Evidence for the Existence of Ions

Evidence for ions is electrolysis. If a direct current is passed through green copper(II) chromate (VI) solution the blue Cu²⁺ cations migrate to the cathode and the yellow negative CrO₄^2- anion goes to the anode.Migration of ionsA strip of filter paper on a microscope slide is soaked in a colourless electrolyte and a drop of copper(II) chromate(VI) solution is placed in the centre. The ends are connected to a d.c. supply. After some time a blue colour moves towards the negative electrode and a yellow colour moves towards the positive electrode.The green colour separates into blue and yellow, so the compound must contain two differently coloured particles that move independently. They move in opposite directions in an electric field, so they must be oppositely charged: this can only be explained by ions.Electron density mapsX-ray diffraction of an ionic crystal such as sodium chloride gives an electron density map, in which contour lines join points of equal electron density. The map for NaCl shows that:

• the contours around each particle are almost spherical
• the electron density falls almost to zero between neighbouring particles, so no electrons are shared
• the contours around each chloride ion enclose a larger region than those around each sodium ion, because Cl^– has 18 electrons and Na⁺ has 10

This shows the particles are separate ions. In a covalent molecule, by contrast, there is a high electron density between the nuclei.