Arrangement of Electrons in the Atom - Summary Chapter 3

Arrangement of Electrons in the Atom

Introduction

• Niels Bohr provided insights into the arrangement of electrons in the atom.

Bohr's Study of Spectra

• White light through a prism creates a continuous spectrum.

• A hydrogen discharge tube produces a line spectrum.

• Each element has a unique emission line spectrum, like a fingerprint.

• Spectrometers are used to measure spectra, while spectroscopes are used to observe spectra.

Flame Tests

• Different metals emit characteristic colors when heated, allowing for identification.

• Examples:

  • Lithium: Crimson

  • Sodium: Yellow

  • Potassium: Lilac

  • Barium: Green

  • Strontium: Red

  • Copper: Blue-green

Explaining the Evidence: The Bohr Theory

• Bohr's Theory introduced the concept of quantization of energy, where electrons can only have fixed amounts of energy.

• Electrons revolve around the nucleus in fixed paths called orbits or energy levels.

• Energy levels are represented by the letter n (n=1, n=2, etc.).

• Electrons in the ground state occupy the lowest available energy levels.

• When an atom absorbs energy, electrons jump to higher energy levels, entering an excited state.

• When electrons fall back to lower energy levels, they release energy in the form of photons of light.

• The frequency of emitted light is related to the energy difference: E2 - E1 = hf, where h is Planck's constant and f is the frequency.

• Each element has a unique emission line spectrum due to different electron arrangements and transitions.

• Bohr calculated energy levels and wavelengths, matching experimental measurements.

Atomic Absorption Spectrometry (AAS)

• Atoms can absorb light; atomic absorption spectrum shows dark lines where light is absorbed.

• Atomic absorption spectrometry detects elements in water and measures their concentrations.

• The amount of light absorbed is directly proportional to the concentration of the element.

• AAS is used in various applications, including water analysis and forensic science.

• The principle is based on:

  • Atoms absorbing light at characteristic wavelengths.

  • Absorbed light being proportional to the element's concentration.

• Lamps emitting the element's line spectrum are used instead of white light.

Energy Sublevels

• Emission spectra reveal that single lines are often composed of multiple closely spaced lines.

• Each main energy level (except the first) consists of sublevels close in energy.

• The number of sublevels equals the value of n for the main energy level.

• Sublevels are labeled s, p, d, and f in order of increasing energy.

Wave Nature of the Electron

• Louis de Broglie suggested moving particles have wave motion (wave-particle duality).

• Heisenberg's Uncertainty Principle states that it is impossible to simultaneously measure both the velocity and position of an electron.

• We can only talk about the probability of finding an electron at a particular position.

Atomic Orbitals

• An orbital is a region in space with a high probability of finding an electron.

• Chemists draw a boundary within which there is a very high probability (approximately 95%) of finding an electron.

• All s orbitals are spherical.

• p orbitals are dumbbell-shaped and oriented along three main axes (px, py, p_z).

• A sublevel is a subdivision of a main energy level and consists of one or more orbitals of the same energy.