Structure et modèles de l’atome Practice Quiz (18 questions)

1. Which contrast best describes the ancient views of Aristotle and Democritus on matter?

A. Aristotle viewed matter as continuous, while Democritus viewed it as indivisible particles separated by void.

B. Aristotle viewed matter as indivisible particles, while Democritus viewed it as continuous.

C. Both viewed matter as particles, but disagreed about whether the particles could combine.

D. Both viewed matter as continuous, but disagreed about whether it contained positive charge.

Answer: (A)

Explanation

Aristotle argued for continuous matter without empty space, whereas Democritus described matter as indivisible particles separated by void. The reversed contrast is a plausible mix-up, but it assigns each philosopher the other’s view.

2. What did Dalton propose about matter and chemical reactions in 1803?

A. Matter consists of atoms that combine during chemical reactions.

B. Matter consists of positive material in which electrons are embedded.

C. Matter consists of electron probability regions with conserved energy levels.

D. Matter consists of a tiny positive nucleus surrounded by electrons.

Answer: (A)

Explanation

Dalton proposed that matter is made of atoms that combine during chemical reactions. The embedded-electron model is associated with Thomson rather than Dalton.

3. Which description identifies Thomson’s 1904 atomic model?

A. Negative electrons are described by probability regions rather than fixed paths.

B. Negative electrons occupy fixed shells and move between them by exchanging energy.

C. Negative electrons orbit a small, dense, positively charged nucleus.

D. Negative electrons are embedded throughout a positively charged substance.

Answer: (D)

Explanation

Thomson’s model places negative electrons within a positively charged substance, often compared to pudding with embedded raisins. The orbiting-electron description belongs to Rutherford’s planetary model, not Thomson’s.

4. In Schrödinger’s 1927 model, how are electrons represented?

A. As probability regions forming an electron cloud, without fixed trajectories.

B. As particles occupying fixed shells and changing them by absorbing or emitting photons.

C. As particles following fixed planetary paths around the nucleus.

D. As particles embedded in a continuous positive substance.

Answer: (A)

Explanation

Schrödinger’s model describes electrons using probability regions that form an electron cloud, rather than fixed trajectories. Fixed shells and energy-changing transitions characterize Bohr’s model.

5. What pattern did Rutherford observe when positive alpha particles were directed at a gold foil?

A. Most were deflected, a few were turned back, and some passed through.

B. Most were turned back, a few passed through, and some were deflected.

C. Most passed through, a few were deflected, and some were turned back.

D. Most were absorbed, a few passed through, and some remained in the foil.

Answer: (C)

Explanation

In Rutherford’s experiment, most alpha particles passed through the foil, very few were deflected, and some were sent back. The pattern with most particles turned back reverses the observed proportions.

6. What did Rutherford infer from the fact that most alpha particles passed through the gold foil?

A. The atom’s positive charge is spread evenly throughout its volume.

B. The electron cloud contains most of the atom’s mass.

C. The nucleus occupies most of the atom’s volume.

D. The atom consists largely of empty space.

Answer: (D)

Explanation

The easy passage of most alpha particles indicated that most of an atom’s volume is empty space. A nucleus occupying most of the atom’s volume would not explain why the particles passed through so readily.

7. What did the backward deflection of some alpha particles suggest about the nucleus?

A. It is similar in size to the electron cloud, much lighter, and negatively charged.

B. It is very small relative to the electron cloud, much lighter, and negatively charged.

C. It is similar in size to the electron cloud, much heavier, and positively charged.

D. It is very small relative to the electron cloud, much heavier, and positively charged.

Answer: (D)

Explanation

The return of some positively charged alpha particles supported the existence of a tiny, dense, positively charged nucleus that is much heavier than the surrounding electron cloud. A negative nucleus would have the wrong charge to account for the observed repulsion.

8. Which statement correctly describes Chadwick’s discovery in 1932?

A. He discovered the uncharged neutron, which is much lighter than a proton and forms the electron cloud.

B. He discovered the uncharged neutron, which is as heavy as a proton and contributes to nuclear cohesion.

C. He discovered the positively charged neutron, which is as heavy as an electron and forms the nucleus.

D. He discovered the negatively charged neutron, which is lighter than a proton and contributes to nuclear cohesion.

Answer: (B)

Explanation

Chadwick discovered the neutron, a particle with no charge and a mass comparable to that of a proton; it also plays a role in nuclear cohesion. The claim that the neutron is negative confuses it with the negatively charged electron.

9. Which description captures what an atom is?

A. A positively charged nucleus that constitutes matter, with electrons excluded from the atom.

B. The smallest entity constituting matter, with a nucleus and electron cloud, and an overall neutral charge.

C. A group of electrons that constitutes matter, with protons and neutrons outside the atom.

D. The smallest entity constituting matter, with a nucleus and electron cloud, and an overall positive charge.

Answer: (B)

Explanation

An atom is the smallest entity constituting matter and comprises a nucleus and an electron cloud; overall, it is neutral. Describing the whole atom as positive mistakes the charge of the nucleus for the charge of the complete atom.

10. What must be true about the particles in a neutral atom?

A. The number of positively charged protons equals the number of uncharged neutrons.

B. The number of positively charged protons is greater than the number of negatively charged electrons.

C. The number of negatively charged electrons equals the number of uncharged neutrons.

D. The number of positively charged protons equals the number of negatively charged electrons.

Answer: (D)

Explanation

A neutral atom has equal numbers of positive protons and negative electrons, so their charges balance. Equal numbers of protons and neutrons do not establish neutrality because neutrons have no charge.

11. A particle is found in the electron cloud and has a negative charge; which identification matches the atom’s structure?

A. It is an electron; protons are in the electron cloud and neutrons are in the nucleus.

B. It is an electron; protons and neutrons are nucleons in the nucleus.

C. It is a proton; neutrons and electrons are nucleons in the nucleus.

D. It is a neutron; protons and electrons are nucleons in the nucleus.

Answer: (B)

Explanation

Electrons carry negative charge and occupy the electron cloud, while positively charged protons and uncharged neutrons make up the nucleus. A neutron is uncharged, so it does not fit the observed particle’s negative charge.

12. In the notation $^{A}_{Z}X$ for an atom, what do $A$ and $Z$ represent?

A. The total number of electrons and the number of neutrons, respectively

B. The total number of nucleons and the number of protons, respectively

C. The number of neutrons and the number of electrons, respectively

D. The number of protons and the total number of nucleons, respectively

Answer: (B)

Explanation

The mass number $A$ counts protons and neutrons together, while the atomic number $Z$ counts protons. The tempting reversal is incorrect because it assigns the total nucleon count to $Z$.

13. A neutral atom has mass number $A=23$ and atomic number $Z=11$; how many electrons and neutrons does it have?

A. 23 electrons and 12 neutrons

B. 11 electrons and 23 neutrons

C. 11 electrons and 12 neutrons

D. 12 electrons and 11 neutrons

Answer: (C)

Explanation

A neutral atom has the same number of electrons as protons, so it has 11 electrons, and its neutron count is $N=A-Z=12$. The alternative assigning 12 electrons and 11 neutrons reverses the electron and neutron counts.

14. Why is an atom's mass practically equal to the mass of its nucleus?

A. Protons and neutrons have negligible mass compared with electrons

B. Electrons lose their mass when the atom is neutral

C. Electrons have negligible mass compared with protons and neutrons

D. The nucleus contains nearly all of an atom's electrons

Answer: (C)

Explanation

Protons and neutrons account for practically all the atom's mass because electron mass is negligible in comparison. Neutrality balances electric charge; it does not mean electrons lose their mass.

15. When filling electron shells under the rule presented, what capacity pattern applies?

A. Every shell follows an octet, with up to 8 electrons including the first shell

B. The first shell follows a duet, then later shells follow an octet with up to 8 electrons each

C. Every shell follows a duet, with up to 2 electrons in each shell

D. The first shell follows an octet, then later shells follow a duet with up to 2 electrons each

Answer: (B)

Explanation

The first two electrons satisfy the duet rule, after which the model applies the octet rule, allowing up to 8 electrons per subsequent shell. Treating the first shell as an octet overlooks the duet rule.

16. What makes an atom become an ion?

A. It rearranges its electrons without gaining or losing any

B. It gains or loses neutrons and changes its electrical charge

C. It gains or loses one or more electrons and becomes electrically charged

D. It gains or loses one or more protons and changes its atomic number

Answer: (C)

Explanation

An ion forms when an atom gains or loses one or more electrons, giving the resulting entity an electrical charge. Changing the number of neutrons does not by itself account for ion formation.

17. An atom becomes positively charged after losing electrons; what type of ion has formed?

A. A cation, because electron loss leaves a positive charge

B. An anion, because electron gain leaves a negative charge

C. A cation, because electron gain leaves a positive charge

D. An anion, because electron loss leaves a negative charge

Answer: (A)

Explanation

A cation forms when an atom loses electrons, leaving it positively charged. An anion is also an ion, but it forms through electron gain and carries a negative charge.

18. Why do atoms tend to gain or give up electrons when forming ions?

A. To change the number of protons and acquire a different atomic number

B. To create an unsaturated outer shell with fewer electrons

C. To make the nucleus contain the same number of protons and neutrons

D. To obtain a saturated outer shell and satisfy the octet rule

Answer: (D)

Explanation

Atoms tend to gain or give up electrons to obtain a saturated outer shell and satisfy the octet rule. Changing the proton count would change the atomic number rather than achieve this electron-shell arrangement.