The atom

Atoms are the fundamental building blocks of matter, and understanding their structure is crucial in physics. In this section, we will explore the atomic structure, the nucleus, atomic numbers and nuclear notation.

Structure of the atom

Atoms consist of two main components:

1. The nucleus is the dense center of an atom, made up of protons and neutrons.

• Protons are positively charged particles (charge of +1). The number of protons in the nucleus determines the identity of the element.

• Neutrons are neutral, meaning they have no charge (charge of 0).

• The nucleus holds nearly all the mass of an atom because both protons and neutrons are much heavier than electrons.

2. Electrons:

   • Negatively charged particles (charge of -1) that orbit the nucleus in specific energy levels (or shells).

   • Their movement balances the positive charge of the nucleus, making the atom electrically neutral overall.

   • Have only 1/1840th of the mass of a proton or neutron.

The positive charge of protons and the negative charge of electrons balance each other out, making an atom electrically neutral overall.

The structure of an atom

Charge of the nucleus and relative mass

The proton number determines the charge of the nucleus since protons are positively charged. For example, a nucleus with 5 protons has a charge of +5.

Since protons and neutrons each have significant mass compared to electrons, the nucleon number is directly related to the relative mass of the nucleus.

Proton number and nucleon number

• The Proton Number (often called the Atomic Number, Z) is the number of protons in the nucleus of an atom. This number defines the element (e.g., hydrogen has 1 proton, helium has 2). 

• The Nucleon Number (or Mass Number, A) is the total number of protons and neutrons in the nucleus. 

• To find the number of neutrons, you can use the formula:

Number of neutrons = A − Z

• For example, if an element has a mass number (A) of 23 and a proton number (Z) of 11, then it has:

23 − 11 = 12 neutrons

Nuclear notation

Nuclear notation is a standardised way to represent information about an atom.

• It is often written as:

Where:

• A is the Nucleon Number (total number of protons and neutrons).

• Z is the Proton Number (number of protons).

• X is the chemical symbol of the element.

For example, the nuclear notation for carbon-12 is: