Electropositive

Salt-like. These are the hydrides of the most electropositive elements (e.g. Na) and contain H " ions. 

When an electropositive or electronegative adsorbate adaches itself to a surface, there is usually a change in the surface dipole, which, in turn, affects the surface work ftmction. Thus, very small coverages of adsorbates can be used to modify fhe surface work ftmcfion in order to ascertain the role that the work function plays in a given process. Conversely, work ftmction measurements can be used to accurately detennine the coverage of these adsorbates. 

Group II elements can be seen to follow a pattern very like that found in Group I. Note, however, that the energy required to attain a noble gas configuration is considerably higher indicating that the elements will be less metallic or electropositive in their chemistry (Chapter 6). 

REACTIONS WITH NON-METALS AND WEAKLY ELECTROPOSITIVE METALS... 

A non-metal or weakly electropositive metal X in Group III of the periodic table would be expeeted to form a covalent volatile hydride XHj. In fact, the simplest hydride of boron is BjHf, and aluminium hydride is a polymer (AlHj) . 

Only the carbon atom can gain four electrons this only happens when it is combined with extremely electropositive elements and this state may be regarded as exceptional. Bonding in carbides is almost invariably predominantly covalent. 

The gradual increase in electropositive character down the group is clearly shown in that, unlike both carbon and silicon, germanium very readily dissolves in both concentrated nitric and sulphuric acids the hydrated germanium(IV) oxide is produced ... 

Reactions with electropositive metals. Ammonia gas reacts with strongly electropositive metals to form the amide, for example... 

Hydrated nitrates, and anhydrous nitrates of very electropositive metals (for example Na, K), contain the ion NOJ which has the structure ... 

Many metals are oxidised by water. At ordinary temperatures the more electropositive metals, for example, sodium, calcium (or their amalgams with mercury), react to give hydrogen, for example ... 

---