Electrons in the Atom

Chapter 5

Section 5.1 Light and Quantized Energy

1. The Nuclear Atom and Unanswered Questions

• Recall that in Rutherford's model, the atom’s mass is concentrated in the nucleus and electrons move around it.

• The model doesn’t explain how the electrons were arranged around the nucleus.

• The model doesn’t explain why negatively charged electrons aren’t pulled into the positively charged nucleus.

• In the early 1900s, scientists observed certain elements emitted visible light when heated in a flame.

• Analysis of the emitted light revealed that an element’s chemical behavior is related to the arrangement of the electrons in its atoms.

2. The Wave Nature of Light

• Visible light is a type of electromagnetic radiation, a form of energy that exhibits wave-like behavior as it travels through space.

• All waves can be described by several characteristics.

• The wavelength (λ) is the shortest distance between equivalent points on a continuous wave.

• The amplitude is the wave’s height from the origin to a crest.

The Wave Nature of Light (cont.)

• The frequency (ν) is the number of waves that pass a given point per second.

• One hertz (Hz), the SI unit of frequency, equals one wave per second

• The speed of light (3.00 108 m/s) is the product of it’s wavelength and frequency c = λν.

• Although the speed of all electromagnetic waves is the same, waves may have different wavelengths and frequencies.

The Wave Nature of Light (cont.)

• Sunlight contains a continuous range of wavelengths and frequencies.

• A prism separates sunlight into a continuous spectrum of colors.

• Continuous means there is no portion of it that does not correspond to a unique wavelength and frequency of light

• The electromagnetic spectrum (EM) includes all forms of electromagnetic radiation.

• Energy increases with increasing frequency.

3. The Particle Nature of Light

• The wave model of light cannot explain all of light’s characteristics.

• Matter can gain or lose energy only in small, specific amounts called quanta.

• A quantum is the minimum amount of energy that can be gained or lost by an atom.

Equantum = h

• Planck’s constant has a value of 6.626 10–34 J ● s.

• The Joule (J) is the SI unit of energy

The Particle Nature of Light (cont.)

• The photoelectric effect is when electrons are emitted from a metal’s surface when light of a certain frequency shines on it.

• Albert Einstein proposed in 1905 that light has a dual nature.

• A beam of light has wavelike and particlelike properties.

• A photon is a particle of electromagnetic radiation with no mass that carries a quantum of energy.

Ephoton = h Ephoton represents energy.h is Planck's constant. represents frequency.

4. Atomic Emission Spectra

• Light in a neon sign is produced when electricity is passed through a tube filled with neon gas and excites the neon atoms.

• The excited atoms emit light to release energy.

• The atomic emission spectrum of an element is the set of frequencies of the electromagnetic waves emitted by the atoms of the element.

• Each element’s atomic emission spectrum is unique.

End of Section 5.1

Section 5.2 Quantum Mechanics and the Atom

5. Bohr's Model of the Atom

• Bohr correctly predicted the frequency lines in hydrogen’s atomic emission spectrum. The lowest allowable energy state of an atom is called its ground state.

• When an atom gains energy, it is in an excited state.

• Bohr suggested that an electron moves around the nucleus only in certain allowed circular orbits.

• Each orbit was given a number, called the quantum number.

Bohr's Model of the Atom (cont.)

• Hydrogen’s single electron is in the n = 1 orbit in the ground state.

• When energy is added, the electron moves to the n = 2 orbit.

• Bohr’s model explained the hydrogen’s spectral lines, but failed to explain any other element’s lines.

• The behavior of electrons is still not fully understood, but it is known they do not move around the nucleus in circular orbits.

6. The Quantum Mechanical Model of the Atom

• Louis de Broglie (1892–1987) hypothesized that particles, including electrons, could also have wavelike behaviors.

• The figure illustrates that electrons orbit the nucleus only in whole-number wavelengths.

• The de Broglie equation predicts that all moving particles have wave characteristics.

The Quantum Mechanical Model of the Atom (cont.)

• Heisenberg showed it is impossible to take any measurement of an object without disturbing it.

• The Heisenberg uncertainty principle states that it is fundamentally impossible to know precisely both the velocity and position of a particle at the same time.

• The only quantity that can be known is the probability for an electron to occupy a certain region around the nucleus.

The Quantum Mechanical Model of the Atom (cont.)

• Schrödinger treated electrons as waves in a model called the quantum mechanical model of the atom.

• Schrödinger’s equation applied equally well to elements other than hydrogen.

• The wave function predicts a three-dimensional region around the nucleus called the atomic orbital.

• Principal quantum number (n) indicates the relative size and energy of atomic orbitals.

7. Hydrogen Atomic Orbitals

• n specifies the atom’s major energy levels, called the principal energy levels.

• Energy sublevels are contained within the principal energy levels.

• Each energy sublevel relates to orbitals of different shape.

End of Section 5.2

Section 5-3 Electron Configuration

8. Ground-State Electron Configuration

• The arrangement of electrons in the atom is called the electron configuration.

• The aufbau principle states that each electron occupies the lowest energy orbital available.

Ground-State Electron Configuration (cont.)

• The Pauli exclusion principle states that a maximum of two electrons can occupy a single orbital, but only if the electrons have opposite spins.

• Hund’s rule states that single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same energy level orbitals.

Ground-State Electron Configuration (cont.)

• Noble gas notation uses noble gas symbols in brackets to shorten inner electron configurations of other elements.

• The electron configurations (for chromium, copper, and several other elements) reflect the increased stability of half-filled and filled sets of s and d orbitals.

9. Valence Electrons

• Valence electrons are defined as electrons in the atom’s outermost orbitals—those associated with the atom’s highest principal energy level.

• Electron-dot structure consists of the element’s symbol representing the nucleus, surrounded by dots representing the element’s valence electrons.

End of Section 5.3

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Figure 5.11 Balmer Series

Figure 5.12 Electron Transitions

Table 5.4 Electron Configurations and Orbital Diagrams for Elements 1–10

Table 5.6 Electron Configurations and Dot Structures

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