Question

Given that the only naturally occurring isotope of sodium is \({ }^{23} \mathrm{Na},\) determine its mass from the periodic table.

Short answer

The mass of \(^{23} \mathrm{Na}\) is 23 amu.

Step-by-step solution

Understanding Isotopes

Isotopes of an element have the same number of protons but different numbers of neutrons, hence different mass numbers. The notation \({ }^{23} \mathrm{Na} \) represents a sodium atom with a mass number of 23, meaning it has 23 protons and neutrons combined.

Determine Sodium's Atomic Mass

The atomic mass of an element on the periodic table is usually an average of the mass numbers of all its naturally occurring isotopes. However, since sodium has only one naturally occurring isotope, \({ }^{23} \mathrm{Na} \), its atomic mass is equal to the mass number of this isotope, 23, in atomic mass units (amu).

Key concepts

Atomic Mass

Atomic mass refers to the weighted average mass of the atoms in a naturally occurring element sample. Typically, elements have more than one isotope. These isotopes are versions of the same element differing in the number of neutrons, which affects the mass number without changing the chemical properties.
The atomic mass takes into account both the mass and abundance of each isotope. For example, if an element had two isotopes—the lighter one being less abundant compared to the heavier— then the atomic mass would reflect a value closer to the weight of the heavier isotope.

• Measured in atomic mass units (amu): This unit is defined with carbon-12 as a reference, wherein one atom of carbon-12 has a mass of exactly 12 amu.
• Found on the periodic table: The atomic masses listed are averages for the isotopes naturally found on Earth.

In the case of sodium, since there is only one abundant isotope, \(^{23}\text{Na} \), the atomic mass is essentially the same as the mass number of this isotope, 23 amu.

Mass Number

The mass number of an element, often found in its isotope notation, is the total count of protons and neutrons in the atom's nucleus. It is denoted as a superscript on the upper left of an element's symbol, such as in \(^{23}\text{Na} \).
Knowing the mass number helps scientists understand the structure of the atom and its stability.

• Integral and non-average: Unlike atomic mass, mass number is always a whole number. It is the sum of protons and neutrons without accounting for isotope distribution or average.
• Represents atomic weight for specific isotopes: Not to be confused with atomic mass, which represents a weighted average, the mass number applies to individual isotopes.

Each isotope of an element has its own specific mass number, which directly influences the overall atomic mass if the element has multiple isotopes.

Periodic Table

The periodic table is an organized chart displaying all known chemical elements arranged by increasing atomic number. Elements are ordered based on their atomic numbers, which indicate the number of protons in the nucleus. Columns are called groups, and rows are called periods.
The periodic table groups elements with similar chemical properties into groups, making it easy to predict the characteristics and behaviors of elements based on their position.

• Provides atomic structure information: Besides atomic numbers, it lists atomic masses which, on average, include all natural isotopes.
• Essential tool for chemists and physicists: Used to quickly reference elemental properties and investigate chemical behaviors.
• Shows isotopic compositions: While not always visible directly inline, elements often have isotopic patterns or annotations near their atomic mass in a detailed periodic table.

The periodic table is vital for understanding relationships between elements, primarily how isotopes play a central role in the averaged atomic masses displayed on it.