Which one among the following pairs is not correctly matched ?
- A.Nitrogen : Three unpaired electrons
- B.Carbon : Tetravalent
- C.Chlorine : Two stable isotopic forms
- D.Helium : Diatomic gas
Correct answer
Explanation
The correct answer is D, Helium : Diatomic gas. Helium is a noble gas whose atoms exist singly, so it is a monatomic gas, not a diatomic one.
Helium has a completely filled first shell (1s²), so its atoms have no tendency to share electrons or bond with each other. All noble gases, from helium to radon, occur as single atoms. Gases such as hydrogen, nitrogen, oxygen and chlorine, by contrast, exist as diatomic molecules (H₂, N₂, O₂, Cl₂).
A is correctly matched: nitrogen (2s² 2p³) has three unpaired electrons in its 2p orbitals, which is why it forms three bonds, as in ammonia.
B is correctly matched: carbon has four valence electrons and forms four covalent bonds, so it is tetravalent.
C is correctly matched: chlorine has two stable isotopes, chlorine-35 and chlorine-37, which is why its average atomic mass is about 35.5.
Exam tip: noble gases are monatomic; H, N, O, F, Cl, Br and I form diatomic molecules.
Practice Questions
View allConsider the following actions :
1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
An accelerometer measures acceleration, including the pull of gravity, and each of the three actions depends on reading it. An accelerometer is a small sensor, today usually a MEMS (micro-electro-mechanical system) chip, that senses changes in speed and the direction of gravity.
Read the full explanation →Consider the following statements :
1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Show answer
Carbon fibre is widely used in cars and aircraft for its strength and low weight, and it can be recovered and recycled.
Read the full explanation →Which one of the following cell organelles contains DNA?
- A.Golgi apparatus
- B.Mitochondrion
- C.Lysosome
- D.Endoplasmic reticulum
Show answer
Mitochondria carry their own small, circular DNA and their own ribosomes. Mitochondria are the powerhouses of the cell, where aerobic respiration makes ATP.
Read the full explanation →Which set of vegetables is underground stem or modified stem of a plant?
- A.Potato, artichoke, round gourd
- B.Onion, garlic, carrot
- C.Potato, onion, garlic
- D.Potato, garlic, turnip
Show answer
All three are underground modified stems: the potato is a stem tuber, and onion and garlic are bulbs. A stem can be told from a root by its nodes and buds.
Read the full explanation →A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :
- A.0.5 s
- B.1 s
- C.1.5 s
- D.2 s
Show answer
A wave moves forward by one wavelength in each cycle, so 150 wavelengths take 150 cycles, and 150 cycles at 300 per second take half a second.
Read the full explanation →