QCM : Fundamentals of Chemistry — 8 questions

Questions et réponses du QCM

1. Which statement correctly describes the mass number of an atom?

It is the number of electrons in the outer shell.
It is the number of protons in a neutral atom.
It is the sum of protons and neutrons in the nucleus.
It is the average mass compared with carbon-12.

It is the sum of protons and neutrons in the nucleus.

Explication

The mass number is the total count of protons plus neutrons. The number of protons alone is the atomic number, and the average mass compared with carbon-12 is the relative atomic mass.

2. What is the primary purpose of the atomic number (Z) in the periodic table?

It determines the element's position and identifies the number of protons in an atom.
It measures the electronegativity of an atom.
It shows the average atomic mass of an element.
It indicates the total number of neutrons in an atom.

It determines the element's position and identifies the number of protons in an atom.

Explication

The atomic number (Z) uniquely identifies an element and indicates the number of protons in its nucleus, which also determines its position in the periodic table. The other options refer to different atomic properties or are incorrect.

3. What can be determined from an element’s position in the periodic table group and period?

Its total number of protons and electrons
Its exact number of neutrons and mass number
Its number of valence electrons and outer shell
Its chemical formula in a compound

Its number of valence electrons and outer shell

Explication

An element’s group and period tell you the number of valence electrons and the outer shell. Neutrons and mass number require atomic number and mass number information, not just position.

4. What does the atomic number (Z) of an element represent in the periodic table?

The total number of neutrons in the nucleus of an atom
The total number of protons in the nucleus of an atom
The total number of electrons in a neutral atom
The average mass of an atom compared to carbon-12

The total number of protons in the nucleus of an atom

Explication

The atomic number (Z) indicates the number of protons in an atom's nucleus, which uniquely identifies the element. It is not related to neutrons, electrons, or atomic mass.

5. What is a simple substance?

A substance that contains only ions
A substance made of two or more chemical elements
A substance made of only one chemical element
A substance named using Greek prefixes

A substance made of only one chemical element

Explication

A simple substance consists of only one chemical element. A compound substance contains more than one element.

6. What is the primary function of the valence electrons in an atom?

They are involved in the atom's radioactive decay.
They largely influence the atom's chemical behavior.
They determine the atom's mass.
They are responsible for the atom's nuclear stability.

They largely influence the atom's chemical behavior.

Explication

Valence electrons are the electrons in an atom's outermost shell and largely determine its chemical behavior, including bonding and reactivity. The other options relate to different atomic properties but are not functions of valence electrons.

7. If a substance has the brute formula CO2, how many carbon atoms are present in one molecule?

Four carbon atoms
Three carbon atoms
Two carbon atoms
One carbon atom

One carbon atom

Explication

In CO2, the missing subscript for carbon means one carbon atom is present, while the 2 applies only to oxygen. The formula shows one C and two O atoms.

8. When was the fundamental law stating that mass remains constant during a chemical reaction formally established?

At the beginning of the 20th century, around 1905
In the mid-19th century, around 1850
In the early 19th century, around 1803
During the late 18th century, around 1789

In the early 19th century, around 1803

Explication

The law of conservation of mass was formulated by Antoine Lavoisier in 1789, making the early 19th century the period when it was formally established and widely recognized.

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Periodic table — role?

Organizes elements by atomic structure

Neutrons: charge

No electric charge in nucleus

Substance formulas — purpose?

Represent element or compound composition

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