Question #4973e

Dec 10, 2014

It depends on the structure of the element. Giant structures like diamond, graphite and metals would have no number, just the state that they are at the temperature of the reaction. e.g.

$M {g}_{\left(g\right)} , {C}_{\left(g\right)}$

If the element is molecular in structure you state the number of atoms per molecule as a subscript. e.g.

${O}_{2 \left(g\right)} , {P}_{4 \left(s\right)} , {I}_{2 \left(s\right)}$

Noble gases are monotomic e.g.

$H {e}_{\left(g\right)} , N {e}_{\left(g\right)}$

Dec 10, 2014

Subscripts are only added to diatomic elements, the elements that form diatomic molecules; there are seven such elements, all gases:

Hydrogen - ${H}_{2}$
Nitrogen - ${N}_{2}$
Oxygen - ${O}_{2}$
Flourine - ${F}_{2}$
Chlorine - $C {l}_{2}$
Iodine - ${I}_{2}$
Bromine - $B {r}_{2}$

These are called homonuclear molecules, and should not be confused with heteronuclear diatomic molecules ($C O$, $H C l$, $C l F$, $H F$ - molecules consisting of two elements).

Now, single-replacement reactions are a type of redox reactions in which one element or ion is replaced by another in a compound. This is done in two ways:

(1) either a cation replaces another cation, or
(2) an anion replaces another anion;

The single-replacement reaction's general form is:

$A + B X \to A X + B$

However, a single-replacement reaction will only take place if a more reactive element is replacing a less reactive one. This is determine by looking at the reactive series for metals (wikipedia link: http://en.wikipedia.org/wiki/Reactivity_series)

Now, here are a couple of examples:

$C u \left(s\right) + 2 A g N {O}_{3} \left(a q\right) \to C u {\left(N {O}_{3}\right)}_{2} \left(a q\right) + 2 A g \left(s\right)$ - notice how no subscripts are used for $A g$ and $C u$, since they do not form diatomic molecules;

$2 N a \left(s\right) + 2 {H}_{2} O \left(l\right) \to 2 N a O H \left(a q\right) + {H}_{2} \left(g\right)$ -> we use the subscript for hydrogen since it forms diatomic molecules;

$C {l}_{2} \left(g\right) + 2 K B r \left(a q\right) \to 2 K C l \left(a q\right) + B {r}_{2}$ -> notice the subscripts for both chlorine, and bromine, as both form diatomic molecules;

$2 A l \left(s\right) + F {e}_{2} {O}_{3} \left(a q\right) \to A {l}_{2} {O}_{3} \left(a q\right) + 2 F e \left(s\right)$ -> no subscript for $A l$ and $F e$ because they do not form diatomic molecules;