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Within the main group (s- and p-block) elements of the Periodic Table ( [link] ) there are some general trends that we can observe for the elemental form, as well as the hydrides, oxides, and halides.
Within the main group (s- and p-block) elements there are some general trends that we can observe.
As an example of these changes [link] shows the trends across one Period.
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
| Properties | Electro-positive metal | Electro-positive metal | Metal but forms covalent bonds | Metalloid semiconductor metal/non-metal character | E-E bonding in elements | E-E bonding in elements | Simple molecule | Mono atomic gas |
The properties of main group hydrides are dependant on the difference in electronegativity between the element and hydrogen ( [link] ). Elements on the left of the Periodic Table are highly electropositive and form ionic hydrides, while those of the center and right are covalent in character. However, of those with covalent E-H bonds, there is a change from polymeric hydrides to molecular compounds. For example, the Group 13 element hydrides (i.e., BH 3 ) form hydrogen-bridged oligomers (i.e., B 2 H 6 ). In contrast, HCl is a diatomic molecule.
| Hydride | Element electronegativity | Hydrogen electronegativity | E-H polarity | Structure | Comments |
| NaH | 0.9 | 2.1 | M + H - | Ionic | Reacts with H 2 O to liberate H 2 |
| BH 3 | 2.0 | 2.1 | B δ + -H δ - | Oligomeric and polymeric | Reacts slowly with H 2 O |
| CH 4 | 2.5 | 2.1 | C δ - -H δ + | Molecular | Insoluble in H 2 O |
| HCl | 3.0 | 2.1 | Cl δ - -H δ + | Molecular | Dissolves in H 2 O to form H + and Cl - |
As with hydrides the properties of main group oxides are dependant on the difference in electronegativity between the element and oxygen. Highly electropositive metals for ionic oxides, while other elements for covalent bonds (albeit polar in character) with oxygen. In addition, the aggregation of covalent oxides decreased across the Period from left to right ( [link] ). As may also be seen from [link] , oxides of elements on the left of the Periodic Table dissolve in water to form basic solutions, while those on the right form acidic solutions. There is a class of oxides (especially those of Group 13 and 14) that can react as either an acid or a base. These are known as amphoteric substances.
| Oxide | Bonding | Reactivity with H 2 O | Description |
| Na 2 O | Ionic | Dissolves to give a strong base | Basic |
| Al 2 O 3 | Covalent polymeric | Dissolves in both acidic and basic solution | Amphoteric |
| SiO 2 | Covalent polymeric | Dissolves in both acidic and basic solution | Amphoteric |
| CO 2 | Covalent molecular | Dissolves to give a weak acid | Acidic |
| SO 3 | Covalent molecular | Dissolves to give a strong acid | Acidic |
In summary, oxides of the main group elements show two trends.
The trend between ionic and non-ionic/covalent in moving across a Period is also true for the chlorides of the main group elements. Those on the left (i.e., Group 1 and 2) are ionic and soluble in water, while those to the right tend to give acidic solutions due to reactions with the water and the formation of hydrochloric acid, e.g., [link] .
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