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Period 3
A5
1
PERIOD 3 ELEMENTS AND THEIR COMPOUNDS
Introduction - the first two periods in the periodic table are not typical
- the first contains only two elements (H, He)
- the second ( $\mathrm{Li}-\mathrm{Ne}$ ) contains the top elements of each group; these have small sizes and relatively high ionisation energies so are atypical
- Period 3 is best for studying periodic trends.
ELEMENTS
Structure As you move from left to right the elements go from highly electropositive metals through metalloids with giant structures to the simple molecular structure of non-metals.
$\left.\begin{array}{lccclcll}\mathrm{Na} | \mathrm{Mg} | \mathrm{Al} | \mathrm{Si} | \mathrm{P}_{4} | \mathrm{~S}_{8} | \mathrm{Cl}_{2} | \mathrm{Ar} <-- | \text { metals } | \ldots-> | \text { metalloid } | & <- \text { non } | \text { metals (simple } | \text { molecules) }\end{array}\right)=>$
Reactions with...
Water As you move from left to right across a period the metals become less reactive
Sodium vigorous reaction with cold water; strong alkaline solution formed
\[
\mathbf{2 N a}_{(\mathbf{s})}+\mathbf{2 H}_{\mathbf{2}} \mathbf{O}_{(\mathbf{g})} \longrightarrow \underset{\text { very soluble }}{\mathbf{2} \mathbf{N a O H}_{(\mathbf{a q})}}+\mathbf{H}_{\mathbf{2 ( g )}}
\]
Magnesium very slow reaction with cold water
\[
\mathbf{M g}_{(\mathbf{s})}+\mathbf{2 H}_{\mathbf{2}} \mathbf{O}_{(\mathbf{g})} \longrightarrow \underset{\text { sparingly soluble }}{\mathbf{M g}(\mathbf{O H})_{\mathbf{2}(\mathbf{s})}}+\mathbf{H}_{\mathbf{2 ( g )}}
\]
vigorous reaction with steam
\[
\mathrm{Mg}_{(\mathrm{s})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})} \longrightarrow \mathrm{MgO}_{(\mathrm{s})}+\mathrm{H}_{2(\mathrm{~g})}
\]
Oxygen - elements must be heated to react; however...
- dry phosphorus can ignite spontaneously which is why it is stored under water
- the reactivity depends a lot on the state of subdivision
Sodium vigorous reaction with ignited sodium ionic sodium oxide formed
\[
4 \mathrm{Na}_{(\mathrm{s})}+2 \mathrm{O}_{2(\mathrm{~g})} \longrightarrow 2 \mathrm{Na}_{2} \mathrm{O}_{(\mathrm{s})}
\]
Magnesium vigorous reaction with ignited magnesium ionic magnesium chloride formed
\[
2 \mathbf{M g}_{(\mathbf{s})}+\mathbf{O}_{\mathbf{2 ( g )}} \longrightarrow 2 \mathrm{MgO}_{(\mathbf{s})}
\]