S1.2.1 Atomic Structure

Components of the Atom and Nuclear Symbols

The Components of the Atom

Atoms are the fundamental building blocks of matter, but they are not indivisible.
They consist of three types of subatomic particles: protons, neutrons, and electrons.
Each of these particles has distinct properties and plays a specific role in the atom.

1.The Nucleus: Protons and Neutrons

At the heart of the atom lies the nucleus.

Definition

Nucleus

A nucleus is a dense, positively charged core that contains protons and neutrons, collectively called nucleons.

Protons:
- These are positively charged particles with a relative charge of $+ 1$ and a relative mass of $1$ .
- The number of protons in the nucleus determines the atomic number (Z), which defines the element.
Neutrons:
- These particles are neutral, with no charge, and have a relative mass of $1$ .
- Neutrons help to stabilize the nucleus by offsetting the repulsion between protons.
- The number of neutrons can vary between atoms of the same element, leading to isotopes.

Note

Protons and neutrons are approximately 1,836 times more massive than electrons, meaning nearly all the mass of an atom is concentrated in its nucleus.

2.The Energy Levels

Surrounding the nucleus is a vast region of space occupied by electrons, which are negatively charged particles with a relative charge of $- 1$ .
Electrons have a negligible mass compared to protons and neutrons.
- Electrons are arranged in regions of space called orbitals, which define the probability of finding an electron in a certain area.
- In a neutral atom, the number of electrons equals the number of protons, balancing the positive and negative charges.

Analogy

Think of the atom as a stadium: the nucleus is like a golf ball at the center of the field, and the electrons are like tiny flies buzzing around the entire stadium. The vast majority of the atom is empty space!

The structure of atom, where green circles are electrons, blue circles are protons and red circles are neutrons. There are two energy levels illustrated using black line.

Relative Masses and Charges of Subatomic Particles

To understand the behavior of atoms, it’s important to know the relative masses and charges of their subatomic particles.
These are summarized in the table below:

Particle	Relative Mass	Relative Charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	Negligible	-1	Outside the nucleus

Tip

Remember: The mass of an electron is so small compared to protons and neutrons that it is often considered negligible in calculations.

Nuclear Symbols: Decoding the Atom’s Identity

Chemists use nuclear symbols to summarize the structure of an atom or ion.
A nuclear symbol is written in the form:

$_{Z}^{A} X$

Where:

$X$ = Chemical symbol of the element (e.g., $Au$ for gold).
$Z$ = Atomic number (number of protons).
$A$ = Mass number (total number of protons and neutrons).

Example

Gold

For gold, the nuclear symbol is:

$_{79}^{197} Au$

This tells us:

$Z = 79$ : Gold has 79 protons.
$A = 197$ : Gold has a total of 197 nucleons.
Number of neutrons = $A - Z = 197 - 79 = 118$ .

Self review

Can you determine the number of protons, neutrons, and electrons in $_{11}^{23} Na$ ?

Atoms vs. Ions: What Happens When Electrons Change?

While atoms are neutral, they can gain or lose electrons to form ions.

Definition

Ion

An ion is an atom or group of atoms that has gained or lost electrons, resulting in a net electrical charge.

This does not affect the number of protons or neutrons but changes the overall charge of the species.

Cations: Formed when an atom loses electrons, resulting in a positive charge.

Example

Magnesium ion ( ${Mg}^{2 +}$ ) has 12 protons but only 10 electrons.

Anions: Formed when an atom gains electrons, resulting in a negative charge.

Example

An oxide ion ( $O^{2 -}$ ) has 8 protons and 10 electrons.

Example question

Deduce the nuclear symbol for an ion with:

24 protons
21 electrons
28 neutrons

Solution

The atomic number ( $Z$ ) is 24, so the element is chromium ( $Cr$ ).
The mass number ( $A$ ) is $24 + 28 = 52$ .
The ion has $24 - 21 = + 3$ charge.

The nuclear symbol is:

$_{24}^{52} {Cr}^{3 +}$

Example

Magnesium forms a ${Mg}^{2 +}$ ion by losing two electrons. Its nuclear symbol is $_{12}^{24} {Mg}^{2 +}$ . This indicates 12 protons, 12 neutrons ( $24 - 12 = 12$ ), and 10 electrons.

Common Mistake

Students often confuse the mass number ( $A$ ) with the atomic number ( $Z$ ). Remember: $A$ includes both protons and neutrons, while $Z$ is just the number of protons.

Reflection

Theory of Knowledge

How does the development of atomic models reflect the evolving nature of scientific knowledge? Consider how Rutherford’s gold foil experiment overturned the plum-pudding model.

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