Chapter 5 Electrons in Atoms

Bohr model of the atom

Valence electrons are the electrons in the outer energy level of an atom. No atom has more than eight electrons in its outer level. Atoms with eight valence electrons are exceptionally stable.

The Bohr model of the atom is not correct The Bohr model of the atom is not correct!! There is substantial evidence that indicates electrons do not move around the nucleus in perfect circular orbits.

Heisenberg Uncertainty Principle Heisenberg concluded that it is impossible to make any measurement on an object without disturbing the object. 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.

Quantum Mechanical Model of the Atom Erwin Schrodinger (1887-1961) derived an equation that treated the hydrogen atom’s electron as a wave. This wave mechanical model is also known as the Quantum Mechanical Model of the atom, and is what is used today.

The wave function predicts the three dimensional region around a nucleus called an atomic orbital which describes an electron’s probable location.

Principal energy levels are the atoms major energy levels Principal energy levels are the atoms major energy levels. An atom’s lowest principal energy level is assigned a principal quantum number of 1. Principal energy levels contain energy sublevels. Principal energy level 1 contains 2 sublevel; principal energy level 2 contains 2 sublevels; principal energy level 3 contains 3 sublevels, and so on.

Do energy levels really have energy? The energy associated with an energy level increases as the distance from the nucleus increases. The lower the number of the principal energy level, the closer the negatively charged electron in it is to the positively charged nucleus and the more difficult it is to remove this electron from the atom.

Energy sublevels Sublevels are labeled s, p, d, or f according to the shapes of the atom’s orbitals. All ‘s’ sublevels are spherical and all ‘p’ sublevels are dumbbell shaped.

Each principal energy level has one sublevel containing one orbital, an s orbital, that can contain a maximum of two electrons. Electrons in this orbital are called s electrons and have the lowest energy of any electrons in that principal energy level. The first principal energy level contains only an s sublevel; therefore, it can hold a maximum of two electrons.

Each principal energy level above the first contains one s orbital and three p orbitals. A set of three p orbitals, called the p sublevel, can hold a maximum of six electrons. Therefore, the second level can contain a maximum of eight electrons - that is, two in the s orbital and 6 in the three p orbitals.

Each principal energy level above the second contains, in addition to one s orbital and three p orbitals, a set of five d orbitals, called the d sublevel. The five d orbitals can hold up to 10 electrons. Thus, the third level holds a maximum of 18 electrons: 2 in the s orbital, 6 in the three p orbitals, and 10 in the five d orbitals.

The fourth and higher levels also have an f sublevel, containing seven f orbitals, which can hold a maximum of 14 electrons. Thus, the fourth level can hold up to 32 electrons: 2 in the s orbital, 6 in the three p orbitals, 10 in the five d orbitals, and 14 in the seven f orbitals.

One further, important note about orbital shapes: These shapes do not represent the path of an electron within the atom; rather, they represent the region of space in which an electron of that sublevel is most apt to be found. Thus, a p electron is most apt to be within a dumbbell-shaped space in the atom, but we make no pretense of describing its path.

Electron configuration is the arrangement of electrons in an atom Electron configuration is the arrangement of electrons in an atom. Because low-energy systems are more stable than high-energy systems, electrons in an atom tend to assume the arrangement that gives the atom the lowest possible energy. The most stable, lowest-energy arrangement of the electrons in atoms of each element is called the element’s ground-state electron configuration.

The Aufbau principle Each electron occupies the lowest energy orbital available. Following is an aufbau diagram In order of increasing energy, the sequence of energy sublevels within a principal energy level is s, p, d, and f

Aufbau diagram

The Pauli exclusion principle A maximum of two electrons may occupy a single atomic orbital, but only if the electrons have opposite spins. A toy top can spin only 2 directions, similar to electrons in atomic orbitals, they can spin only one direction.

Hund’s rule Single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same orbitals.

Back to Valence Electrons… Only certain electrons, called valence electrons determine the chemical properties of an element. Valence electrons are defined as electrons in the atom’s outermost orbitals, generally those orbitals associated with the atom’s highest principal energy level.

Electron Dot Structures This consists of an elements symbols, which represents the atomic nucleus and inner-level electrons, surrounded by dots representing the atom’s valence electrons. The American chemist G.N. Lewis (1875-1946) devised the method while teaching a college chemistry class in 1902.

Example of Lewis dot