1.5 More about electron arrangements in atoms

• In 1913, Niels Bohr proposed that electrons could only exist in fixed energy levels. The main energy levels are called principal energy levels and are given a number called the principal quantum number (n) with the lowest number being 1.
• Ideas about the structure of atom have changed over the years. The Bohr theory thought of it as a small nucleus of protons and neutrons surrounded by circulating electrons.

  Each shell or energy level could hold a maximum number of electrons.

  The energy of levels became greater as they got further from the nucleus and electrons filled energy levels in order.

  The theory didn't explain certain aspects of chemistry.
• During studies of the spectrum of hydrogen it was shown that the energy levels were not equally spaced. The energy gap between successive levels got increasingly smaller as the levels got further from the nucleus. The importance of this discussed later.

  A study of ionisation energies and the periodic properties of elements suggested that the main energy levels were split into sub levels.

  Level 1 was split into 1 sub level.

  Level 2 was split into 2 sub levels.

  Level 3 was split into 3 sub levels.

  Level 4 was split into 4 sub levels.
• There are four sub-levels, labelled in order of increasing energy: s, p, d, and f. Each holds a different number of electrons.
• The Aufbau principle states that the lowest energy sub-levels are occupied first. This means the 1s sub-level is filled first, followed by 2s, 2p, 3s and 3p. However the 4s sub-level is lower in energy than the 3d, so this will fill first.
• It is impossible to exactly locate the position of an electron within an energy sub-level. By measuring the electron density around the nucleus, it is possible to define regions where electrons are most likely to be found at any one time. These regions are called orbitals.

  Each energy sub-level has one or more orbitals, each of which can contain a maximum of two electrons.
• Maximum number of electrons in s block: 2

  Maximum number of electrons in p block: 6

  Maximum number of electrons in d block: 10

  Maximum number of electrons in f block: 14
• An orbital is a region in space where one is likely to find an electron.
• Orbitals can hold up to 2 electrons as long as they have opposite spin; this is known as Pauli's exclusion principal.
• Two electrons in the same orbital must have opposite spins (usually represented by arrows pointing up or down to show directions of the spin).
• s orbitals are spherical. One occurs in every principal energy level.
• p orbitals are dumb-bell shaped. Three occur in energy levels except the first.
• d orbitals come in various shapes. Five occur in energy levels except the first and second.
• Hund's rule states that single electrons occupy all empty orbitals within a sub-level before they start to form pairs in orbitals.
• If two electrons enter the same orbital there is repulsion between them due to their negative charges. The most stable configuration is within single electrons in different orbitals.