[imd321 (OP)]

I understand that metals have a series of properties - sonorous, conduct heat & electricity etc etc.  But do the properties of the element define it as a metal or is there something about the structure of the element that makes it metal and have these properties.  So if an imaginary new element was found, could you tell just from the structure of the atom whether it was going to be classified as a metal or would you need to look at its properties first.

[graham.d]

Take a look at these sites:

http://www.chemguide.co.uk/atoms/structures/metals.html
http://www.chemguide.co.uk/atoms/bonding/metallic.html#top

Generally it would be possible to tell if a "new" element was likely to behave like a metal but it would depend on what you know about its atomic composition. The periodic table shows distinct groupings:
http://en.wikipedia.org/wiki/Periodic_table_(metals_and_non-metals)

[Soul Surfer]

When in the solid state metals have a crystalline structure that does not restrict electron movement to the immediate vicinity of the atom at room temperature.  That is electrons can move freely through the structure.  This means that the outer electrons are relatively losely bound to the atoms.

Let me explain in a little more detail.  Electrons are fermions and as such only two fermions of opposite spins may reside in any one quantum state (Pauli exclusion principle   PEP) now this is OK as long as atoms are well separated from each other and individual atoms are essentially identical, but as atoms get closer together and form liquids or solids the quantum states start to be effected by each other and to comply with the PEP the quantum states become slightly separated at different energy levels according to the crystal structure and proximity of the atoms so some of the outer electrons of the atoms are at slightly higher energy levels  now the energy level at which the electrons are bound to the individual atoms is called the Fermi level and it may be that some of the higher level energy states of the electrons are above this level and can therefore move through the crystal lattice.  This gives the substance the property of being a metal.  Non metals have strongly bound electrons. 

More interesting are the semiconductors where the fermi level is very close to the top energy level of the electrons and the properties can be radically changed with small quantities of impurities and electric fields.

[syhprum]

Why do astronomers refer to all elements other than Hydrogen and Helium as metals ?

[Soul Surfer]

It is just a perverse sort of tradition.  They are referring to generically elements that did not form part of the big bang (although it did make a tiny bit of lithium) but were formed in the nuclear syntheses processes in stars that have formed evolved and distributed themselves (by explosion and "evaporation") at the end of their lives which only started quite some time after the big bang had cooled off a bit.  most of these more complex elements are in fact metals.

[Monox D. I-Fly]

Why do astronomers refer to all elements other than Hydrogen and Helium as metals ?

Wait, they think oxygen is metal?

[chiralSPO]

Why do astronomers refer to all elements other than Hydrogen and Helium as metals ?

Wait, they think oxygen is metal?

Yup! Astronomers are weird!

All kidding aside, as long as the terms are well-defined within a field, they are useful--no matter how confusing they are to people in other fields.

For instance: I have a fair amount of experience in electrochemistry, from a chemistry/fuel cell background, so I am used to the convention of calling the electrode which pulls electrons out of beaker the anode, and the electrode that pushes electrons into the beaker thee cathode. To my great distress, electrical engineers and battery folks use the exact opposite convention, because they consider the outside of the battery (beaker) the environment of interest, so the electrode that carries electrons from the beaker to the outside world is the cathode, and the electrode that pulls electrons from the outside world into the beaker is the anode (I was on a conference call between fuel cell folks and battery folks that got really awkward for what felt like 20 minutes before we realized that we were talking about the same thing using totally opposite language!)

[evan_au]

conduct heat & electricity
Soul Surfer established that it is electron mobility through the crystal structure that defines a metal.

Electrical conductivity derives from this mobility: apply a voltage across a metal, and the electrons will promptly migrate towards the positive terminal.

The metallic sheen derives from the shared cloud of electrons: when an electromagnetic wave (light) approaches the metal surface, the electric and magnetic fields of the light cause the electrons to move in a way that cancels the electric field within the metal. This causes the light to reflect, forming a mirror surface (if it is smooth enough).

I understand that thermal conductivity also derives from the shared cloud of electrons that binds the metal ions together in the crystal: heat up one part of the metal, and the vibrations (phonons) propagate easily through this cloud.

Would someone else like to tackle ductility, the sound metals make when you hit them, etc?

[Bored chemist]

Because the ions in a metal are surrounded by a "sea" of electrons, the bonds between them are more or less equally strong in any direction. If the ions get pushed a bit out of place then the electron cloud can shift to accommodate them until they get moved into "the next position along" in the lattice.
So, even while the atoms slide around, they are still held together.
Which is essentially why they are  ductile.
As for "sonorous".
Lead isn't but glass is.
So it's not strictly a  property of metals.
It's a property of reasonably rigid things.
While the bonds in a metal aren't very directional, they are strong- so most metals are fairly stiff.
That elasticity is responsible for the sound.

[syhprum]

Astronomers also use strange and often wildly inappropriate units they talk about the output of supernovas in ergs and wavelengths in Angstroms not to mention solar masses.