Basic Definitions for Atomic Structure

Atomic Structure Overview

The constituent particles forming an atom are protons, neutrons, and electrons. Protons and neutrons are known as nucleons and form the nucleus of the atom. Protons have a positive charge, neutrons are neutral, and electrons have a negative charge. In comparison to electrons, protons and neutrons have a relatively large mass, exceeding the electron mass by a factor of almost 2000. The ratio of the proton mass to electron mass is given as:

The following general definitions apply to atomic structure:

• Atomic Number (Z): The number of protons and the number of electrons in an atom.
• Atomic Mass Number (A): The number of nucleons in an atom, i.e., the sum of protons (Z) and neutrons (N): $A=Z+N$.
• Atomic Mass (m_a): The mass of a specific isotope expressed in atomic mass units (u), where 1 u is defined as 1/12th the mass of a carbon-12 atom at rest. This mass is also approximately equal to 931.5 MeV/c²: $1u\approx \mathrm{931.5}\mathrm{MeV/c}²$.

The atomic mass is smaller than the sum of the individual masses of the nucleons (protons and neutrons) due to the intrinsic binding energy within the nucleus. The atomic mass is larger than the nuclear mass (M) because it also includes the mass of the orbital electrons, which is ignored in the definition of nuclear mass.

The atomic mass number (A) and the atomic mass (m_a) are very similar for most elements, but they are not exactly the same. For example, for carbon-12, the atomic mass is exactly 12 u. However, for other elements, the atomic mass (m_a) may slightly differ from A.

In addition, the mole (mol) is the amount of substance that contains exactly 6.022 × 10²³ atoms (Avogadro's constant $N$_A) and corresponds to the atomic mass of an element in grams. For example, natural cobalt has an atomic mass of 58.9332 u, so 1 mole of cobalt weighs 58.9332 grams and contains exactly $N$_A atoms.

The number of atoms (N_a) per mass of an element can be given by:

Similarly, the number of electrons per volume of an element is given by:

And the number of electrons per mass of an element is:

For most elements, the ratio of atomic number to atomic mass number (Z/A) is approximately 0.5, with some exceptions. For example, for hydrogen, Z/A = 1, and for helium, Z/A = 0.5. As Z increases, Z/A gradually decreases, e.g., for ⁴He, Z/A = 0.5, for ⁶⁰Co, Z/A = 0.45, and for ²³⁵U, Z/A = 0.39.

For a molecule, the mass is the sum of the masses of the constituent atoms. The number of molecules (N_A) per mole of a compound is equal to Avogadro’s number, and the mass of the molecule in grams is the sum of the atomic mass numbers of the constituent atoms. For example, 1 mole of water (H₂O) is 18 g, and 1 mole of carbon dioxide (CO₂) is 44 g. These contain exactly $N$_A molecules.