[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. 

[hamdani yusuf]

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

In certain condition hydrogen can be metallic. So I guess that metal is not a property attached to the elements independent of other factors.
AFAIK, organic superconductors such as fullerene and graphene can easily conduct electricity but they are still not called metal.

[alancalverd]

A metal is an element in the lower left of the periodic table. The boundary is not distinct. Hydrogen can conduct electricity at high pressures, and the useful stuff in the middle includes semiconductors like Ge and Si.

More broadly, of course, a metal is "any substance with metallic properties". Most of the metals we use are not pure elements but alloys.

[Bored chemist]

A metal is an element

Most of the metals we use are not pure elements

There seems to be an elementary error there.
Very few people would try to argue that bronze and steel are not metals.

The definition's not that easy
Anyone for tungsten bronze or molybdenum bronze?
https://en.wikipedia.org/wiki/Molybdenum_bronze

[hamdani yusuf]

A metal (from Greek μέταλλον métallon, "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically malleable (they can be hammered into thin sheets) or ductile (can be drawn into wires). A metal may be a chemical element such as iron; an alloy such as stainless steel; or a molecular compound such as polymeric sulfur nitride.

In physics, a metal is generally regarded as any substance capable of conducting electricity at a temperature of absolute zero.[1] Many elements and compounds that are not normally classified as metals become metallic under high pressures. For example, the nonmetal iodine gradually becomes a metal at a pressure of between 40 and 170 thousand times atmospheric pressure. Equally, some materials regarded as metals can become nonmetals. Sodium, for example, becomes a nonmetal at pressure of just under two million times atmospheric pressure.

In chemistry, two elements that would otherwise qualify (in physics) as brittle metals—arsenic and antimony—are commonly instead recognised as metalloids, on account of their predominately non-metallic chemistry. Around 95 of the 118 elements in the periodic table are metals (or are likely to be such). The number is inexact as the boundaries between metals, nonmetals, and metalloids fluctuate slightly due to a lack of universally accepted definitions of the categories involved.

In astrophysics the term "metal" is cast more widely to refer to all chemical elements in a star that are heavier than the lightest two, hydrogen and helium, and not just traditional metals. A star fuses lighter atoms, mostly hydrogen and helium, into heavier atoms over its lifetime. Used in that sense, the metallicity of an astronomical object is the proportion of its matter made up of the heavier chemical elements.[2]

Metals, as chemical elements, comprise 25% of the Earth's crust and are present in many aspects of modern life. The strength and resilience of some metals has led to their frequent use in, for example, high-rise building and bridge construction, as well as most vehicles, many home appliances, tools, pipes, and railroad tracks. Precious metals were historically used as coinage, but in the modern era, coinage metals have extended to at least 23 of the chemical elements.[3]

The history of metals is thought to begin with the use of copper about 11,000 years ago. Gold, silver, iron (as meteoric iron), lead, and brass were likewise in use before the first known appearance of bronze in the 5th millennium BCE. Subsequent developments include the production of early forms of steel; the discovery of sodium—the first light metal—in 1809; the rise of modern alloy steels; and, since the end of World War II, the development of more sophisticated alloys.

https://en.wikipedia.org/wiki/Metal
It appears that the term "metal" has different meanings depends on its context.