Atomic structure
5.1.1 The nucleus
The nucleus is made up ofprotons, which are positively charged, andneutrons, which
have no charge. Neutrons and protons have about the same mass, and together account for
most of the mass of the atom.
5.1.2 Electrons
The electrons are negatively charged particles. The mass of an electron is about 2000 times
smaller than that of a proton or neutron at 0.00055 amu. Electrons circle so fast that it
cannot be determined where electrons are at any point in time. The image depicts the
old Bohr model of the atom, in which the electrons inhabit discrete ”orbitals” around the
nucleus much like planets orbit the sun. This model is outdated. Current models of the
atomic structure hold that electrons occupy fuzzy clouds around the nucleus of specific
shapes, some spherical, some dumbbell shaped, some with even more complex shapes.
5.2 Shells and Orbitals.
5.2.1 Electron shells
Electrons orbit atoms in clouds of distinct shapes and sizes. The electron clouds are layered
one inside the other into units calledshells, with the electrons occupying the simplest
orbitals in the innermost shell having the lowestenergy stateand the electrons in the
most complex orbitals in the outermost shell having the highest energy state. The higher
the energy state, the moreenergythe electron has, just like a rock at the top of a hill has
more potential energy than a rock at the bottom of a valley. The main reason why electrons
exist in higher energy orbitals is because only two electrons can exist in any orbital. So
electrons fill up orbitals, always taking the lowest energy orbitals available. An electron can
also be pushed to a higher energy orbital, for example by a photon. Typically this is not
a stable state and after a while the electron descends to lower energy states by emitting a
photon spontaneously. These concepts will be important in understanding later concepts
like optical activity of chiral compounds as well as many interesting phenomena outside the
realm of organic chemistry (for example, how lasers work).
5.2.2 Electron orbitals
Eachdifferentshell is subdivided intoone or moreorbitals, whichalso havedifferentenergy
levels, although the energy difference between orbitals is less than the energy difference
between shells.
Longer wavelengths have less energy; the s orbital has the longest wavelength allowed for
an electron orbiting a nucleus and this orbital is observed to have the lowest energy.
Each orbital has a characteristic shape which shows where electrons most often exist. The
orbitals are named using letters of the alphabet. In order of increasing energy the orbitals
are:s,p,d, andforbitals.