5.1.2 The Nucleus

This section explores the structure of the atom, focusing specifically on the nucleus and how it is represented using nuclide notation.

The Structure of the Nucleus

The nucleus is the dense, central part of an atom. It contains positively charged protons and neutral particles called neutrons. The number of protons determines the element.

Protons are subatomic particles with a positive electrical charge (+1). Neutrons have no electrical charge (neutral).

The number of protons in the nucleus is also known as the atomic number, denoted by Z.

The total number of protons and neutrons in the nucleus is known as the mass number, denoted by A.

Nuclide Notation

Nuclide notation is a shorthand way of representing a specific type of atom, including the number of protons, neutrons, and the element itself.

The general form of nuclide notation is:

$$ _Z^A X $$

Where:

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

Examples of Nuclide Notation

Here are some examples of how to write nuclide notation:

• Carbon-12: $$ _6^{12} C $$ (6 protons, 6 neutrons, Carbon)
• Oxygen-16: $$ _8^{16} O $$ (8 protons, 8 neutrons, Oxygen)
• Uranium-238: $$ _92^{238} U $$ (92 protons, 146 neutrons, Uranium)

Isotopes

Atoms of the same element (same number of protons, Z) but with different numbers of neutrons (different A) are called isotopes.

For example, Carbon-12 and Carbon-14 are isotopes of carbon. They both have 6 protons but different numbers of neutrons (12 and 14 respectively).

Ions

Atoms that have gained or lost electrons are called ions. Ions have a net electrical charge.

If an atom loses electrons, it becomes a positive ion (cation). The number of protons is greater than the number of electrons.

If an atom gains electrons, it becomes a negative ion (anion). The number of electrons is greater than the number of protons.

Ion notation uses a superscript to indicate the charge:

$$ _Z^A X^+ $$

Represents a positive ion with charge +1.

$$ _Z^A X^{2+} $$

Represents a positive ion with charge +2.

$$ _Z^A X^- $$

Represents a negative ion with charge -1.

$$ _Z^A X^{2-} $$

Represents a negative ion with charge -2.

Table Summary

Notation	Meaning
$$ _Z^A X $$	General nuclide notation (Z = atomic number, A = mass number)
$$ _Z^A X^+ $$	Positive ion (cation) with charge +1
$$ _Z^A X^{2+} $$	Positive ion (cation) with charge +2
$$ _Z^A X^- $$	Negative ion (anion) with charge -1
$$ _Z^A X^{2-} $$	Negative ion (anion) with charge -2