Fundamental Particles

01Fundamental Particles

Development of the atomic model

This section covers how experimental evidence changed ideas about atomic structure from early indivisible atoms to nuclear and energy-level models.

How ideas about atoms changed

Successive experimental findings led chemists to replace or refine earlier pictures of atomic structure.

Early model: the atom was represented simply as a solid sphere with no internal structure.
Plum-pudding model: after the electron was discovered, electrons were placed throughout an atom whose positive charge was spread over its volume.
Nuclear model: alpha-particle scattering provided evidence for a tiny central nucleus containing the positive charge and most of the atomic mass.
Bohr model: electrons were restricted to particular energy levels, or shells, at specific distances from the nucleus. The success of the model was supported by agreement between its predictions and experimental observations.

Evidence from alpha-particle scattering

Most particles continued through the foil: only a small fraction of the atom's volume is occupied by concentrated matter.
Some particles changed direction: they had encountered a concentrated region of positive charge.
Very few were sent back: the central region must be extremely small and contain most of the atomic mass.

Exam Tip: Present scattering explanations as observation → conclusion. When comparing models, name each model explicitly rather than referring vaguely to an “old” and a “new” model.

02Fundamental Particles

Subatomic particles

This section covers the positions, relative charges and relative masses of protons, neutrons and electrons.

Structure of the atom

The nucleus occupies a tiny central region of the atom. It contains protons and neutrons, which are collectively called nucleons. Electrons occupy orbitals in the surrounding space, so much of the atom's volume is empty.

Only the protons contribute electrical charge inside the nucleus, because neutrons are neutral. The nucleus is therefore positively charged.

Protons and neutrons each have a relative mass close to 1, whereas an electron has a much smaller relative mass. Consequently, almost all of an atom's mass is associated with its nucleus.

The positive nucleus and negative electrons attract one another electrostatically.

Relative mass and charge

Particle masses and charges are expressed on relative scales so that the three fundamental particles can be compared without quoting extremely small values in conventional units.

Particle	Position	Relative charge	Relative mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0	1
Electron	Orbitals around the nucleus	−1	Very small, about $\dfrac{1}{1840}$

03Fundamental Particles

Atomic and mass number

This section covers atomic number, mass number, nuclear notation and relationships between the numbers of protons, neutrons and electrons.

Atomic number and mass number

Atomic number (proton number), $Z$ : the count of protons in the nucleus. This value identifies the element.

Mass number (nucleon number), $A$ : the number of nucleons in the nucleus, found by adding the numbers of protons and neutrons.

^{A}_{Z}\mathrm{X}

$X$ is the element symbol.
$A$ gives the mass number.
$Z$ gives the atomic number.

Quantity	How to determine it
Protons	$Z$
Neutrons	$A-Z$
Electrons in a neutral atom	$Z$

Example: lithium-7

For $\ce{^{7}_{3}Li}$ , the lower number shows that there are 3 protons and the upper number shows that there are 7 nucleons altogether.

Protons $=3$
Neutrons $=7-3=4$
Electrons $=3$ because the atom has no overall charge

number of neutrons

=A-Z

Exam Tip: Define mass number by counting the nucleons in the nucleus: protons + neutrons. Do not describe it as an average or as a measured mass.

04Fundamental Particles

Atoms and ions

This section covers how gaining or losing electrons forms ions and how particle numbers are determined from atomic number, mass number and charge.

How charge changes the electron count

An atom is neutral when its proton and electron numbers are equal. Changing the number of electrons upsets this balance and produces an ion; the number of protons remains unchanged.

Positive ions

Electron loss leaves more protons than electrons, giving the species a positive charge.

Negative ions

Electron gain leaves more electrons than protons, giving the species a negative charge.

Species	Protons	Electrons	Neutrons
Neutral atom	$Z$	$Z$	$A-Z$
Ion with charge $n+$	$Z$	$Z-n$	$A-Z$
Ion with charge $n-$	$Z$	$Z+n$	$A-Z$

Worked example: Find the numbers of protons, neutrons and electrons in $\ce{^{56}_{26}Fe^{2+}}$ .

1. Protons: $Z=26$ , so the ion has 26 protons.

2. Neutrons:

$56-26=30$

The nucleus therefore contains 30 neutrons.

3. Electrons: before ion formation there would be 26 electrons. A $2+$ charge corresponds to two fewer electrons:

$26-2=24$

The ion has 24 electrons.

Exam Tip: Treat the ionic charge as the difference from the neutral electron count. For example, $2+$ means $Z-2$ electrons, while $2-$ means $Z+2$ electrons. Never alter the proton number when calculating an ion's composition.