The Structure of the Atom and Atomic Spectra

The particles in the nucleus are collectively called nucleons. The total number of protons and neutrons is called the nucleon number ($A$). The relationship is expressed as:

$$\begin{aligned} A &= Z + N \end{aligned}$$

where $N$ is the number of neutrons.

Understanding Isotopes

Not all atoms of the same element are identical in mass. Isotopes are nuclei of the same element that have the same number of protons (and thus the same chemical properties) but different numbers of neutrons. For example, Hydrogen has three isotopes: the standard Hydrogen ($^1\text{H}$), Deuterium ($^2\text{H}$), and Tritium ($^3\text{H}$).

Exam Tip: Because isotopes have the same number of protons and electrons, they behave identically in chemical reactions. They only differ in their physical properties, such as density.

Spectra: The Elemental Fingerprint

We can identify elements and study their structure by analyzing light using a spectroscope, a device that separates light into its constituent wavelengths, much like a prism.

Emission and Absorption

When a gas is subjected to high voltage, it undergoes ionization, where electrons are stripped or excited, causing the gas to emit light.

• Emission Spectrum: This consists of bright, discrete colored lines against a dark background. Because each element emits specific wavelengths, it acts as an "elemental fingerprint".
• Absorption Spectrum: This occurs when a continuous light source passes through a cool gas. The gas absorbs specific wavelengths, leaving dark lines in the spectrum.

Energy Levels and Photons

The existence of discrete lines in a spectrum provides direct evidence that electrons exist in specific, "quantized" energy levels. When an electron absorbs energy (via a photon), it jumps to a higher energy level, becoming an excited atom. When it falls back to a lower level (de-excitation), it releases that energy as a photon of a specific wavelength.

The energy of the emitted photon ($E$) is related to its frequency ($f$) by Planck's constant ($h$):

$$\begin{aligned} E &= hf \end{aligned}$$

And since $c = \lambda f$ (where $c$ is the speed of light and $\lambda$ is wavelength):

$$\begin{aligned} E &= \frac{hc}{\lambda} \end{aligned}$$

Fundamental Forces in the Nucleus

If the nucleus contains multiple protons, they should repel each other due to their positive charges (the Coulomb force). What keeps the nucleus from flying apart?

The answer is the strong attractive nuclear force. This force is incredibly powerful, but it is also very short-range. It acts effectively only within the nucleus; once particles are separated by a distance roughly equal to the diameter of an atom ($\approx 10^{-10}\text{ m}$), the strong force drops to zero.