145 practice questions covering Journey Inside the Atom, organised into 2 topics.
145 questions include a written explanation.
Pick a topic below, or try the samples first.
The nucleus of a sodium atom contains $11$ protons. How many electrons must move around that nucleus if the atom is to show no charge overall, and why?
A$10$, because one electron is always given away to leave a stable outer shell
B$11$, so that the total negative charge balances the total positive charge
C$1$, because only the outermost electron matters
D$22$, because every proton needs two electrons to balance it
Show answer and explanation
Correct answer: B - $11$, so that the total negative charge balances the total positive charge
Each proton carries one unit of positive charge and each electron one unit of negative charge, so the charges cancel only when the two numbers are equal. Pairing two electrons with each proton would leave the atom strongly negative. A sodium atom that has given away an electron is no longer neutral, and the inner electrons carry charge just as the outer one does.
Q2
medium
For which shell does the rule $2n^{2}$ give a maximum of $32$ electrons?
AThe L-shell
BThe K-shell
CThe N-shell
DThe M-shell
Show answer and explanation
Correct answer: C - The N-shell
Putting $2n^{2} = 32$ gives $n^{2} = 16$ and so $n = 4$, which is the N-shell. The M, L and K shells have $n = 3$, $2$ and $1$, giving maximum capacities of $18$, $8$ and $2$ electrons.
Q3
hard
In a certain atom the K-shell holds $2$ electrons, the L-shell holds $8$ and the M-shell holds $8$, and no shell beyond these contains any electron. How many electrons has the atom, and could that M-shell ever have held more?
A$18$ electrons; the M-shell could have held up to $18$ once a shell beyond it began to fill
B$16$ electrons; the M-shell could have held up to $18$
C$18$ electrons; the M-shell can never hold more than $8$ in any atom, since $8$ fills an outer shell
D$18$ electrons; the M-shell could have held up to $32$ once the shells beyond it began to fill
Show answer and explanation
Correct answer: A - $18$ electrons; the M-shell could have held up to $18$ once a shell beyond it began to fill
Adding the shells gives $2 + 8 + 8 = 18$ electrons, and dropping the K-shell electrons would wrongly give $16$. The M-shell is held to $8$ only because it is the outermost shell here; its capacity from $2n^{2}$ is $18$, which it can reach in a larger atom where the N-shell has started to fill. A capacity of $32$ belongs to the N-shell.
Q4
medium
Assertion (A): A fluorine atom, which has $9$ electrons, has $7$ valence electrons. Reason (R): Electrons fill the K-shell first and only then the L-shell, so fluorine's shells hold $2$ and $7$ electrons.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: A - Both A and R are true, and R is the correct explanation of A
(A) is true: fluorine's configuration is $2,7$, so its outermost shell holds $7$ electrons. (R) is true as well, and it is the reason for (A), because the filling order is what forces $2$ into the K-shell and leaves the other $7$ in the L-shell, which is then the valence shell.
Q5
medium
Why do scientists write short symbols instead of the full names of elements?
AWriting the full name would change the chemical behaviour of the element, while a symbol leaves it unaffected in every reaction
BOnly a symbol can be written beginning with a capital letter
CThe symbols show how heavy the atoms of each element are compared with hydrogen
DThe symbols are recognised everywhere, so scientists can follow one another's work whatever language they use
Show answer and explanation
Correct answer: D - The symbols are recognised everywhere, so scientists can follow one another's work whatever language they use
A symbol agreed worldwide lets a chemist in any country read a formula written in any other, which a name in one language cannot do. A symbol carries no information about the mass of an atom. Writing something down cannot alter how a substance behaves, and ordinary names begin with capital letters just as easily.
Q6
medium
Acharya Kanada in India and Leucippus and Democritus in Greece all reached the idea that matter is built from particles that cannot be divided further. On what were these very early ideas based?
AObservations made through early microscopes
BMeasurements of the charge carried by cathode rays
CReasoning and imagination rather than experiments
DCareful weighing of substances before and after a reaction
Show answer and explanation
Correct answer: C - Reasoning and imagination rather than experiments
These thinkers had no laboratories; the idea of the atom began as an imagined answer to the question of what everything is made of, not as a conclusion drawn from measurements. Weighing substances in reactions belongs to much later chemistry, microscopes cannot show atoms at all, and cathode rays were only studied at the end of the nineteenth century.
Practice the full Journey Inside the Atom bank free
All 145 questions with timed practice, instant scoring, and explanations. Free forever.