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Atoms, Elements & Compounds

General Science · Chemistry

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Overview

Matter is made of atoms; elements contain one kind of atom, compounds have elements combined in fixed ratios.

Key points

Atomic number Z = number of protons; mass number A = protons + neutrons.
Isotopes: same Z, different A (e.g. hydrogen: protium, deuterium, tritium).
Isobars: same A, different Z.
Compounds have fixed composition; mixtures do not.
Valency is the combining capacity of an atom.

Explanation

Electrons fill shells 2, 8, 8 for the first elements.
Noble gases have full outer shells and are inert.
Molecules can be of elements (O₂) or compounds (H₂O).
Physical separation works for mixtures, not compounds.

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

Sodium: Z = 11, A = 23 → neutrons = 12.
Isotopes of hydrogen: protium, deuterium, tritium.
Water is a compound; air is a mixture.

Traps and advanced points

Trap: Calling air a compound.
Trap: Mixing isotopes and isobars.
Trap: Using mass number as protons.

Quick revision

Z = protons; A = p + n.
Isotopes same Z.
Compound fixed ratio.

Practice MCQs

Which law relates force, mass and acceleration as F = ma?
1. Ohm’s law
2. Newton’s first law only
3. Boyle’s law
4. Newton’s second law
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Answer: (d) Newton’s second law. Newton’s second law (law) relates force, mass and acceleration as F = ma.
Isotopes of an element have the same number of what?
1. Isobars
2. Nucleons
3. Protons
4. Neutrons
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Answer: (c) Protons. Isotopes of an element have the same number of Protons.
Revision question: Sound cannot travel through which medium?
1. Vacuum
2. Air
3. Water
4. Steel
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Answer: (a) Vacuum. Revision question: Sound cannot travel through Vacuum.
Sound cannot travel through which medium?
1. Vacuum
2. Air
3. Steel
4. Water
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Answer: (a) Vacuum. Sound cannot travel through Vacuum.
Static-GK drill: Cholera is caused by which bacterium?
1. Salmonella typhi
2. Vibrio cholerae
3. HIV
4. Mycobacterium tuberculosis
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Answer: (b) Vibrio cholerae. Cholera is caused by Vibrio cholerae.
Which instrument is used to measure the intensity of earthquakes?
1. Anemometer
2. Hygrometer
3. Seismograph
4. Barometer
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Answer: (c) Seismograph. A seismograph records seismic waves; magnitude is expressed on the Richter/moment-magnitude scale.
Which option correctly answers this General Science check: chemical formula of common salt?
1. NaCl
2. vitamin K
3. radiation
4. ohm
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Answer: (a) NaCl. Common salt is sodium chloride.
Which option correctly answers this General Science check: pH below 7 generally indicates?
1. 3 × 10^8 m/s
2. negative charge
3. acidic solution
4. force per unit area
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Answer: (c) acidic solution. At 25°C, pH 7 is neutral; lower values are acidic.
Which mirror is used by dentists to see enlarged images of teeth?
1. Convex mirror
2. Concave mirror
3. Cylindrical mirror
4. Plane mirror
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Answer: (b) Concave mirror. A concave mirror gives a magnified, erect image when the object is within its focus.
Which option correctly answers this General Science check: an electron carries?
1. cos 90° = 0
2. negative charge
3. NaCl
4. H₂O
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Answer: (b) negative charge. Electrons are negatively charged; protons are positive.
Which option correctly answers this General Science check: SI unit of electrical resistance?
1. cos 90° = 0
2. equal and opposite action-reaction forces
3. ohm
4. nephron
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Answer: (c) ohm. Ohm’s law: V = IR.
Which option correctly answers this General Science check: gas released during photosynthesis?
1. negative charge
2. 3 × 10^8 m/s
3. oxygen
4. cos 90° = 0
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Answer: (c) oxygen. Plants use CO₂ and water to form carbohydrates and release oxygen in the overall process.
Which option correctly answers this General Science check: green pigment chiefly responsible for photosynthesis?
1. chlorophyll
2. convex lens
3. vitamin C
4. mitochondria
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Answer: (a) chlorophyll. Chlorophyll absorbs light energy for photosynthesis.