Atomic Structure, Isotopes, and Mass Calculation

Posted on May 2, 2025 in Chemistry

Sub-atomic Particles & Atomic Structure

  • Protons, neutrons, and electrons are the sub-atomic particles of an atom.
  • Scientists developed notations to easily identify the numbers and arrangement of these particles within the atom.

Atomic number (Z): The number of protons in the nucleus.

Mass number (A): The total number of protons and neutrons in an atom.

A = Z + N (where N is the number of neutrons)

Finding the Number of Neutrons

To find the number of neutrons, subtract the atomic number (Z) from the mass number (A).

What Are Isotopes?

Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. They generally have the same chemical properties. For example, Carbon-12 (C-12) and Carbon-14 (C-14) are isotopes of carbon.

Naming Isotopes

Isotopes of an element are named by writing the element’s name followed by a hyphen and the isotope’s mass number. For example, the most common isotope of oxygen (Z = 8) is Oxygen-16. An atom of Oxygen-16 contains 8 protons and 8 neutrons.

Isotopic Notation

When chemists want to indicate a specific isotope in a chemical expression, they use a special notation called isotopic notation. This notation is based on the element’s symbol and includes the atomic number as a leading subscript and the mass number as a leading superscript.

The standard format for isotopic notation is:

AZX

Where X represents the element symbol, A is the mass number, and Z is the atomic number. Note that the atomic number (Z) is often omitted because the element symbol (X) already implies the atomic number.

An isotope name can be written as Oxygen-16 or O-16 (shortened form).

Weighted Average Atomic Mass

A weighted average is an average that gives more importance to certain values than to others.

In the case of atomic mass, more common isotopes are given more weight than less common isotopes. Thus, the listed atomic mass is closer to the mass of a typical atom than a simple average would be.

For example, a sample of chlorine is made of two chlorine isotopes: 75.77% Cl-35 (with a mass of 34.969u) and 24.23% Cl-37 (with a mass of 36.966u). The simple average mass of these isotopes is (34.969 + 36.966) / 2 = 35.968u. However, because Cl-35 is three times as common, a randomly chosen atom is much more likely to have the smaller mass. Thus, the weighted average atomic mass of a sample containing innumerable atoms will be closer to 35u than to 37u.

Calculating Average Atomic Mass

Let’s calculate the average atomic mass of chlorine atoms in nature, given that 75.77% of chlorine atoms have a mass of 34.969u per atom (Cl-35) and 24.23% of Cl atoms have a mass of 36.966u per atom (Cl-37).

To find the weighted average:

  1. Convert the percentages to decimals.
  2. Multiply the decimal percentage by the mass of each isotope in the sample.

Mass contribution from Cl-35: 0.7577 x 34.969u = 26.50u

Mass contribution from Cl-37: 0.2423 x 36.966u = 8.957u

  1. Add the results to get the weighted average mass of the sample.

26.50u + 8.957u = 35.46u (Weighted average mass of the Cl atom)

Valence Electrons Explained

Valence electrons are the electrons in the outermost principal energy level of an atom. They are the electrons primarily involved in chemical bonding and largely determine an element’s chemical properties.

Valence electrons are always found in the outermost ‘s’ sublevel, or in both the outermost ‘s’ and ‘p’ sublevels. Since the total number of electrons possible in the s and p sublevels of a given energy level is eight (2 in s + 6 in p), there can be no more than eight valence electrons.

Example: Carbon

Electron Configuration of Carbon is 1s2 2s2 2p2.

The outermost principal energy level is n=2, which contains 2s2 and 2p2 electrons. Therefore, Carbon has 2 + 2 = 4 valence electrons.

  • If an atom loses electrons, it forms a positively charged ion (cation). For example, losing one electron results in a +1 charge.
  • If an atom gains electrons, it forms a negatively charged ion (anion). For example, gaining one electron results in a -1 charge.
  • Valence electrons are the outer shell electrons that are involved in chemical reactions.

Electron Dot Notation

Sometimes scientists represent the valence electrons of an atom using electron dot notation (also known as Lewis dot structures). Dots representing valence electrons are placed around the element’s symbol.

Since valence electrons are typically limited to the outermost s and p sublevels, which hold a maximum of eight electrons, no more than eight dots are placed around the symbol.

  • Electron dot notation visually represents the number of valence electrons around an atom.