formed during secondary growth (phellogen, phellem, phelloderm).

Q5. What are companion cells associated with?
A. Sieve tube elements in phloem.

UNIT 7: Structural Organisation in Animals

Q1. Name the epithelial tissue specialized for absorption.
A. Columnar epithelium.

Q2. What is the contractile unit of muscle?
A. Sarcomere.

Q3. What is the function of areolar connective tissue?
A. Binds skin to underlying organs.

Q4. Name the protein present in bone matrix.
A. Ossein.

Q5. Which type of neuron has one axon and one dendrite?
A. Bipolar neuron.

UNIT 8: Cell – The Unit of Life

Q1. Who discovered the cell?
A. Robert Hooke (1665).

Q2. What are prokaryotic cells?
A. Cells without a true nucleus and membrane-bound organelles.

Q3. What is the function of ribosomes?
A. Protein synthesis.

Q4. What is the function of lysosomes?
A. Intracellular digestion (contain hydrolytic enzymes).

Q5. What are plasmodesmata?
A. Cytoplasmic connections between adjacent plant cells.

UNIT 9: Biomolecules

Q1. What are the building blocks of proteins?
A. Amino acids.

Q2. Name the polysaccharide stored in animals.
A. Glycogen.

Q3. What are enzymes chemically?
A. Proteins.

Q4. Which bond joins two monosaccharides?
A. Glycosidic bond.

Q5. What is the most abundant biomolecule on Earth?
A. Cellulose.

UNIT 10: Cell Cycle and Cell Division

Q1. What are the two main phases of the cell cycle?
A. Interphase and M phase.

Q2. In which phase does DNA replication occur?
A. S phase (Synthesis phase).

Q3. What is the significance of mitosis?
A. Produces identical daughter cells for growth and repair.

Q4. What happens during crossing over?
A. Exchange of genetic material between homologous chromosomes in Prophase I.

Q5. How many daughter cells are formed after meiosis?
A. Four haploid cells.

UNIT 1: SOLID STATE (XII CHEMISTRY KEY NOTES) PART VI CBSE

 

1. Defects in Solids

• Defects (or imperfections) are irregularities in the arrangement of constituent particles (atoms, ions, or molecules) in a crystal.
• Types of Defects

• Point Defects – Irregularities at single lattice points.

• Line Defects – Irregularities along a line (dislocations).

Point Defects

These occur due to the absence or displacement of atoms/ions at lattice points.
They are of two main types:

• 1. Stoichiometric Defects
• Do not change the stoichiometry (chemical composition) of the compound.
• Common in ionic crystals.
• (a) Schottky Defect

• Definition: A pair of vacancies (one cation and one anion missing) from the lattice.

• Condition: Maintains electrical neutrality.

• Example: NaCl, KCl, CsCl, AgBr

• Characteristics:

  • Decreases density of the crystal.

  • Common in ionic compounds with high coordination numbers and similar cation & anion sizes.

  • Number of defects ∝ exp(–E/2kT) (increases with temperature).

Normal lattice:   + - + - + -

With defect:      + -   - + -    (missing + and - ions)

(b) Frenkel Defect

• Definition: An ion (usually a smaller cation) leaves its regular site and occupies an interstitial site.

• Condition: No ions are missing from the crystal → stoichiometry remains unchanged.

• Example: ZnS, AgCl, AgBr, AgI

• Characteristics:

  • Density remains unchanged (no ions lost).

  • Common in ionic crystals with large size difference between cation and anion.

  • Produces both a vacancy and an interstitial defect.

Normal lattice:   + - + - + -

With defect:      +   + - + -   (one + ion moved to interstitial site)

2. Non-Stoichiometric Defects

     Change the ideal ratio of cations to anions.
→ Often due to metal excess or metal deficiency.

(Examples: F-centres, variable valency ions, etc., but not required here if only Schottky & Frenkel are asked.)

Property	Schottky Defect	Frenkel Defect
Nature	Vacancy defect	Displacement defect
Density	Decreases	Unchanged
Electrical Neutrality	Maintained	Maintained
Ionic Size	Similar sizes	Large size difference
Common in	NaCl, KCl, CsCl, AgBr	ZnS, AgCl, AgBr, AgI

Key Formula

Number of Schottky defects (Ns) at temperature T:

$$N_s = N \, e^{-E_s / (2kT)}$$

where

• $\mathrm{<span\; class="katex"><span\; style="font-size:\; medium;"><span\; aria-hidden="true"\; class="katex-html"><span\; class="base"><span\; class="mord\; mathnormal">N</span></span></span>=\; total\; number\; of\; lattice\; sites</span></span>}$

• ${}_{}$Es​ = energy required to form a defect

• $\mathrm{<span\; style="font-size:\; medium;">k\; =\; Boltzmann\; constant</span>}$

• $\mathrm{<span\; style="font-size:\; medium;">T\; =\; absolute\; temperature</span>}$

SCIENCE AND TECHNOLOGY (LAST 3 MONTHS CURRENT AFFAIRS FOR COMPETITIVE EXAMS)

 

1.Which joint satellite mission by NASA and ISRO sent its first radar images of Earth’s surface in August/September 2025?

Answer: NISAR

2. According to one weekly current-affairs list, which of the following was reported as a “Science & Tech” item in September 2025?

A) India launched a National Geothermal Energy Policy.
B) India’s volcano Barren Island erupted twice.
C) ISRO transferred SSLV technology to HAL.
Options: (a) A only (b) B only (c) C only (d) All of A, B & C
Answer: (d) All of A, B & C. 

3. What significant scientific item was introduced by ICMR in August 2025 for rural/emergency care brain-injury diagnosis?

A) Device named “CEREBO”.
B) Device named “CEREBRUS”.
C) Device named “CEREBRAIN”.
D) Device named “CEREB-X”.

Answer: A) CEREBO

4. Which organisation’s “Science & Technology Current Affairs” archive lists separate months such as August 2025, September 2025 etc?

A) GKToday
B) Adda247
C) Testbook
D) All of the above

Answer: A) GKToday.

5. A “Weekly Current Affairs” report for the week 31 Aug-6 Sept 2025 mentioned which astronomical discovery?

A) First direct image of baby planet WISPIT 2 b.
B) Discovery of new quasar at edge of universe.
C) Detection of new gravitational wave source.
D) Launch of new telescope into orbit.

Answer: A) First direct image of baby planet WISPIT 2 b. 

6. In October 2025, which Indian shipyard launched the 6th ASWC SWC ‘INS Magdala’?

A) Mazagon Dock Shipbuilders Limited
B) Cochin Shipyard Limited
C) Garden Reach Shipbuilders & Engineers
D) Hindustan Shipyard

Answer: B) Cochin Shipyard Limited.

7. Which major global index hit a record level of around 424 ppm in 2024, as reported in October 2025?

A) Atmospheric CO₂ levels
B) Methane atmospheric concentration
C) Ozone layer UV index
D) Sea-level rise (mm)

Answer: A) Atmospheric CO₂ levels. 

8. The weekly current affairs for the week ­12–18 October 2025 included which item relating to weather/disaster-forecasting technology?

A) CSIR-AMPRI launching its indigenous SODAR system at IMD Delhi
B) India launching a new cyclone warning satellite
C) National tsunami warning system overhaul
D) India’s first autonomous flood-forecasting drone

Answer: A) CSIR-AMPRI launching its indigenous SODAR system at IMD Delhi. 

9. According to current affairs, India and the EU launched an “ideathon” to fight what?

A) Marine plastic pollution
B) Cyber-security threats
C) Space debris
D) AI misuse in healthcare

Answer: A) Marine plastic pollution. 

10. Which sector’s workers were targeted (in India) by a roadmap released by NITI Aayog in October 2025 to drive inclusive development via AI?

A) Informal workers (~490 million)
B) Formal manufacturing workers (~100 million)
C) Agricultural labourers (~50 million)
D) IT services professionals (~10 million)

Answer: A) Informal workers (~490 million). 

11. What did the weekly report say about India’s only active volcano in September 2025?
A) Barren Island erupted twice.
B) It erupted once and issued major ash cloud warnings.
C) It showed no activity for the whole month.
D) It entered dormancy and was declared inactive.

Answer: A) Barren Island erupted twice. 

12. Which plant (cold-desert “wonder plant”) was featured in August 2025 Science & Tech Current Affairs notes?
A) Seabuckthorn
B) Cactus
C) Edelweiss
D) Artemisia

Answer: A) Seabuckthorn

13.  In August 2025, researchers found that which antibody type can stabilise and neutralise bacterial toxins?

A) IgG
B) IgA
C) IgM
D) IgE
Answer: C) IgM.

14. Which monthly MCQ quiz provider specifically lists “Science & Technology” as a topic heading in August 2025?

A) GKToday
B) Testbook
C) Adda247
D) All of the above
Answer: A) GKToday.

15. A major collaborative Earth-observing satellite (NISAR) will have dual-band radars and can detect changes as small as ___ cm in Earth’s surface?

A) 10 cm
B) 1 cm
C) 5 cm
D) 0.1 cm
Answer: B) 1 cm.

16. According to a weekly current-affairs list, six new dragonfly species were discovered in which location in October 2025?

A) Silent Valley National Park, Kerala
B) Sundarbans, West Bengal
C) Kaziranga National Park, Assam
D) Deccan Plateau region

Answer: A) Silent Valley National Park, Kerala

SOLID STATE – NEET & JEE Asked Questions with Answers CBSE

 

1. In a face-centred cubic (FCC) arrangement, atoms touch each other along the:

A. Body diagonal
B. Edge
C. Face diagonal
D. None of these

Answer: C. Face diagonal

Explanation:
In FCC, atoms are at the corners and face centres. The face diagonal = 4r = √2a.

2. The packing efficiency of a body-centred cubic (BCC) structure is:

A. 52.4%
B. 68%
C. 74%
D. 60%

Answer: B. 68%

Explanation:
In BCC, 2 atoms/unit cell, √3a = 4r → Packing efficiency = 68%.

3. The number of atoms in a face-centred cubic unit cell is:

A. 2
B. 4
C. 6
D. 8

Answer: B. 4

Explanation:
8 corner atoms × 1/8 + 6 face atoms × 1/2 = 1 + 3 = 4.

4. The coordination number of an atom in a hexagonal close-packed (hcp) structure is:

A. 6
B. 8
C. 12
D. 4

Answer: C. 12

Explanation:
Each atom touches 6 in its own layer, 3 above, and 3 below.

5. The number of tetrahedral and octahedral voids per atom in a cubic close-packed structure is:

A. 1 and 1
B. 2 and 1
C. 1 and 2
D. 4 and 2

Answer: B. 2 and 1

Explanation:
For every atom, 2 tetrahedral voids and 1 octahedral void exist.

6. Radius ratio for octahedral void is:

A. 0.225
B. 0.414
C. 0.732
D. 1.0

Answer: B. 0.414
Explanation:
Radius ratio = r+/r– = 0.414 for octahedral geometry.

7. A solid has a cubic structure where A atoms are at corners and B atoms at body centre. The formula of the compound is:

A. AB
B. AB₂
C. A₂B
D. AB₃

Answer: A. AB
Explanation:
A: 8 corners × 1/8 = 1 atom;
B: 1 at body centre = 1 atom → Formula AB.

8. In a crystal lattice, the ratio of number of octahedral voids to tetrahedral voids is:

A. 1 : 1
B. 1 : 2
C. 2 : 1
D. 1 : 4

Answer: B. 1 : 2
Explanation:
Always 1 octahedral and 2 tetrahedral voids per atom.

9. The number of nearest neighbours of an atom in BCC is:

A. 4
B. 6
C. 8
D. 12

Answer: C. 8
Explanation:
Each atom in BCC touches 8 others — coordination number = 8.

10. In which type of crystal defect density decreases?

A. Schottky defect
B. Frenkel defect
C. Interstitial defect
D. Metal deficiency defect

Answer: A. Schottky defect
Explanation:
Both cation and anion are missing → reduces mass but not volume → density ↓.

11. Which of the following is a metallic crystal?

A. Diamond
B. NaCl
C. Graphite
D. Copper

Answer: D. Copper
Explanation:
Metallic solids have delocalised electrons → good conductors.

12. The number of atoms per unit cell in BCC structure is:

A. 1
B. 2
C. 4
D. 6

Answer: B. 2
Explanation:
8 corners × 1/8 + 1 centre = 1 + 1 = 2.

13. If ‘r’ is the radius of atom and ‘a’ is the edge length in FCC, then relation is

A. a = 2r
B. a = 4r
C. a = 2√2r
D. a = √2r

Answer: D. a = √2r

14. What fraction of volume is empty in simple cubic lattice?

A. 26%
B. 32%
C. 47.6%
D. 68%

Answer: C. 47.6%
Explanation:
Packing efficiency = 52.4%, so voids = 100 – 52.4 = 47.6%.

15. Which defect does NOT change density of crystal?

A. Schottky
B. Frenkel
C. Metal excess
D. Vacancy

Answer: B. Frenkel defect
Explanation:
Ion leaves normal site but occupies interstitial → mass and volume constant → density unchanged.

UNIT 1 SOLID STATE- (TYPE OF VOIDS) XII CHEMISTRY PART V KEY NOTES CBSE

 

• When spheres (atoms/ions) are packed, empty spaces are left — called voids.

A. Tetrahedral Voids

• Formed when a sphere of one layer rests in the depression of three spheres of another layer.

• Number of tetrahedral voids: 2 × number of atoms.

• Radius ratio:

​rsmall/rlarge=0.225

B. Octahedral Voids

• Formed at the centre of six spheres — 3 in one layer and 3 in the adjacent one.

• Number of octahedral voids: Equal to number of atoms.

• Radius ratio:

• rsmall/rlarge=0.414

C. Relationship Between Voids and Atoms

​

Type of Void	No. of Voids per Atom	Radius Ratio	Coordination No.

Tetrahedral

2

0.225

4

Octahedral

1

0.414

6

4. Examples

Structure Type	Example Compounds
SC	Polonium (Po)
BCC	α-Iron, Na, K
FCC	Cu, Al, Ag, Au
HCP	Zn, Mg, Ti

​5. Important Points

• Close packing = maximum efficiency = 74% (in FCC & HCP).

• Sequence of layers:

  • HCP: ABAB...

  • CCP: ABCABC...

• Tetrahedral voids are smaller than octahedral voids.

• Ionic crystals: smaller ions occupy these voids depending on their radius ratio.