Atomic Number, Mass Number, Isotope, Isobar, Isotone, Atomic Mass
Atomic Number (Z)
The atomic number of an element is the number of protons in the nucleus of its atoms. It determines the identity of the element and its position in the periodic table. For example: Helium The atomic number of helium is 2 because the number of protons in helium nucleus is 2.
Mass Number (A)
The mass number of an atom is the sum of its protons and neutrons. It represents the total number of nucleons (protons and neutrons) in the nucleus of an atom.
Mass Number (A) = Number of Protons (p) + Number of Neutron (n)
For example: The mass number of helium-4 (most common isotope of helium) refers to the total number of nucleons (protons + neutrons) present in the nucleus of a helium atom. Helium has an atomic number of 2, indicating it has two protons in its nucleus. As well as helium has two neutrons in its nucleus.
Thus the atomic mass number of helium 4 will be 4 (2 protons + 2 neutrons = 4 nucleons).
However, there are other isotopes of helium, such as helium-3, which have different numbers of neutrons and, therefore, different mass numbers.
Isotopes
Isotopes (iso means equal and topos means place) are atoms of the same element that have the same number of protons but different numbers of neutrons.
- Isotopes have the same atomic number but different mass number.
- Isotopes of an element have similar chemical properties but different physical properties due to differences in mass.
- Almost every chemical substance has an isotope. Approximately 275 isotopes of approximately 81 stable chemical components have been confirmed.
- Isotopes are of two types: Stable isotopes and Radioactive isotopes
Example: Isotopes of helium: Helium has two stable isotopes: helium-3 and helium-4.
- Helium-3: Helium-3 has two protons like all helium atoms, but it contains only one neutron in its nucleus. It is less common on Earth than helium-4.
- Helium-4: Helium-4, also known as alpha particles, is the most common and stable isotope of helium. It has two protons and two neutrons in its nucleus. Commonly it is found in natural gas deposits.
Isobars
Isobar (iso means equal, and baros means weight) are the atoms of different chemical element have the same mass number but different atomic numbers. The term “isobars” was coined by Alfred Walter Stewart in 1918.
Example: Argon, potassium, and calcium
- Argon (Ar): Atomic number of Argon is 18 and Mass Number is 40. It has 18 protons and 22 neutrons in its nucleus. Chemically it is a noble gas.
- Potassium (K): Atomic number of Potassium is 19 and Mass Number is 40. It has 19 protons and 21 neutrons in its nucleus. Chemically it is an alkali metal.
- Calcium (Ca): Atomic number of calcium is 20 and Mass Number is 40. It has 20 protons and 20 neutrons in its nucleus. Chemically it is an alkaline earth meta.
Isotones
Isotones are two or more different chemical element that have the same number of neutrons but different number of protons. The value of (A – Z) for these elements is the same even when the atomic number Z and the mass number A are different.
Example: Carbon, nitrogen, and oxygen 
- Carbon (C): Carbon has 6 protons and 8 Neutron in its nucleus
- Value of A-Z = 14-6 = 8
- Nitrogen (N): Nitrogen has 7 protons and 8 Neutron in its nucleus
- Value of A-Z = 15-7 = 8
- Oxygen (O): Carbon has 8 protons and 8 Neutron in its nucleus
- Value of A-Z = 16-8 =8
Isoelectronic
Isoelectronic are atoms, ions, or molecules that have the same number of electrons. Despite potentially having different numbers of protons, they have identical electron configurations and thus similar chemical properties.
Example: Sodium (Na⁺ ) and fluorine (F⁻)
Sodium (Na): Atomic number of Sodium is 11.
- Electron configuration of sodium is 1s² 2s² 2p⁶ 3s¹.
- Sodium has 11 electrons and the outermost electron is in the 3s orbital.
- To form a stable Sodium Na⁺ ion sodium loses its outermost electron resulting in an electron configuration of 1s² 2s² 2p⁶
Fluorine (F): Atomic number of Fluorine is 9.
- Electron configuration of fluorine is 1s² 2s² 2p⁵.
- Fluorine has 9 electrons and the outermost electron is in the 2p orbital.
- Fluorine gains one electron to form a stable F⁻ ion resulting in an electron configuration of 1s² 2s² 2p⁶
Atomic Mass
Atomic mass is the total mass of the subatomic particles (proton, neutron and electron) in an atom and is measured in atomic mass units (amu).
Atomic mass of an element = Mass of protons + Mass of electrons + Mass of neutrons
The calculated value for atomic mass is a whole number.- Atomic Mass Unit: One atomic mass unit (u) is defined as one twelfth of the mass of a carbon-12 atom. (The International Union of Chemists adopted Carbon-12 as the standard for comparing the atomic and molecular weights of elements and compounds in 1961.)
Example: Carbon: the atomic mass of carbon is 12 u.
