The atom Revision Notes

An atom is composed of three fundamental subatomic particles: protons, neutrons, and electrons.

Protons are positively charged particles found within the nucleus of the atom.

Neutrons have no electric charge (they are neutral) and are also located in the nucleus alongside protons, contributing to the atom's mass but not its charge. Their relative mass is approximately equal to that of protons (about 1 atomic mass unit). Electrons are negatively charged particles that orbit the nucleus in defined energy levels or shells. Their mass is much smaller than that of protons or neutrons (approximately 1/1836 of a proton's mass).

• Proton: charge +1e, relative mass ≈ 1
• Neutron: charge 0, relative mass ≈ 1
• Electron: charge −1e, relative mass ≈ 1/1836

The nucleus is the dense central core of the atom, containing protons and neutrons tightly bound together.

Electrons move around the nucleus in regions called electron shells or energy levels, which are arranged at increasing distances from the nucleus.

The nucleus is very small compared to the overall size of the atom but contains nearly all of the atom's mass because protons and neutrons are much heavier than electrons.

• Nucleus contains protons and neutrons
• Electrons orbit nucleus in shells
• Atom size mostly empty space

Protons have a positive charge (+1 elementary charge), and electrons have an equal but opposite negative charge (−1 elementary charge).

Neutrons have no charge and do not affect the electrical charge of the atom.

In a neutral atom, the number of protons equals the number of electrons, so their charges cancel out, making the atom electrically neutral.

• Charge of proton = +1.6 × 10⁻¹⁹ C
• Charge of electron = −1.6 × 10⁻¹⁹ C
• Neutral atom: number of protons = number of electrons

Protons and neutrons each have a relative mass of approximately 1 atomic mass unit (u), where 1 u = 1.66 × 10⁻²⁷ kg.

Electrons have negligible mass compared to protons and neutrons, about 1/1836 of a proton's mass.

The radius of the nucleus is about 10⁻¹⁵ m, whereas the radius of the whole atom is about 10⁻¹⁰ m, meaning the atom is mostly empty space.

• Mass of proton ≈ 1.67 × 10⁻²⁷ kg
• Mass of neutron ≈ 1.67 × 10⁻²⁷ kg
• Mass of electron ≈ 9.11 × 10⁻³¹ kg
• Nucleus radius ≈ 10⁻¹⁵ m
• Atom radius ≈ 10⁻¹⁰ m

To draw a simple atomic model: 1. Draw a small circle at the centre to represent the nucleus. 2. Inside the nucleus, label 'p⁺' for protons and 'n⁰' for neutrons. 3. Around the nucleus, draw concentric circles to represent electron shells. 4. Place small dots or crosses on these shells to represent electrons (e⁻).

This diagram shows the nucleus containing protons and neutrons, with electrons orbiting in shells around it.

• Nucleus: central circle with p⁺ and n⁰
• Electron shells: concentric circles around nucleus
• Electrons: dots or crosses on shells

An atom consists of a nucleus containing protons and neutrons, surrounded by electrons in orbitals. Protons have a positive charge, electrons have a negative charge, and neutrons are neutral. In a neutral atom, the number of electrons equals the number of protons, so the overall charge is zero.

Electrons are the subatomic particles involved in chemical reactions and bonding because they can be transferred or shared between atoms. The outermost electrons, called valence electrons, are the most important in these processes.

• Protons: positive charge (+1)
• Electrons: negative charge (-1)
• Neutrons: no charge
• Neutral atom: equal protons and electrons

When an atom loses one or more electrons, it has more protons than electrons, resulting in a net positive charge. This positively charged particle is called a positive ion or cation.

For example, a sodium atom (Na) has 11 protons and 11 electrons. If it loses one electron, it becomes a sodium ion (Na⁺) with 11 protons and 10 electrons, giving it a charge of +1.

The loss of electrons requires energy, often supplied during chemical reactions, and is common in metals which tend to form positive ions.

• Positive ion = atom - electrons
• Charge = number of protons - number of electrons
• Example: Na → Na⁺ + e⁻
• Metals tend to form cations

When an atom gains one or more electrons, it has more electrons than protons, resulting in a net negative charge. This negatively charged particle is called a negative ion or anion.

For example, a chlorine atom (Cl) has 17 protons and 17 electrons. If it gains one electron, it becomes a chloride ion (Cl⁻) with 17 protons and 18 electrons, giving it a charge of -1.

Non-metals commonly gain electrons to form negative ions during chemical reactions.

• Negative ion = atom + electrons
• Charge = number of protons - number of electrons
• Example: Cl + e⁻ → Cl⁻
• Non-metals tend to form anions

The charge on an ion is equal to the difference between the number of protons and electrons:

Charge (in elementary charge units) = Number of protons − Number of electrons

Since protons are fixed in the nucleus, ions form by changing the number of electrons only. Losing electrons produces positive charge; gaining electrons produces negative charge.

• Charge = protons − electrons
• Electrons transferred during chemical reactions
• Ions have net electric charge

Ions are essential in the formation of ionic compounds, where positive and negative ions attract each other by electrostatic forces to form neutral compounds.

For example, sodium ions (Na⁺) and chloride ions (Cl⁻) combine to form sodium chloride (NaCl), common table salt.

Understanding ion formation helps explain electrical conductivity in solutions and the behaviour of substances in chemical reactions.

• Ionic compounds formed by attraction of cations and anions
• Example: Na⁺ + Cl⁻ → NaCl
• Ions conduct electricity in solution

Alpha particles are helium nuclei consisting of two protons and two neutrons, carrying a +2e positive charge. In the early 20th century, Ernest Rutherford and his colleagues conducted experiments by directing a beam of alpha particles at a very thin metal foil, typically gold, to investigate atomic structure.

The experiment involved a source emitting alpha particles aimed at the thin foil. A fluorescent screen or detector surrounded the foil to observe the deflection angles of the scattered alpha particles. The setup allowed measurement of how many alpha particles passed straight through, were slightly deflected, or were deflected at large angles, including some bouncing back.

• Alpha particles: helium nuclei, charge +2e, mass approximately 4 atomic mass units.
• Thin metal foil: usually gold, a few atoms thick to allow passage of alpha particles.
• Detection: fluorescent screen or detector records scattering angles.

Most alpha particles passed straight through the foil with little or no deflection, indicating that most of the atom is empty space. A small fraction of alpha particles were deflected at large angles, and a very few were scattered backwards.

These observations contradicted the then-accepted plum pudding model, which predicted only small deflections due to a diffuse positive charge. Instead, the large-angle deflections suggested a concentrated positive charge within the atom.

• Majority of alpha particles pass through undeflected.
• Some alpha particles deflected at small angles.
• Very few alpha particles deflected at angles greater than 90°, i.e., backwards.

The large-angle deflections occur because alpha particles experience strong electrostatic repulsion when they come close to a small, dense, positively charged nucleus. The nucleus contains most of the atom's mass, explaining why alpha particles can be deflected sharply without the atom being destroyed.

Since most alpha particles pass through undeflected, the nucleus must be very small compared to the overall size of the atom, which is mostly empty space occupied by electrons.

The positive charge of the nucleus is due to protons, which repel the positively charged alpha particles. This explains the observed scattering pattern.

• Nucleus is very small compared to the atom's size.
• Nucleus contains nearly all the atom's mass.
• Nucleus has a positive charge causing repulsion of alpha particles.

The electrostatic force between the alpha particle and the nucleus is described by Coulomb's law:

\[ F = \frac{1}{4\pi\epsilon_0} \frac{Q_1 Q_2}{r^2} \]

where \( Q_1 \) and \( Q_2 \) are the charges of the alpha particle and nucleus, \( r \) is the separation distance, and \( \epsilon_0 \) is the permittivity of free space (\(8.85 \times 10^{-12} \ \mathrm{C^2 N^{-1} m^{-2}}\)). The strong repulsive force at close distances causes large deflections of alpha particles that approach the nucleus closely.

• Coulomb's law governs the repulsive force between alpha particles and nucleus.
• Large deflections occur when alpha particles approach within a very small distance of the nucleus.
• The nucleus radius is on the order of 10⁻¹⁴ m, much smaller than the atom's radius (~10⁻¹⁰ m).

To draw a diagram illustrating alpha particle scattering:

1. Draw a thin vertical rectangle to represent the thin metal foil (gold foil).

2. Draw several straight arrows (alpha particle paths) approaching the foil from the left, mostly passing straight through to the right side with little or no deflection, labelled as 'Most alpha particles pass through undeflected'.

• Include a few arrows deflected at small angles to the right, labelled 'Small angle deflections'.
• Include one or two arrows deflected at large angles, even backwards, labelled 'Large angle deflections due to nucleus'.
• Draw a small circle inside the foil representing the nucleus, labelled 'Small, dense, positively charged nucleus'.