How many atoms are in a molecule of talc?

Answer 1

There are several different types of feldspar, but they all contain aluminum, oxygen, silicon and 1 or more alkaline earth metals or alkali metals. The feldspars are not molecular, they have giant polymeric anions with cations in voids in the crystal.

The feldspars are minerals, some have a definite chemical composition but many do not. What is common to all of them is that the number of (Si + Al) atoms is half that of oxygen- becuse tetrahedral [AlO4 ] units replace tetrahedral [SiO4] units in silica SiO2 framework and because the net effect is replacing Si4+ with Al3+ this creates a polymeric anion , and the charge balancing cations sit in the voids in the structure.

You just take the formula and count how many atoms you see, paying attention to whatever subscripts and parentheses there might be. Because these are minerals the formulas can a look a little strange to chemists. For example labradorite, (Ca,Na)Al(Al,Si)Si2O8 In this the ratio ratio of Si:Al overall is between 3:1 and 1:1. The more Al in the mineral the more negative charge on the polymeric anion and the more balancing calcium and sodium cations there will be.

The polymeric anion has 8 oxygen atoms so we expect there to be 4 balancing silicon and aluminium atoms. The formula shows there are three definites 2 silicon atoms and one aluminium, and the fourth is a mixture of Si and Al shown by the (Si, Al) part. If the fourth atom were all Si the ratio is 3:1 , if it were all aluminium it would be 1:1.

Answer 2

The chemical formula for gypsum is CaSO4·2H2O , which means that there are 12 atoms in one molecule of gypsum.

Answer 3

Salt (NaCl) is formed by ionic bonds, so it does not come in molecules. Instead, it is called a formula unit, and there are two atoms in this particular one.