As shown in the figure, A, B, C and D are the structural diagrams of four kinds of particles, and E is the information of silver in the periodic table of elements. Please answer: (1) A, B, C and D in the diagram. Element (1) is the general name of a kind of atoms with the same proton number (i.e. nuclear charge number), and it is the proton number (i.e. nuclear charge number) that determines the type of element. The number of protons in the nuclei of particles A, B, C and D are 13, 17 and 18, respectively, representing three elements.

(2) In an atom, the number of protons = the number of electrons outside the nucleus, 18=2+8+x, and x = 8.

(3) The cation represented in A, B, C and D is A, which is aluminum ion; The anion is c, which is chloride ion; The compound formed is aluminum chloride, in which the valence of aluminum is +3, the valence of chlorine is-1, and the chemical formula is AlCl3. ..

So, the answer is: (1) three; (2)8; (3) aluminum trichloride.