Atomic Foundations of Matter MCQs for UPSC Prelims
125 practice questions covering Atomic Foundations of Matter, organised into 1 topics.
125 questions include a written explanation.
Pick a topic below, or try the samples first.
A hydrogen atom has one electron in its K-shell. How do two hydrogen atoms form a hydrogen molecule?
AEach shares its single electron, so both reach the two electrons a K-shell needs
BEach picks up an extra electron from the air, so both end with two
CThe two nuclei join together so that one larger atom is formed
DOne gives its electron to the other, making a positive and a negative ion
Show answer and explanation
Correct answer: A - Each shares its single electron, so both reach the two electrons a K-shell needs
The K-shell holds two electrons and each hydrogen atom has only one, so the two atoms share one electron each; the shared pair then counts for both and both become stable. Transfer would leave one atom as a bare proton, which is not how $\text{H}_2$ forms. Electrons are not collected from the air, and joining nuclei would change the element altogether.
Q2
medium
A beaker holding $50\ \text{mL}$ of water is placed on a digital balance and the balance is set to zero with the tare button. Common salt of mass $5\ \text{g}$ is then tipped in and stirred until it all dissolves. What will the balance read afterwards?
ALess than $5\ \text{g}$, because part of the salt breaks down in the water
B$0\ \text{g}$, because the salt is no longer there as a solid
CMore than $5\ \text{g}$, because salt and water join to form a new compound
D$5\ \text{g}$, because dissolving does not change the total mass
Show answer and explanation
Correct answer: D - $5\ \text{g}$, because dissolving does not change the total mass
Dissolving is a physical change: the salt spreads right through the water but not a grain of it leaves the beaker, so the balance still shows the $5\ \text{g}$ that was added. The salt has become invisible, not absent. It does not break down, and a solution is a mixture rather than a new compound, so no mass can be gained either.
Q3
medium
A student uses the criss-cross method for a metal of valency $3$ combining with a non-metal of valency $1$. What formula should she obtain?
AOne metal atom with three non-metal atoms
BOne metal atom with one non-metal atom
CThree metal atoms with three non-metal atoms
DThree metal atoms with one non-metal atom
Show answer and explanation
Correct answer: A - One metal atom with three non-metal atoms
Crossing over puts the non-metal valency of $1$ onto the metal and the metal valency of $3$ onto the non-metal, so one metal atom joins three non-metal atoms, as in $\text{AlCl}_3$. Reversing the swap gives three metal atoms to one, the classic criss-cross error. Three to three reduces to one to one, and one to one ignores the valencies completely.
Q4
medium
An atom is stable when its outermost shell is complete. How many electrons complete that shell for an atom whose K-shell is its only shell, and for an atom that has further shells beyond the K-shell?
ATwo for the K-shell, and eight for an outermost shell beyond it
BTwo in both cases
CEight in both cases
DEight for the K-shell, and two for an outermost shell beyond it
Show answer and explanation
Correct answer: A - Two for the K-shell, and eight for an outermost shell beyond it
The K-shell holds only two electrons, so hydrogen and helium are stable with a duplet. Where the outermost shell lies beyond the K-shell, stability comes with an octet of eight electrons. Eight will not fit in the K-shell at all, and settling for two in a later outermost shell contradicts the octet that sodium and chlorine both work towards.
Q5
hard
A student argues that when wood burns in the open the ash weighs far less than the wood, so atoms must have been destroyed. Using Dalton's theory, what is wrong with this argument?
AAsh is a lighter kind of material, so the same atoms weigh less inside it
BWood is a mixture, and Dalton's theory applies only to pure elements
CBurning is a physical change, so Dalton's theory does not apply to it
DThe carbon dioxide and water vapour that carry most of the atoms away were never weighed
Show answer and explanation
Correct answer: D - The carbon dioxide and water vapour that carry most of the atoms away were never weighed
Most of the atoms from the wood leave as carbon dioxide and water vapour, which the student never put on a balance; weighing those would account for every gram. Burning is a chemical change, so the theory certainly applies. Dalton's theory covers mixtures too, since they are built from atoms as well, and an atom carries the same mass into whatever substance it joins.
Q6
hard
Sodium chloride made in a laboratory and sodium chloride taken from sea water both contain sodium and chlorine in the mass ratio $23:35.5$. Which chain of reasoning from Dalton's theory accounts for that?
ASea water and laboratory chemicals happen to contain exactly the same impurities
BSodium atoms and chlorine atoms are identical to each other, so they combine equally
CAtoms rearrange without being destroyed, so no mass is lost and the ratio must be fixed
DSodium and chlorine atoms join in a fixed whole number ratio, and each kind of atom has a fixed mass, so the mass ratio is fixed
Show answer and explanation
Correct answer: D - Sodium and chlorine atoms join in a fixed whole number ratio, and each kind of atom has a fixed mass, so the mass ratio is fixed
Two postulates are needed together: a fixed whole number ratio of atoms inside the compound, and a fixed mass for every atom of a given element. Put side by side they give the same mass ratio in every sample. Atoms not being destroyed explains conserved mass, a different law. Impurities are removed before analysis, and sodium and chlorine atoms are certainly not identical, since their masses differ.
Practice the full Atomic Foundations of Matter bank free
All 125 questions with timed practice, instant scoring, and explanations. Free forever.