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I came a across a passage in my textbook on Formula Mass topic which mentioned something about discrete molecules.

Some substances such as sodium chloride do not contain discrete molecules as their constituent units. In such compounds, positive (sodium) and negative (chloride) entities are arranged in a three dimensional structure.

What does the author mean by discrete molecules? What are the properties of discrete molecules? I came across this term for the first time. What does the author want to convey in the above context?enter image description here

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  • $\begingroup$ It's an ionic compounds. There are no molecules. $\endgroup$ – Zhe Aug 11 '18 at 13:16
  • $\begingroup$ @Zhe Can compound be formed both ways? i.e. It can be either ionic or a molecule? $\endgroup$ – Saksham Sharma Aug 11 '18 at 13:19
  • $\begingroup$ I'm not sure I understand what you are asking? If you are asking if there are covalent compounds and ionic compounds, the answer is yes. $\endgroup$ – Zhe Aug 11 '18 at 13:20
  • $\begingroup$ @Zhe And both these are different concepts? Covalent compounds and ionic compounds? $\endgroup$ – Saksham Sharma Aug 11 '18 at 13:21
  • $\begingroup$ @SakshamSharma Yes. There are a variety of different types of solids. $\endgroup$ – matt_black Aug 11 '18 at 13:24
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What the author is pointing out is that not all solids are made up of distinct units (in his terminology "discrete molecules").

Consider the difference between the following solids: copper, ice, iodine, common salt, diamond.

In copper the units are copper atoms held together (in effect) in a sea of electrons but with a fixed arrangement of the atoms.

In ice, we have a rigid array of water molecules largely held in a 3D array in specific orientations by hydrogen bonds between the water molecules.

In iodine the structure consists of I2 molecules held together by weak dispersion forces.

In common salt the units of the crystals are individual ions of Cl- and and Na+ ions in an infinite 3D array as described in the picture.

In diamond the structure consists of an infinite array of tetrahedral carbon atoms bonded to each other in an infinite network by covalent carbon-carbon bonds.

In ice and iodine the discrete units of the structure are molecules and when the solids fall apart the molecules retain their individual, discrete identity: ice melts to give water which also contains water molecules; iodine sublimes to give a vapour that still contains iodine molecules.

None of the other solids can be decomposed into molecules. If, for example, salt is dissolved in water, the result is a solution of the individual ions.

The point the author is making is that some solids contain units that are preserved when the solid is broken up and some do not. In many chemicals the units are discrete molecules that retain their identity when not in the solid state. In others the units that make up the crystal are either atoms or ions and there are no discrete components at any other level.

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  • $\begingroup$ does the author mean to say that sodium chloride is not a molecule but a combination of metal and non metal? And this metal and non metal aren't molecules either? And hence it is an ionic compound? $\endgroup$ – Saksham Sharma Aug 11 '18 at 13:28
  • $\begingroup$ @SakshamSharma Sodium chloride is not a molecule. it consists of sodium ions and chloride ions. It is made from combining a metal and a non-metal (an electron transfers from sodium to chlorine to create two ions). The resulting compound is an ionic compound. But be careful, some ions can be molecules as well, just not in this case. $\endgroup$ – matt_black Aug 11 '18 at 13:40
  • $\begingroup$ can you give a simple example for the latter? $\endgroup$ – Saksham Sharma Aug 11 '18 at 13:42
  • $\begingroup$ @SakshamSharma Sodium acetate consists of a sodium ion (as in salt) and an acetate ion (a molecule formed from the acetic acid molecule when it is deprotonated). $\endgroup$ – matt_black Aug 11 '18 at 14:07

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