The atom consists of a central nucleus surrounded by a cloud of negatively charged electrons. Most of the mass of the atom is concentrated in the atomic nucleus, which consists of protons and neutrons. Here, we explain key concepts such as atomic number, atomic mass number, isotopes, and related definitions.
Atomic Structure
The atom is composed of a nucleus and a cloud of electrons. The nucleus contains:
- Protons (positively charged) and Neutrons (no charge).
- The number of protons is called the atomic number \(Z\), and the total number of nucleons (protons and neutrons) is the atomic mass number \(A\).
The radius of an atom is around 0.1 nm, whereas the radius of the nucleus is much smaller, about \(10^{-5}\) nm.
Atomic Number and Atomic Mass Number
The atomic number \(Z\) represents the number of protons in an atom and is equal to the number of electrons in a neutral atom. The atomic mass number \(A\) represents the total number of protons and neutrons, so we have:
Isotopes
An element may have atoms with the same number of protons but different numbers of neutrons. These are known as isotopes. For example, the isotope of cobalt with 60 nucleons is denoted as:
Strong Force and Nucleons
Protons and neutrons are collectively referred to as nucleons. They are bound in the nucleus by the strong force, which is a short-range force active only at distances on the order of a few femtometers (fm). The strong force is much stronger than other forces, such as electrostatic and gravitational forces, at these short distances.
Atomic Mass and Units
The unified atomic mass unit \( \mu \) is used to express the mass of atoms. It is defined as 1/12 of the mass of the carbon-12 atom and is approximately equal to 931.5 MeV/c². The atomic mass \(M\) of a specific isotope is smaller than the sum of the masses of the individual nucleons due to binding energy:
Atomic Weight and Avogadro's Constant
The atomic weight \( A_r \) is the average mass of an element’s atoms compared to the unified atomic mass unit. The average is weighted according to the relative abundance of the element's isotopes. The gram-atom is the number of grams corresponding to Avogadro's constant (\(N_A = 6.022 \times 10^{23}\)) atoms of an element.
For any element, there are \(N_A\) atoms per gram-atom. This leads to the following relationships for the number of atoms and electrons per unit mass and volume of an element:
Empirical Relation Between \(A\) and \(Z\)
While there is no simple fundamental relationship between the atomic mass number \(A\) and the atomic number \(Z\), an empirical relation provides a good approximation for stable nuclei: