The hardness of a material or mineral is directly related to its atomic structure.. consider as an example the element carbon. Carbon (pure C) comes in different forms called polymorphs (poly means many and morph means form...). Now consider two forms of carbon: One is the graphite in your so called lead pencil. But it is not Pb (lead ) in the pencil...it is a form of carbon called graphite. Graphite is one of the softest materials known because the carbon atoms are linked into hexagonal sheets. Between the sheets the forces are very, very weak. This is why it good for pencils: just a little force of your hand is enough to make the sheets come undone and hence the carbon goes unto the paper.
Now consider another form of carbon: Diamonds like your mom's diamond ring! In diamond (again pure carbon) each carbon atom is linked to 4 other carbon atoms forming a covalent type bond. This bond is very, very strong and diamond is one of the hardest substances. Is very hard to scratch a diamond.So the moral of the story is that it all has to do with the type of bonding between atoms in a material.
Now gold (Au) -like most metals- has what is called metallic bonding. This is pretty weak and is related to the large nucleous of the atoms and the extended cloud of electrons surrounding the nucleus. Without going into the details, the forces between Au atoms in gold metal are quite weak and most importantly, bonds are easy to break and rebuild.

Gold is soft because the atomic nuclei in gold is held together by a cloud of electrons.This cloud drifts through the gold structure instead of having one a set of electrons dedicated to one nucleus. The force between the nuclei and the cloud of electrons is called a metallic bond. That's the scientific explanation. A simple picture explains it better, but I can't draw over e-mail. Maybe your teacher can draw it for me. Have him draw an ordered set of circles (5x5 grid of circles, see below) on the board with pluses in the center and gaps between the circles, representing atomic nuclei.
Now randomly draw a pathway through these circles, like a maze. The pathway represents the cloud of electrons drifting through the nuclei.
This is a metallic bond. Because the electrons aren't "stuck" to the nuclei, the bond is easier to break apart, thus, metals like gold and aluminum tend to be soft. Other bonds (covalent, ionic, etc.) for harder minerals (ruby, diamond, etc.) have electrons partnered with a nucleus (instead of a maze through the circles, each circle with a plus has a set of electrons connected to it). They are harder to separate and so the minerals are harder.
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