Electronic Postcards The views below are taken from three dimensional computer generated models representing ionisation energies, atomic radii, atomic mass or density of the elements across sections of the periodic table. For further information about how these images were generated click here Choose a thumbnail image to view and then send an e-postcard Density Densities of solid elements at 298 K Density is defined as mass per unit volume. The volume of a sphere is proportional to the cube of its radius. Across the periodic table atomic masses and atomic radii vary differently so its swings and roundabouts what happens to the densities! Incidentally lead is only the twenty third most dense of the elements. Atomic Radii The atomic radius is half the distance apart of two atoms of the element in its normal state. The atomic radius of a metal depends on its coordination numbers, or number of nearest neighbors. For a non-metal the atomic radius is half the distance between a pair of bonded atoms of the element. Molecules such as oxygen and nitrogen are multiply bonded so the atomic radii of these elements are calculated from molecules such as hydrogen peroxide (HOOH) and hydrazine (NH2NH2) which contain only single bonds. Gases like neon, which are atomic rather than molecular and are not known to form compounds, are more of a problem. Their radii are calculated from the structures the elements adopt at a temperature of absolute zero. The size of an atom depends on the control the positively charged nucleus can exercise over the negatively charged electrons. From an electron's point of view going down a group is like changing from a double bass to a violin. The nucleus has progressively less control over the 'high pitched' electrons, consequently the atomic radii increase. Across a series the electron tunes are more complex. In general the atomic radii decrease from left to right across the periodic table. Ionisation Energies Here 'ionisation' refers to the amount of energy which is needed to prize away from an atom in the gas phase its most loosely held electron. The ionisation energy values are given as amount of energy for a mole of atoms of each element. Ionisation energy may be used to predict an element's properties because the electron being removed is one of the outermost electrons, called valence electrons, which take part in chemical bonding. For example metallic elements generally have lower ionisation energies than non-metals. The actual value of the ionisation energy for a particular element depends heavily on its position in the periodic table. A single ionisation energy is therefore less useful in accounting for chemical properties than observing the trends in ionisation energies going down a group or across a series. Relative Atomic Mass At the turn of the nineteenth century Dalton extended and refined Prout's remarkable conclusion that compounds were of fixed composition by proposing that atoms of the same element had the same atomic weight. Today, following the discovery of isotopes in this century, we define an atomic weight for a blend of isotopes of an element as 'the ratio of the average mass per atom of the element to one-twelfth the mass of an atom of 12C'. As the atomic weights measured this way are numbers without units an element's atomic weight is more correctly referred to as its 'mean relative atomic mass'. The atomic weights of the elements might be expected to increase steadily with increasing atomic number and so they do, with a few exceptions which caused much confusion among those early chemists. For further information about how these images were generated click here