Group IV. Metal, metalloid, non-metal?

Question

In group IV, the upper elements, carbon can form a giant covalent structure with pure carbon. Under carbon, silicon can form a giant covalent structure with oxygen. Under it, metals come out and they can only form metallic bond.
It is non-metal----> metalloid----> metal when down a group.
I think that metals are metals not because they are metals. But they got the properties of metals. That means they got the properties of metals first, so people classify them into metal. And I think non-metal and metalloid are the same.
What I want to ask is elements in same group, got the same number of outermost shell electrons, but why their properties change down a group? From non-metal to metal. Why elements in the upper of group IV covalent bond together but elements under them become metallic bond? What cause this?
Last, only group IV can form giant covalent structure, so there would be giant covalent structure form by carbon and silicon only, right?

And I think non-metal and metalloid are the same *with it*.

Answer 1

A lot of insightful questions here. I would like to know what stage of study you are at, and what resources you have access to. That would help me give appropriate answers.

Going down the group, the principal quantum number (and distance from the nucleus) increases, so the outer electrons are less tightly held.

There are two slightly different definitions of metal. The physical definition is in terms of electrical conductivity, and results from what is called the "band structure", and delocalised bonding (cations in a sea of electrons). This usually goes with structures in which each atom has a large number of neighbours, since the bonding is not directional, and also with malleability because layers of atoms can be pushed past each other and still be bonded at each stage.

The chemical definition of a metal is, roughly, something which forms cations. Both physical and chemical definitions depend on the atoms having only a loose hold on their electrons, which is why "metallic character" increases down a group.

I never liked the term "metalloid". It seems to mean intermediate between metal and non-metal, but like you I am dissatisfied with this definition.

Chemically, tin is a metal, since it forms cations. In the solid state it has two different crystalline forms; the form stable at low temperatures has the diamond structure, and is a semiconductor, while the high temperature form is a metal.

You are right in saying that you need four bonds to make a three-dimensional giant structure. However,three bonds to neighbours are enough to form a giant sheet covalent structure. Such structures are found in one crystalline form of phosphorus, and in crystalline forms of arsenic, antimony, and bismuth. As you go down that group, the sheets get closer together, and localised bonding within the sheets becomes progressively less important compared with delocalised bonding between them, so that solid bismuth is physically (just about) a metal.

Answer 2

okay i don't know all the answers to your qns, but all i can say is, non-metals and metalloids are definitely different. metalloids have properties of BOTH metals and non-metals. while non-metals are just... well, non-metals.

as for elements of the same group with varying properties...
this is because as you go down the periodic table, the number of electron layers increase. when the number of electron shell layers increase, the amount of energy that binds them to the core of the atom decreases. Therefore, this changes the reactivity of the different elements.

Answer 3

15

Answer 4

Because d orbitals apear. By the elements that have d orbitals apear the metallic properties.