Question

Distinguish between atomic number and mass number.

Short answer

Atomic number is the count of protons; mass number is the sum of protons and neutrons.

Step-by-step solution

Understand Atomic Number

The atomic number of an element is the number of protons present in the nucleus of an atom. It is denoted by the symbol \(Z\). Each element has a unique atomic number because each type of atom has a specific number of protons. For example, the atomic number of carbon is 6, which means there are 6 protons in the nucleus of a carbon atom.

Explore Mass Number

The mass number of an atom is the total number of protons and neutrons in its nucleus. It is represented by the symbol \(A\). Mass number can vary even within the same element due to the presence of isotopes, which are atoms of the same element with different numbers of neutrons. For instance, one isotope of carbon is carbon-12, having a mass number of 12 (6 protons and 6 neutrons), and another is carbon-14, which has a mass number of 14 (6 protons and 8 neutrons).

Compare Atomic Number and Mass Number

The atomic number \(Z\) strictly refers to the count of protons, which defines the identity of an element. Meanwhile, the mass number \(A\) includes both protons and neutrons, representing the total nucleon count in an atom's nucleus. Consequently, while the atomic number is unique to each element, the mass number can differ among isotopes of the same element.

Summarize the Distinction

The atomic number identifies the type of element through its number of protons, while the mass number accounts for the sum of protons and neutrons, including the varying neutrons in isotopes. Thus, the atomic number establishes the element's identity, and the mass number provides information about the isotopic form of the element.

Key concepts

Mass Number

The mass number provides important information about an atom's nucleus by indicating the total number of protons and neutrons present. In scientific notation, the mass number is represented by the symbol \(A\). Calculating the mass number is simple: you add the number of protons and the number of neutrons in the nucleus. This understanding helps you identify specific isotopes of an element.
This leads us to naturally consider: how do we distinguish between atoms if they all have the same element? The answer lies in isotopes, which we'll discuss further below. For example, carbon-12 and carbon-14 are isotopes of carbon; carbon-12 has a mass number of 12 (6 protons and 6 neutrons), while carbon-14 has a mass number of 14 (6 protons and 8 neutrons).
Thus, the mass number gives insight into the variation of isotopes by showing differences in the neutron count. It reflects the nuclear composition beyond the fixed identity provided by the atomic number.

Protons and Neutrons

Understanding the composition of an atom begins with knowing about protons and neutrons. Together, they are called nucleons and make up the atomic nucleus. Protons are positively charged particles, crucial in defining the atomic number and, consequently, the element itself.

• Protons define the element: For example, all atoms with 6 protons are carbon atoms.
• Charge: Protons have a positive charge.

Neutrons, on the other hand, are neutral particles with no electric charge, but they play a key role in adding stability to the nucleus.

• Role in stability: Neutrons help offset the repulsive forces due to the electric charges of protons.
• Isotopic differences: Neutrons' number can differ, leading to isotopes.

Therefore, both protons and neutrons are essential for understanding chemical identity and the stability of atoms.

Isotopes

Isotopes are different forms of the same element that have equal numbers of protons but different numbers of neutrons. This means that while the atomic number \(Z\) remains constant, the mass number \(A\) changes among isotopes. Isotopes play a vital role in scientific research and various applications, including medicine and archaeology.
For instance, carbon has two naturally occurring isotopes, carbon-12 and carbon-14. Both have 6 protons (thus, an atomic number of 6), but they have 6 and 8 neutrons respectively. This neutron difference gives them their different mass numbers and denotes them as distinct isotopes.
The significance of isotopes lies in their application. Carbon-14, for example, is used in radiocarbon dating to determine the age of archaeological artifacts. Additionally, isotopes can have different properties for stability; for example, radioactive isotopes can undergo decay over time, making them useful in nuclear medicine for treatment and diagnostics.