Atomic Structure & Subatomic Particles

Modern Atomic Theory – From Dalton to the Quantum Model

Atomic Structure

Now that we know the history, let's look at the modern understanding of atomic structure.

Subatomic Particles

Every atom is made of three types of subatomic particles:

Atomic Number and Mass Number

Notation

An atom is represented as:

$^{A}_{Z}X$

where $X$ is the element symbol, $A$ is the mass number (top), and $Z$ is the atomic number (bottom).

Calculating Subatomic Particles

Ions – Cations and Anions

Isotopes, Isobars & Isotones

Isotopes

Hydrogen Isotopes – The Classic Example

All three are hydrogen (1 proton each), but they differ in neutron count and mass.

Carbon Isotopes

Oxygen Isotopes

Isobars

Isotones

Isoelectronic Species

Summary Comparison

Average Atomic Mass

The atomic mass listed on the periodic table is NOT the mass of any single atom – it's the weighted average of all naturally occurring isotopes, based on their relative abundance.

Radioactive Radiation – Alpha, Beta & Gamma

Some nuclei are unstable – they have too many or too few neutrons relative to protons, or they are simply too large. These unstable nuclei undergo radioactive decay, spontaneously emitting particles or energy to become more stable. The three main types of radiation are alpha (α), beta (β), and gamma (γ).

Alpha Radiation (إشعاع ألفا)

Properties of alpha radiation:

• Charge: +2
• Mass: 4 amu (heaviest of the three)
• Speed: ~5–7% the speed of light
• Penetrating power: Very low – stopped by a sheet of paper or a few centimetres of air
• Ionising power: Very high – strong ability to knock electrons off atoms (because of large charge and mass)
• Deflection: Deflected by electric and magnetic fields (toward the negative plate)

Alpha decay equation – General form:

$^{A}_{Z}X \rightarrow ^{A-4}_{Z-2}Y + ^{4}_{2}\text{He}$

The parent atom loses 2 protons and 2 neutrons, so the atomic number drops by 2 (new element!) and the mass number drops by 4.

Beta Radiation (إشعاع بيتا)

Properties of beta radiation:

• Charge: −1
• Mass: ~$\frac{1}{1836}$ amu (negligible compared to nucleons)
• Speed: Up to ~99% the speed of light
• Penetrating power: Medium – stopped by a few millimetres of aluminium or a few metres of air
• Ionising power: Medium – less than alpha (smaller charge and mass) but more than gamma
• Deflection: Deflected by electric and magnetic fields (toward the positive plate – opposite to alpha)

Beta decay equation – General form:

$^{A}_{Z}X \rightarrow ^{A}_{Z+1}Y + ^{0}_{-1}e$

The mass number stays the same (a neutron became a proton – total nucleons unchanged), but the atomic number increases by 1 (one more proton → new element).

Gamma Radiation (إشعاع جاما)

Properties of gamma radiation:

• Charge: 0
• Mass: 0
• Speed: Speed of light ($3 \times 10^8$ m/s)
• Penetrating power: Very high – requires several centimetres of lead or metres of concrete to significantly reduce intensity
• Ionising power: Very low (no charge, no mass – interacts weakly with matter)
• Deflection: NOT deflected by electric or magnetic fields (no charge)

Gamma emission:

$^{A}_{Z}X^* \rightarrow ^{A}_{Z}X + \gamma$

The asterisk (*) indicates an excited nuclear state. Gamma emission does NOT change the atomic number or mass number – it just releases excess energy. Gamma rays often accompany alpha or beta decay.

Comparison of the Three Types

Skills Practised in This LO

Vocabulary

Key Takeaways

Test Yourself

Reference Materials

This confirmed electrons are incredibly light – about $\frac{1}{1836}$ the mass of a hydrogen atom.

Rutherford's Nuclear Model:

• The atom has a tiny, dense, positively charged nucleus at the centre (containing most of the mass)
• Electrons orbit the nucleus at a distance, like planets orbiting the sun
• Most of the atom is empty space
• The nucleus is ~$10^{-15}$ m in diameter; the whole atom is ~$10^{-10}$ m – the nucleus is 100,000 times smaller than the atom

Limitation: Rutherford's model couldn't explain why electrons don't spiral into the nucleus (a moving charged particle should radiate energy and collapse into the centre according to classical physics).

where $h$ is Planck's constant and $f$ is the frequency of the emitted/absorbed photon.

4. Electrons in their lowest possible energy level are in the ground state (الحالة المستقرة).

What Bohr explained: The hydrogen emission spectrum – the specific colours of light emitted by excited hydrogen atoms. His model correctly predicted the wavelengths of hydrogen's spectral lines.

Limitation: Bohr's model only works well for hydrogen (one electron). It fails for multi-electron atoms because it doesn't account for electron-electron repulsion.

Properties of the neutron:

• Mass ≈ mass of a proton ($1.675 \times 10^{-27}$ kg ≈ 1 amu)
• Charge = 0 (neutral)
• Located in the nucleus alongside protons

The discovery of the neutron completed the picture of the three fundamental subatomic particles: proton, neutron, and electron.