Basic Gaussian Function Plot [closed]

Question

I'm new to Mathematica and I'm trying to plot a Gaussian function (actually a sum of three Gaussian functions) using custom x-axis tick marks.

Here's what I have so far:

a0 = QuantityMagnitude[UnitConvert[Quantity[1, "BohrRadius"]]];
c1 = 0.154329;
c2 = 0.535328;
c3 = 0.444635;
a1 = 2.227660;
a2 = 0.405771;
a3 = 0.109818;
z1 = (2 * a1 / pi)^(3/4) Exp[-a1 * (x / a0)^2];
z2 = (2 * a2 / pi)^(3/4) Exp[-a2 * (x / a0)^2];
z3 = (2 * a3 / pi)^(3/4) Exp[-a3 * (x / a0)^2];
Plot[y = a0^(-3/2) * (c1 * z1 + c2 * z2 + c3 * z3),
 PlotRange -> {{-15a0, 15a0}}, 
 Ticks -> {Table[{k a0, Style[If[k > 1, Row[{k, Subscript[a, 0]}], Subscript[a, 0]],
                              Small]},
                 {k, -15, 15}], Automatic}]

I'm getting a really basic error (Plot::argr: Plot called with 1 argument; 2 arguments are expected.), but I'm not sure where I'm going wrong.

EDIT: Thanks to Alexei Boulbitch, here's my final code after edits.

a0 = QuantityMagnitude[UnitConvert[Quantity[1, "BohrRadius"]]];
Subscript[c, 1] = 0.154329;
Subscript[c, 2] = 0.535328;
Subscript[c, 3] = 0.444635;
a1 = 2.227660;
a2 = 0.405771;
a3 = 0.109818;
Subscript[\[Phi], 1] = (2*a1/Pi)^(3/4) Exp[-a1*(x/a0)^2];
Subscript[\[Phi], 2] = (2*a2/Pi)^(3/4) Exp[-a2*(x/a0)^2];
Subscript[\[Phi], 3] = (2*a3/Pi)^(3/4) Exp[-a3*(x/a0)^2];
Subscript[\[Psi], 1s] = a0^(-3/2)*(Subscript[c, 1] * Subscript[\[Phi], 1] + 
                          Subscript[c, 2] * Subscript[\[Phi], 2] + 
                          Subscript[c, 3] * Subscript[\[Phi], 3]);
Subscript[\[Psi], 1sActual] = a0^(-3/2) / Sqrt[Pi] * Exp[-x/a0];
str = ToString[Subscript["\[Psi]", "1s"], StandardForm];
Plot[{Subscript[\[Psi], 1s], Subscript[\[Psi], 1sActual]}, {x, 0, 7a0}, 
PlotLegends-> "Expressions",
Ticks -> {Table[{k a0, Style[If[k > -8, Row[{k, Subscript[a, 0]}], 
Subscript[a, 0]],
                          Small]},
             {k, 0, 7}], Automatic}]
Plot[{Subscript[\[Phi], 1], Subscript[\[Phi], 2], Subscript[\[Phi], 3], 
  (Subscript[c, 1] * Subscript[\[Phi], 1] + Subscript[c, 2] * 
Subscript[\[Phi], 2] + 
   Subscript[c, 3] * Subscript[\[Phi], 3])}, {x, -7a0, 7a0}, 
PlotRange -> {{-7a0, 7a0}, {0, 1.35}},
PlotLegends-> "Expressions",
Ticks -> {Table[{k a0, Style[If[k > -8, Row[{k, Subscript[a, 0]}], Subscript[a, 0]],
                          Small]},
             {k, -7, 7}], Automatic}]

Answer 1

I propose that you start from this:

a0 = 1;
c1 = 0.154329;
c2 = 0.535328;
c3 = 0.444635;
a1 = 2.227660;
a2 = 0.405771;
a3 = 0.109818;
z1 = (2*a1/Pi)^(3/4) Exp[-a1*(x/a0)^2];
z2 = (2*a2/Pi)^(3/4) Exp[-a2*(x/a0)^2];
z3 = (2*a3/Pi)^(3/4) Exp[-a3*(x/a0)^2];
Plot[a0^(-3/2)*(c1*z1 + c2*z2 + c3*z3), {x, -1, 1}]

which gives the answer:

and then one-by-one add your options and change the definitions. Then you will see what's wrong. One tip: this a0 = QuantityMagnitude[UnitConvert[Quantity[1, "BohrRadius"]]] creates a part of your troubles.

Have fun!