Atomic Structure and Periodic Table

A knowledge of atomic structure is fundamental to an understanding of chemistry – as well as being crucial for success in Leaving Certificate!

Basically there are two aspects of Atomic Structure that have to be known:

(i) the simple or Bohr theory of it – pertaining to electron shells or orbits; and
(ii) the more modern theory of orbitals and energy levels that satisfies or agrees with known facts that the Bohr theory cannot explain.

It is also essential to know one’s way around the Periodic Table, for this consolidates so much chemistry – atomic structures, properties and reactions of elements, formulae of compounds and makes sense of many of the aspects of radioactivity.

Atomic Structure: Definitions and Laws

1. Atomic_Structure (ppt from iChemistry)

Atomic Structure Study Cards

Bohr, the atom, the hydrogen spectrum and Bohr’s theory

The Bohr structure of an atom refers to the ideas that Bohr put forward in 1913, electrons rotating around the nucleus in allowed orbits or shells. While the Bohr structure of atoms is unlikely to be asked at Leaving Certificate level, a knowledge of it is essential to the understanding of many of the other topics on the course.

Bohr and the Hydrogen Spectrum – Emission Spectra ppt (from iChemistry)

Bohr’s theory, which resulted from his explaining the line spectrum of hydrogen, is that electrons can only revolve around atoms in certain allowed orbits or energy levels. When energy is given to an atom, electrons can move to higher energy levels, and when they return to their lower levels, energy is given out in the form of radiation – light, ultraviolet or infrared. This is the origin of the various lines in the spectra of the different elements.

Energy Levels and Sub-levels

Energy sub-levels are a more advanced explanation of Bohr’s shells. The main energy levels of atoms are divided into one or more sub-levels – the number of sub-levels being the same as the principle quantum number of the main level. The sub-levels are denoted by the letters s, p, d and f. The Aufbau principle dictates the order in which the sub-levels are filled with electrons.

Atomic Orbitals

An orbital, the modern equivalent of a Bohr’s shell, is defined as the region around the nucleus of an atom where the electrons are most likely to be found. There is an orbital characteristic of each of the sub-energy levels, so the four types of orbital are s, p, d and f. Two more rules summarise how electrons fill the orbitals; these are Pauli’s exclusion principle and Hund’s rule of maximum multiplicity. It is necessary to know both these rules and how to apply them.

The Periodic Table

1.1_Periodic_Table-2-2 (ppt from iChemistry)

Periodic Table trends (ppt from iChemistry)

See video: Introduction to Periodic Table

See video: Atoms, isotopes, ions and the Periodic Table

Periodic Table Definitions

Brainiac: Reaction of alkali metals with water and Alkali explosive reactions

Transition Elements and D-block Elements

Transition elements and d-block elements are not exactly the same. In period 4, the d-block elements run from scandium (At. No. 21) to zinc (At. No. 30). Zinc, however, does not have the characteristic properties of transition elements (i.e., variable valency, coloured compounds, catalytic activity) and so is not classed as a transition element; this is because its d orbital is full, with 10 electrons. Similarly, in periods 5 and 6, cadmium and mercury have full d orbitals and are not therefore transition elements. As would be expected, they do not have variable valencies and do not form coloured compounds.


A knowledge of the development of both atomic theory (and radioactivity) needs to be known. The main people involved were: the ancient Greeks, Dalton, Crookes, Thomson, Stoney, Millikan, Rutherford, Bohr, Heisenberg, Moseley, Chadwick, Schrodinger, Dobereiner, Newlands, Mendeleev.

See video: Mendeleev’s Periodic Table

Video on discovery of electrons, protons and neutrons

The Periodic Table Study Cards


In Chemistry, the main aspects of radioactivity are:

  • Historical outline
  • Types of radiation and their properties
  • Nuclear reactions
  • Half life
  • Radioisotopes and three of their uses

Video: What are radioactive isotopes?

Radioactivity Study Cards




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