Structure of Atom

The One Thing

Atoms consist of a small, dense nucleus of protons and neutrons surrounded by electrons occupying quantized orbitals. Modern atomic structure is explained by the quantum-mechanical model, in which electron energies, locations, and configurations are governed by quantum numbers and established filling rules.

Reactions, Processes and Experiments

Key Terms

• Atom: The smallest electrically neutral unit of an element that retains the element's chemical identity.
• Electron: A negatively charged subatomic particle with a relative mass of about 1/1837 and charge of -1.602 × 10^-19 C.
• Proton: A positively charged particle present in the nucleus, having a relative charge of +1 and relative mass close to 1 u.
• Neutron: An electrically neutral particle in the nucleus with a relative mass close to 1 u.
• Atomic number: The number of protons in the nucleus, represented by Z; it identifies an element.
• Mass number: The total number of protons and neutrons in an atom, represented by A: A = Z + N.
• Isotopes: Atoms of the same element having the same atomic number but different mass numbers because they contain different numbers of neutrons.
• Thomson model: A model in which electrons are embedded in a positively charged sphere; it could not explain the results of alpha-particle scattering.
• Rutherford nuclear model: A model proposing a tiny, positively charged nucleus containing most of the atom's mass, with electrons around it and mostly empty space within the atom.
• Bohr model: A model in which electrons move in permitted stationary orbits with fixed energies and do not continuously lose energy.
• Quantum mechanical model: The modern model that describes electrons using wave functions and probability distributions rather than fixed circular paths.
• Electromagnetic radiation: Energy that travels through space as oscillating electric and magnetic fields.
• Wavelength: The distance between two successive crests or troughs of a wave, represented by lambda.
• Frequency: The number of wave cycles passing a point per second, represented by nu and measured in hertz.
• Photon: A discrete packet of electromagnetic energy with energy E = hν.
• Photoelectric effect: The emission of electrons from a metal surface when light of sufficiently high frequency falls on it.
• Atomic spectrum: The characteristic pattern of wavelengths or frequencies emitted or absorbed by an atom.
• Quantum number: A number that specifies the energy, size, shape, orientation, or spin state of an electron in an atom.
• Principal quantum number: The quantum number n, which identifies the main shell and indicates the approximate energy and size of an orbital; n = 1, 2, 3, ... .
• Azimuthal quantum number: The quantum number l, which identifies the subshell and orbital shape; for a given n, l = 0 to n - 1.
• Magnetic quantum number: The quantum number m_l, which specifies the orientation of an orbital; its values range from -l to +l.
• Spin quantum number: The quantum number m_s, which describes electron spin and has values +1/2 or -1/2.
• Orbital: A region around the nucleus where the probability of finding an electron is high; an orbital can hold a maximum of two electrons.
• s, p, d, and f subshells: Subshells corresponding to l values 0, 1, 2, and 3; they contain 1, 3, 5, and 7 orbitals respectively.
• Heisenberg uncertainty principle: It is impossible to determine simultaneously and exactly both the position and momentum of an electron.
• Aufbau principle: Electrons occupy orbitals in order of increasing energy.
• Pauli exclusion principle: No two electrons in an atom can have the same set of all four quantum numbers; an orbital can contain at most two electrons with opposite spins.
• Hund's rule: Electrons occupy degenerate orbitals singly with parallel spins before pairing begins.
• Electronic configuration: The distribution of electrons among shells, subshells, and orbitals.
• Valence electrons: Electrons in the outermost shell or those that participate most directly in chemical bonding.

Easily Confused

• Atomic number and mass number: Atomic number is the number of protons, Z; mass number is the total number of protons and neutrons, A = Z + N.
• Isotopes and ions: Isotopes differ in neutron number, whereas ions differ in electron number while their nuclear proton number remains unchanged.
• Orbit and orbital: A Bohr orbit is a permitted fixed-energy path in the Bohr model; an orbital is a probability region in the quantum-mechanical model.
• Wavelength and frequency: Wavelength is the distance between successive wave crests or troughs, whereas frequency is the number of cycles passing a point per second; they are related by c = λν.
• Bohr model and quantum mechanical model: The Bohr model uses fixed circular orbits, whereas the quantum mechanical model uses orbitals, wave functions, and probability distributions.
• Aufbau principle and Hund's rule: Aufbau determines the order in which orbitals of increasing energy are filled; Hund's rule determines how electrons occupy orbitals of equal energy.
• Orbital capacity and subshell capacity: An individual orbital holds a maximum of two electrons, while s, p, d, and f subshells hold 2, 6, 10, and 14 electrons respectively.
• Electron and proton charge: An electron has charge -1.602 × 10^-19 C, while the proton has charge +1.602 × 10^-19 C.

What Gets Asked

• Identifying evidence for the nuclear model: Questions may ask what Rutherford's alpha-particle scattering experiment demonstrated. The mark-losing error is failing to state both that most of the atom is empty space and that positive charge and most mass are concentrated in a small nucleus.
• Comparing atomic models: Questions may require distinctions between the Thomson model, Rutherford nuclear model, Bohr model, and quantum mechanical model. The key slip is describing an orbital as a fixed circular path.
• Using radiation equations: Problems may require use of c = λν, E = hν = hc/λ, or ΔE = E_final - E_initial = hν. Errors include confusing wavelength with frequency or omitting the photon-energy relationship.
• Applying quantum numbers: Questions may ask for allowed values of n, l, m_l, and m_s. The required restrictions are n = 1, 2, 3, ...; l = 0 to n - 1; m_l = -l to +l; and m_s = +1/2 or -1/2.
• Counting orbitals and electrons: Questions may use the relations that a shell contains n^2 orbitals and a maximum of 2n^2 electrons, while a subshell contains 2l + 1 orbitals and holds 2(2l + 1) electrons. The common error is applying a shell capacity to a subshell.
• Writing electronic configurations: Questions may test the increasing-energy order, the Aufbau principle, Pauli exclusion principle, Hund's rule, and the exceptions Cr is [Ar] 3d^5 4s^1 and Cu is [Ar] 3d^10 4s^1. The key slip is writing the regular filling pattern without accounting for these exceptions.