The covalent compound: the valence electrons are put in common and belong equally to all atoms that make up that compound forming a covalent compound. Covalent compounds occurs in oxides and non-metallic acids.
The ionic compound: the valence electrons transfer from one atom to another one forming a ionic compound. The atom that gives up electrons turns into a positive ion and the atom that receives turns into a negative ion. Ionic compounds occur in metal oxides and salts. Chemical elements located in groups 1-3 of Mendeleev's table are electro-positives and those located in groups 5-7 are electro-negatives.
Metallic compounds, as the name suggests, occurs in metals. Unlike covalent compounds where the valence electrons are placed in common between atoms that make up a compound, in the metallic compounds, the valence electrons of all atoms making up the metal, belong equally to the entire metal structure. That's why, metals are very good heat and electrical conductors. All very good electrically conductive materials (metals and alloys) can not be electrified. The materials that electrifies create around them a stationary electric field. Those that are very good electrically conductive materials have a moving electric field only if they are under electric tension and around them a magnetic field is formed.
The magnetic field can be highlighted with a magnetic needle that does not show the Geographic north when the electric current is passing through the conductor. Magnetic field lines formed this way are elongated but only for inertial observer. So he sees them from his reference system which remains fixed to the referential of the wave that goes away from it with the speed of light. The electric wave is the one that carries the energy. The electric field is found inside the conductor and has closed field lines as opposed to the open electric field lines which can be found in electrified bodies.
If an observer could move parallel to the conductor, with the speed of light, then the observer wouldn't see the magnetic field lines elongated.
Additional explanations to avoid confusion:
The observer does not mean a man, an object can't move at the speed of light, greater speed = greater mass.
The mass is a measure of inertia and inertial force opposes the motion, having the vector oriented in opposite direction of motion vector. Besides all this, people don't have sense organs for the perception of physical fields, so there is no way for someone to see the magnetic fields lines, they can be viewed with the help of iron filings. Covalent compound can be simple (2 electrons participate in the compound), double (4 electrons participate in the compound) or triple (6 electrons participate in the compound).