The atom is the smallest unit of matter that retains all the chemical properties of a given element. While once considered indivisible, it is now understood that atoms are composed of even smaller subatomic particles. The structure of an atom can be conceptualized as a two-part system: a central, dense nucleus and a vast, orbiting electron cloud.

The nucleus occupies the center of the atom and contains the heaviest particles, which are the protons and neutrons. Due to the significantly greater mass of these particles compared to electrons, the nucleus accounts for virtually all of an atom's mass. Protons within the nucleus carry a positive charge, giving the entire nucleus a net positive charge.

Surrounding the nucleus is the electron cloud, a largely empty space where negatively charged electrons orbit. While not physically located in the nucleus, the electrons are crucial for understanding the atom's overall charge. In a neutral atom, the number of negatively charged electrons is precisely balanced by the number of positively charged protons, resulting in an overall charge of zero. This foundational understanding of the atom as a whole is essential context for appreciating the specific role and composition of the nucleus.

The nucleus is composed of two primary subatomic particles: protons and neutrons, collectively known as nucleons. These particles define the fundamental identity and characteristics of an atom.

3.1 Protons: The Positive Identity

Protons are subatomic particles located within the atomic nucleus. Their properties are central to the identity of an atom:

• Relative Charge: A proton has a relative charge of +1. This positive charge is the reason the nucleus itself has a positive charge, and it creates the electrostatic force that holds the negatively charged electrons in orbit.
• Relative Mass: A proton has a relative mass of 1 atomic mass unit (amu). This is approximately equal to the mass of a neutron.

The number of protons within a nucleus is the most important property of an element. This number is called the atomic number or proton number (Z). The proton number uniquely identifies an element, and no two elements can have the same number of protons. This is why elements are arranged on the periodic table in order of increasing atomic number. For example, every atom with 6 protons is, by definition, a carbon atom, regardless of how many neutrons it has.

3.2 Neutrons: The Neutral Stabilizers

Neutrons are also found in the nucleus, alongside protons. Their properties are as follows:

• Relative Charge: A neutron has a relative charge of 0, meaning it is neutral.
• Relative Mass: A neutron's relative mass is also 1 amu, making it nearly identical in mass to a proton.

The primary function of neutrons is to add mass to the nucleus without changing its overall charge. They also play a critical role in nuclear stability. The positive charges of multiple protons in a small, confined space would cause them to repel each other. Neutrons help to space out the protons and contribute to the strong nuclear force, which binds the nucleus together and overcomes this repulsive electrostatic force.

The properties of these subatomic particles are summarized in the table below for clarity.

To accurately describe a specific nucleus, scientists use a system of numbers and notation that encapsulates its composition.

4.1 The Proton Number (Z) and Nucleon Number (A)

As established, the proton number (Z), also known as the atomic number, is the count of protons in the nucleus. The nucleon number (A), also referred to as the mass number or atomic mass number, is the total count of all the particles in the nucleus—that is, the sum of protons and neutrons. The concepts are universally applied in physics and chemistry.

The number of neutrons in a nucleus can be easily determined using these two numbers. By subtracting the proton number (Z) from the nucleon number (A), one can calculate the number of neutrons. This is expressed by the formula:

N = A - Z

For example, a nucleus with a nucleon number of 37 and a proton number of 17 contains 20 neutrons (37 - 17 = 20).

4.2 Nuclide Notation

A standardized notation, known as nuclide notation, is used to represent a specific nucleus. This notation provides a concise summary of its composition. The format is represented as follows:

A
Z X

In this format, X is the chemical symbol for the element, A is the nucleon number (or mass number) written as a superscript, and Z is the proton number (or atomic number) written as a subscript. For example, the nuclide notation for carbon-14 is ¹⁴₆C. From this single symbol, it is possible to determine that the nucleus has 6 protons (Z=6), 14 total nucleons (A=14), and 8 neutrons (14 - 6 = 8). In a neutral atom, the number of electrons orbiting the nucleus would also be 6, equal to the number of protons.

The composition of the nucleus in terms of protons and neutrons gives rise to the concept of isotopes.

5.1 Defining Isotopes

Isotopes are atoms of the same element that have an identical number of protons but a different number of neutrons. Because the proton number is constant, all isotopes of an element have the same atomic number and thus belong to the same element on the periodic table. They also have the same number of electrons in a neutral state, meaning their chemical properties are identical. The varying number of neutrons, however, results in a different nucleon number and thus a different mass, which affects their physical properties, such as density.

5.2 A Detailed Case Study: Carbon's Isotopes

A common and well-understood example is that of carbon. Carbon has two stable isotopes: carbon-12 and carbon-13, and one radioactive isotope, carbon-14.

• Carbon-12 has a proton number of 6 and a neutron number of 6, giving it a nucleon number of 12. It is a stable isotope.
• Carbon-14 also has a proton number of 6 but contains 8 neutrons, for a nucleon number of 14. The increased neutron count makes it unstable and radioactive. This is why carbon-14 is used in radiocarbon dating to determine the age of ancient organic materials, as its radioactivity provides a measurable rate of decay (half-life).

This example clearly illustrates how the number of neutrons in the nucleus can dramatically alter an atom's physical properties while its chemical identity remains unchanged.

The following table summarizes the composition of key isotopes of several elements.

The core learning objectives for this syllabus topic revolve around the ability to describe the composition of the nucleus and to apply this knowledge to solve problems, often involving nuclide notation and isotopes.

6.1 Common Question Types

Typical exam questions test the understanding of fundamental definitions and calculations. A common question requires students to analyze nuclide notation to determine the number of protons, neutrons, and electrons in a neutral atom. Another asks for a comparison of different isotopes, focusing on their similarities and differences.

6.2 Worked Examples

To illustrate these concepts, consider the following examples based on common exam-style questions:

Example 1: Determining Composition from Notation

A neutral atom of carbon-14 is represented by the nuclide notation ¹⁴₆C. State the number of protons, electrons, and neutrons in the atom.

• Protons: The proton number (Z) is the subscript, which is 6. Therefore, there are 6 protons in the nucleus.
• Electrons: The atom is neutral, which means the number of electrons must equal the number of protons. Thus, there are 6 electrons orbiting the nucleus.
• Neutrons: The number of neutrons is found by subtracting the proton number from the nucleon number (A - Z). The nucleon number (A) is the superscript, 14. So, 14 - 6 = 8 neutrons.

Example 2: Comparing Isotopes

Compare the nucleus of carbon-14 with the nucleus of carbon-12, stating the similarities and differences.

• Similarity: The nuclei of both carbon-12 and carbon-14 have the same number of protons (6 protons), which is what makes them both isotopes of the element carbon.
• Differences: The nucleus of carbon-14 has a different number of neutrons (8 neutrons) compared to carbon-12 (6 neutrons). This difference in neutron count also means they have a different nucleon number (14 vs. 12).

The composition of the nucleus is a cornerstone of nuclear physics. At its most basic, the nucleus is a compact structure of protons and neutrons. The number of protons defines the element and its chemical properties, while the number of neutrons contributes to its mass and helps stabilize the nucleus. The total count of these particles constitutes the nucleon number. The existence of isotopes—atoms of the same element with a varying number of neutrons—highlights the distinct yet interconnected roles of protons and neutrons. A thorough understanding of these fundamental concepts, including the ability to interpret nuclide notation and perform basic calculations, provides the essential foundation for more complex topics in the Cambridge IGCSE 0654 syllabus, such as radioactivity and nuclear decay.