Problem 25 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Define atomic number and mass number. (b) Which of these can vary without changing the identity of the element?

Step-by-Step Solution

Answer

(a) Atomic number (\(Z\)) is the number of protons in the nucleus of an atom, uniquely identifying an element. Mass number (\(A\)) is the total number of protons and neutrons in the nucleus. (b) The mass number can vary without changing the element's identity, as variations in neutron quantity form different isotopes of the same element.

Step 1: Part (a): Define Atomic Number and Mass Number

Atomic number is the number of protons in the nucleus of an atom. It is represented by the symbol \(Z\). The atomic number uniquely identifies an element, since no two elements have the same number of protons. Mass number, also known as nucleon number, is the total number of protons and neutrons in the nucleus of an atom. It is represented by the symbol \(A\). The mass number is not unique to an element, as isotopes of the same element can have different mass numbers due to the varying number of neutrons.

Step 2: Part (b): Determination of Variation

Between atomic number and mass number, the one that can vary without changing the identity of the element is the mass number. This is because the mass number takes into account both protons and neutrons within the nucleus of an atom. The number of neutrons can vary in an element, giving rise to different isotopes, but the number of protons (i.e., the atomic number) remains constant for a particular element. Changing the number of protons would result in a different element altogether.

Key Concepts

Mass Number

Mass number is a fundamental concept in understanding atomic structure. It refers to the total count of protons and neutrons in an atom's nucleus, represented by the symbol \(A\).
The mass number is crucial because it indicates the atom's weight at the atomic level.
While two atoms of the same element can have the same atomic number, they may have different mass numbers due to the presence of extra neutrons.

• This is because protons and neutrons together make up most of the atom's mass.
• The mass number differs from the atomic number, which only counts protons.
• For instance, a carbon atom typically has 6 protons and 6 neutrons, giving it a mass number of 12.

Understanding mass number helps clarify why certain atoms of the same element behave slightly differently in chemical reactions.

Isotopes

Isotopes are different versions of the same element, distinguished by having the same number of protons but different numbers of neutrons. This variance in neutrons leads to different mass numbers.
Despite these differences in mass numbers, isotopes still share identical chemical properties because chemical behaviors are determined by electrons, not neutrons.

• For example, carbon-12 and carbon-14 are both isotopes of carbon. They both have 6 protons but differ in their neutron count; carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.
• Isotopes can also be stable or radioactive, with the latter undergoing decay over time.
• Radioactive isotopes have important uses in medicine and archaeology, such as carbon dating.

By studying isotopes, scientists can uncover information about the stability and history of elements.

Protons and Neutrons

Protons and neutrons are subatomic particles that reside in the nucleus of an atom, each playing crucial roles in its structure.

• Protons, which have a positive charge, define the atomic number of an element. The number of protons does not change for an element, as it determines the element's identity.
• Neutrons are neutral particles, and their presence influences the mass number but not the chemical behavior.
• The difference in neutron count leads to the formation of isotopes, which are variants of the same element with different mass numbers.

By balancing the number of protons and neutrons, the nucleus can be stable. However, certain ratios may lead to radioactive decay, releasing energy in the process.

Atomic Structure

Atomic structure refers to the organization of subatomic particles within an atom.
An atom consists primarily of:

• Protons: Positively charged particles situated in the nucleus, determining the element’s identity.
• Neutrons: Neutral particles also found in the nucleus, contributing to the atom's mass but not its charge.
• Electrons: Negatively charged particles orbiting the nucleus in electron shells or energy levels.

The arrangement of these particles governs the behavior and properties of atoms during chemical reactions and physical interactions.
An understanding of atomic structure not only reveals the nature of elements but also the interactions that lead to molecule and compound formation.