1.2:
Electronic Structure of Atoms
An atom is made of three subatomic particles – electrons, protons, and neutrons. Positively charged protons and electrically neutral neutrons constitute the atomic nucleus, where all of the positive charge and most of the mass of an atom reside.
Electrons are the negatively charged, low-mass constituents located in the empty space around the nucleus. An atom is electrically neutral when it has as many electrons as protons.
Electrons exhibit a dual nature — particle and wave. Their location in space and their energy are complementary properties. Therefore, for an electron with a well-defined energy, its position is uncertain.
The quantum mechanical model of an atom predicts that the probability of finding an electron of a given energy in a three dimensional region in space can be determined from the square of the wavefunction expressing the electron's wavelike nature, also called the electron probability density.
A function describing the probable spatial location of an electron of specific energy in an atom is called an atomic orbital.
Each orbital corresponds to a unique combination of three quantum numbers. The principal quantum number, n, groups orbitals into energy levels or shells.
The angular momentum, or azimuthal, quantum number, l, groups orbitals within shells into subshells. Each value of l corresponds to either an s, p, d, or f orbital of varying shape.
Some orbital shapes may have multiple possible orientations, each associated with a magnetic quantum number, ml. The number of orientations for a given subshell is equal to two-l plus one.
The fourth quantum number — the spin quantum number, ms — is associated with the electrons occupying an orbital and has only two possible values: ‘plus one-half’ and ‘minus one-half’. This ensures that no two electrons in an atom have the same values for all four quantum numbers.
Overlaying the orbitals on top of one another produces a roughly spherical shape, like a ‘cloud’ of electrons surrounding the nucleus. This is why atoms are generally represented as spheres.
1.2:
Electronic Structure of Atoms
An atom comprises protons and neutrons, which are contained inside the dense, central core called the nucleus, with electrons present around the nucleus. Taking into account the wave–particle duality of electrons and the uncertainty in position around the nucleus, quantum mechanics provides a more accurate model for the atomic structure. It describes atomic orbitals as the regions around the nucleus where electrons of discrete energy exist, characterized by four quantum numbers: n, l, ml, and ms.
Quantum mechanical model
An atom consists of a small, positively charged nucleus surrounded by electrons. The nucleus contains protons and neutrons. Protons have a charge of +1 and a mass of 1.0073 amu. Neutrons have zero charge and a mass of 1.0087 amu. Electrons have a charge of −1 and a mass of 0.00055 amu. The number of protons in the nucleus is called the atomic number (Z) and defines an atom’s elemental identity. The sum of the numbers of protons and neutrons in the nucleus is called the mass number and, expressed in amu, is approximately equal to the mass of the atom. An atom is neutral when it contains equal numbers of electrons and protons.
Various models were proposed to describe the atomic structure explaining the distribution of electrons around the nucleus. The de Broglie relation quantified that electrons have properties of both a particle and a wave. Quantifying this wave–particle duality further, Heisenberg gave the uncertainty principle: the position (associated with the particle nature) and the velocity or the energy (associated with the wave nature) of the electron cannot be determined simultaneously with full precision. The quantum mechanical model gives a more accurate picture of the atomic structure. It describes the three-dimensional position of an electron with a specific energy, contained in an atom, in a probabilistic manner in terms of the square of the wavefunction, ψ2. Atomic wavefunctions, or atomic orbitals, describe the regions in an atom where electrons are most likely to be found.
An atomic orbital is characterized by three quantum numbers. The principal quantum number, n, groups orbitals in terms of location of energy level, or ‘shell’. The angular momentum or azimuthal quantum number, l, groups the orbitals based on their shapes into ‘subshells’. It has integer values from 0 to n – 1 and is represented by letters: s (l = 0), p (l = 1), d (l = 2), f (l = 3), g (l = 4), and h (l = 5). The magnetic quantum number, ml, with 2l + 1 values ranging from –l to +l, describes the orientation of the orbital in space. In addition, each electron has a spin quantum number, ms, that can be equal to ±1/2. No two electrons in the same atom can have the same set of values for all four quantum numbers.
This text is adapted from Openstax, Chemistry 2e, Section 6.