Unlike the rest of the Group 1A elements, which exist as metals, elemental hydrogen exists as gaseous H2 molecules. Compounds formed between hydrogen and non-metals are molecular rather than ionic. (i.e., hydrogen forms covalent bonds with non-metals). For example, hydrogen reacts with halogens (Group VIIA) according to:
H2 (g) + X2 → 2 HX(g)
2 H2 (g) + O2(g) → 2 H2O(g)
8 H2 (g) + S8(s) → 8 H2S(g)
3 H2 (g) + N2(g) → 2 NH3(g)
Hydrogen can also form compunds with more active metals to form ionic hydrides. For example, lithium hydride is formed according to:
2 Li(s) + H2 (g) → 2 LiH(s)
The metal (Li in this case) loses an electron to become a cation and H gains an electron to become H- (hydride anion), which has an charge of -1. Here's another example:
Mg(s) + H2 (g) → MgH2(s)
By gaining an electron, the hydride ion obtains the stable electron configuration of a closed n=1 shell, that is, the noble gas configuration of He.
Oxygen is a group 6A element. Elemental Oxygen is found in two forms: oxygen gas (O2) and and ozone gas (O3). Different forms of an element in the same state are called Allotropes.
When oxygen reacts with most metals a metal oxide is formed where oxygen has an oxidation state of -2. For example, zinc oxide is formed when zinc metal reacts with oxygen gas:
2 Zn(s) + O2 (g) → 2 ZnO(s)
and aluminum oxide is formed when aluminum metal reacts with oxygen gas:
4 Al(s) + 3 O2 (g) → 2 Al2O3(s)
There are, however, some exceptions, which we consider next.
Because alkali metals are so active, the product of their reaction with oxygen gas is not what you might expect. While lithium metal reacts with oxygen gas to form lithium oxide, as one might expect:
4 Li(s) + O2 (g) → 2 Li2O(s) ,
2 Na(s) + O2 (g) → Na2O2(s)
K(s) + O2 (g) → KO2(s)
Oxygen reacts with most alkaline earth metals to form a metal oxide:
2 M(s) + O2 (g) → 2 MO(s)
Ca(s) + O2 (g) → CaO(s)
However, oxygen combines with barium metal, the most active of this group, to form a peroxide:
Ba(s) + O2(g) → BaO2 (s)
When oxygen combines with non-metals in their elemental form, the product is a non-metal oxide. For example, oxygen reacts with solid carbon to form carbon monoxide or carbon dioxide, respectively, as shown below (reaction not balanced):
C(s) + O2 (g) → CO(g) or CO2(g)
Similarly, oxygen reacts with solid phosphorus to form tetraphorphorus heptoxide or tetraphorphorus decoxide, respectively, as shown below (reaction not balanced):
P4(s) + O2 (g) → P4O6(g) or P4O10(g)
When reacted with solid sulfur, oxygen forms sulfur dioxide gas:
S8(s) + 8 O2 (g) → 8 SO2(g)
The oxides of non-metals are acidic. If a non-metal oxide dissolves in water, it will form an acid.
Non-Metal Oxide + Water → Acid
SO3(g) + H2O(l) → H2SO4(aq)
N2O3(g) + H2O(l) → HNO2(aq)
The non-metal oxides can be neutralized with a base to form a salt and water.
Non-Metal Oxide + Base → Salt + Water
SO3(g) + Ba(OH)2(aq) → BaSO4(aq) + H2O(l)
P4O10(s) + 12 NaOH(aq) → 4 Na3PO4(aq) + 6 H2O(l)
The oxides of metals are basic. If a metal oxide dissolves in water, it will form a metal hydroxide.
Metal Oxide + Water → Metal Hydroxide
BaO(s) + H2O(l) → Ba(OH)2(aq)
K2O(s) + H2O(l) → 2 KOH(aq)
Like any base, these bases can be neutralized by an acid to form a salt and water.
Metal Oxide + Acid → Salt + Water
CuO(s) + 2 HNO3(aq) → Cu(NO3)2(aq) + H2O(l)
Al2O3(s) + 6 HCl(aq) → 2 AlCl3(aq) + 3 H2O(l)
Generally, the more metallic character an element has, the more basic its oxide will be. Likewise, the more non-metalic character an element has, the more acidic its oxide will be. The metalic character of an element can be determined by its position on the periodic table:
Finally, we note that a salt can also be formed by the direct reaction of a metal and a non-metal.
Metal + Non-Metal → Salt
2 Al(s) + 3 Br2(l) → 2 AlBr3(s)
Chemisty, The Central Science, 10th Ed.
7.53, 7.55, 7.57, 7.59, 7.61, 7.65, 7.67, 7.69, 7.71, 7.73, 7.77
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