Mendeleev left gaps in his Periodic Table for elements that had not been discovered yet. He used the properties of surrounding elements to predict the properties of these undiscovered elements.
He called one of these predicted elements "eka-aluminium".
Table 1: Predicted properties of 'eka-aluminium'
| Property | Predicted Value |
|---|---|
| Appearance | Metal with a very low melting point |
| Melting Point (∘C^\circ\text{C}∘C) | Around 30∘C30^\circ\text{C}30∘C |
| Relative Atomic Mass | 686868 |
| Density (g/cm3\text{g/cm}^3g/cm3) | 5.95.95.9 |
Table 2: Properties of three elements discovered later
| Element | Appearance | Melting Point (∘C^\circ\text{C}∘C) | Relative Atomic Mass | Density (g/cm3\text{g/cm}^3g/cm3) |
|---|---|---|---|---|
| Scandium (Sc) | Silver-white metal | 154115411541 | 45.045.045.0 | 3.03.03.0 |
| Gallium (Ga) | Silver-grey metal | 29.829.829.8 | 69.769.769.7 | 5.915.915.91 |
| Germanium (Ge) | Grey-white metal | 938938938 | 72.672.672.6 | 5.325.325.32 |
Which element matches Mendeleev's predictions for 'eka-aluminium'?
Give two reasons for your answer to part (a).
Mendeleev did not predict the existence of Helium (He). The electron configuration of a helium atom is 222. Describe and explain the reactivity of helium based on this electron arrangement.
Helium has two stable isotopes: 23He{}^{3}_{2}\text{He}23He and 24He{}^{4}_{2}\text{He}24He. State the number of protons, neutrons, and electrons in a neutral atom of each isotope.