In ionic compounds such as table salt, NaCl, the atoms as ions do not bond to specific neighbors. Surrounding each chloride ion in a salt crystal are six sodium ions. Likewise, six chloride ions surround each sodium ion.
This attraction of oppositely charged ions extends throughout the entire crystal. There is no discrete bonding of a particular sodium ion to a specific chloride ion. So the formula for sodium chloride is expressed as the smallest whole number ratio between the Na and Cl ions which is for sodium chloride.
So, NaCl is also a formula but not a molecular formula. The reason for the term "formula unit" is that it is useful when we talk about how much of one substance is required to combine with a particular amount of another substance.
For example: to make the smallest amount of hydrogen carbonate we combine one molecule of water with one molecule of carbon dioxide. But now suppose we want to combine silver nitrate with sodium chloride. Both of these are ionic compounds, they do not form molecules.
In this case you would say that one formula unit FU of sodium chloride combines with one FU of silver nitrate. The one-to-one combining in both these examples is simply coincidence. Because the positively and negatively charged ions in an ionic compound are arranged in a crystal lattice, there is no discrete particle comparable to a molecule. What's the difference between a formula unit and a molecule? Dec 17, Explanation: A molecule is composed of two or more elements that are covalently bonded.
Related questions What is the chemical formula for sugar? Navy, as he had been in World War II. I'm just not sure that the formula continues to work for us at 50 given other uses we might have for that money. The formula for the date of its foundation in may be thus expressed—Harvard College founded; the chum age. When we talk about atoms that have reacted with each other the end product is a molecule. So primarily a molecule is made up of atoms sometimes different types of atoms other times same kinds of atoms.
The atoms that react together share a bond, and there are many types of bonds such as a covalent bond, ionic bond, etc. There two atoms of Oxygen that form one molecule of Oxygen.
Similarly, one atom of Hydrogen and two atoms of oxygen forms a molecule of H2O which is commonly known as water. To define a molecule in a more scientific way the definition would be that the molecule is made up of two or atoms that are bonded to each other. Molecules are the smallest part of any given compound. This molecule may or may not consist of same types of atoms. Molecules are generally discussed in the chemical reactions for finding the molecular weight, moles, etc.
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