Atomic & Mass Number

Atomic Number

The atomic number of an atom is the number of protons in its nucleus. This number is unique to each element and defines the element's identity. For example, all atoms with 6 protons are carbon atoms, and all atoms with 8 protons are oxygen atoms.

In a neutral atom, the number of electrons equals the number of protons. This balance means the positive charge of the protons is exactly cancelled by the negative charge of the electrons, so the atom has no overall charge.

The atomic number is usually represented by the symbol Z.

For instance, carbon has an atomic number of 6, meaning every carbon atom has 6 protons and, if neutral, 6 electrons.

Mass Number

The mass number is the total number of protons and neutrons in the nucleus of an atom. It is always a whole number because both protons and neutrons are whole particles.

The mass number is used to distinguish between different isotopes of the same element because isotopes have the same number of protons but different numbers of neutrons.

The mass number is usually represented by the symbol A.

For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12. Carbon-13, an isotope of carbon, has 6 protons and 7 neutrons, so its mass number is 13.

Isotopes

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. This means they have the same number of protons but different numbers of neutrons.

Because isotopes have the same number of protons, they share chemical properties, but their physical properties can differ due to the difference in mass.

Some isotopes are stable, meaning they do not change over time, while others are radioactive and decay over time (radioactive decay is covered in other topics).

For example, carbon-12 and carbon-14 are both isotopes of carbon. Carbon-12 is stable, while carbon-14 is radioactive.

Notation of Atoms

Atoms are often represented using a notation that includes the atomic number and mass number alongside the chemical symbol. This helps identify the specific isotope of an element.

The format is:

$_{Z}^{A}X$

• X is the chemical symbol of the element.
• Z (subscript) is the atomic number (number of protons).
• A (superscript) is the mass number (protons + neutrons).

For example, the most common isotope of carbon is written as $_{6}^{12}C$, meaning it has 6 protons and a total of 12 protons and neutrons.

This notation is useful for quickly identifying isotopes and understanding their composition.

Learning Example

If an atom has 11 protons and 12 neutrons, what is its atomic number and mass number? What element is it?

The atomic number $Z$ is the number of protons, so $Z = 11$.

The mass number $A$ is protons + neutrons, so $A = 11 + 12 = 23$.

The element with atomic number 11 is sodium (Na).

Therefore, the atom is sodium-23, written as $_{11}^{23}Na$.