46

The circle, if anything, represents the inability of our everyday physical intuition to cope with the quantum phenomena. See, you would often encounter those two pictures with "double bonds this way" and "double bonds that way", intended to give a vague impression that the molecule switches quickly between the two, but that's not true. It does not switch; ...


44

You are correct suggesting that 1 μg/kg implies 1 ppb, however the reverse is not true. For instance, 1 ppb can also be 1 nmol/mol, and the reader will never have a chance to deduce which one is it unless you explicitly define the usage of the "parts per something" in the text. This clutters the manuscript with redundant notes and causes overall ...


43

Looks like in 1966 there were still residues of double prefix notation in use. Here's an entry from Russ Rowlett's compilation of units on ibiblio.org (Rowlett): millimicro- (mμ-) an obsolete metric prefix denoting 10-9 or one billionth. This prefix has been replaced by nano- (n-). Here's what is noted on wikipedia: Double prefixes Double prefixes have ...


39

Yes, you may. It is quite common to convert units into each other. The simplest conversion might be the prefixing of units, e.g. $$\mathrm{\frac{km}{m}}=1000 \Longleftrightarrow \mathrm{1~km = 1000~m}.$$ Another example is the interconversion of units of energy. In some parts of chemistry, it is still quite common to use calories, in others Joule, as a SI ...


36

This is not a simple physics versus chemistry distinction. I taught Physics for 25 years and saw many examples of either usage in multiple textbooks. In fact, at some point in my tenure, the AP Physics committee swapped conventions on the equation sheet for the AP Exam. Just my take here: I've always attributed the work-done-by-the-system camp as being more ...


33

The symbol is called "double dagger" (sometimes also "double cross") and is used to denote transition state (a maximum in an energy diagram; also often denoted with "*" or "TS") or a related physical property. Note, however, that a transition state and an intermediate are two different terms. The symbol has the peculiar origins: as written by H. Eyring, it'...


32

As stated in both the links Geoff and Philipp have kindly commented (1, 2) they are to do with symmetry labels we chemists like to assign to orbitals. Knowing an orbitals symmetry class can lead to a lot of simplifications down the road when you use quantum mechanical calculations and even dictate the reactivity of which orbitals are "allowed" to interact ...


32

I quote the Green Book by IUPAC, 2nd printing (2008), section 1.6, enumeration item 2: The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman. [...] As such, the correct way to write it is $\ce{NO}_x$, because $x$ is a variable. Please note ...


31

There is nothing wrong with either formula. And you can use even more: $\ce{NaC2H3O2}$ $\ce{C2H3NaO2}$ It really depends on which point you want to bring across. $$\ce{NaCH3COO}$$ This formula, being analogous to formulae like $\ce{NaCl}$ stresses the inorganic salt view more. It shows that there is a cation ($\ce{Na+}$) and an anion ($\ce{CH3COO-}$) ...


28

Treating units as if they were algebra is called “dimensional analysis”. One example given in that article is the question of how many seconds are there in two years. $$2\ \mathrm{yr} \times 365\ \mathrm{day}\ \mathrm{yr}^{-1} \times 24\ \mathrm{hr}\ \mathrm{day}^{-1} \times 60\ \mathrm{min}\ \mathrm{hr}^{-1} \times 60\ \mathrm{s}\ \mathrm{min}^{-1} = ...


27

Not only may you, but you absolutely should. As a physics professor, watching students toss away the information contained in units is incredibly frustrating. Typically students drop the units, "do the math," and then put back on whatever final units they think should be there (if they bother with the last step at all). This leads to a shockingly large ...


26

Omitting j when alphabetically enumerating things has a long tradition. First of all, the alphabet did not always exist in the form we know it today. Quoting Wikipedia: After [...] the 1st century BC, Latin adopted the Greek letters ⟨Y⟩ and ⟨Z⟩ [...] Thus it was during the classical Latin period that the Latin alphabet contained 23 letters: [no J, V, W] [.....


23

By writing $\ce{AB.xCD}$ chemists mean that there are CDs are found in the crystalline framework of AB. The most common example of this is water trapped inside the crystal structure of ionic compounds. (See water of crystallization in wikipedia) An example that's often taught is $\ce{CuSO4.5H2O}$. See that 5 that's a representative of $x$? It means that ...


23

Don't get too caught up in notational rigidity. You're "allowed" to use any notation you want, if it gets you to the correct answer. If you really want, go ahead and use $\ce{Na\ - \ e^{-} -> Na+}$, or go all out and use $\ce{-Na^{+}\ - \ e^{-} -> - Na}$. However, as Richard Feynman discovered while learning trigonometry, if you start using too much ...


22

For the azimuthal quantum number (l) of an atom, there is no "j" because some languages do not distinguish between the letters "i" and "j". L is the total orbital quantum number in spectroscopic notation and uses capital letters. The nomenclature just follows suit with the suborbital notation and skips J since there is no corresponding j.


22

IR-3.3.1 Isotopes of an element The isotopes of an element all bear the same name (but see Section IR-3.3.2) and are designated by mass numbers (see Section IR-3.2). For example, the atom of atomic number 8 and mass number 18 is named oxygen-18 and has the symbol $\ce{^{18}_{}O}$. IR-3.3.2 Isotopes of hydrogen Hydrogen is an exception to the rule in Section ...


19

Your answer is the same as the book's. The book wrote the first carbon as $\ce{H3C}$ to stress that the chain continues via a carbon to carbon bond, not bonded a carbon to hydrogen to carbon bond. As for why it's not like this in other examples, technically it is not necessary to write it this way, but it's strange your book lacks that consistency.


19

The second one, $(2)$, $\ce{<->}$, indicates resonance structures. The difference between the second two is explained in another post - What are the correct equilibrium arrows? Loong basically says that $\ce{<-->}$, $(4)$, indicates forward and reverse elementary steps happening in equilibrium, that is that those are the only two things ...


18

Here the bond connecting the substituent with the center of the ring suggests that the substituent may be located on any of the three positions (ortho-, meta-, para-). From Graphical representation standards for chemical structure diagrams (IUPAC Recommendations 2008), section GR-9.1 Small substituents [1, p. 393] (emphasis mine): Parent structures with a ...


18

The at symbol @ universally means "at the site" or "at the rate of". It is routinely used in supramolecular chemistry to denote guest@host relationship because it follows the original designation of denoting the binding site; its shape resembles a guest (letter "a") entrapped within a host shell (circle sign "○") ...


17

Okay, let's tackle at least one problem here. Consider the rotation of 1,2-dichloroethane (BP86/cc-pVDZ): These conformational changes can be further rationalised: C and C' are the same conformation, since these are mirror images. The same applies to B and B'. A and C are local minima and can be referred to as conformes in the above given way B and D ...


16

The -1 means "per" unit. So your first example mol/L-1/s-1 is not correct - it would actually be written as mol L-1 s-1, OR mol/(L s). It is also sometimes written as mol/L/s, but the double division is ambiguous and should be avoided unless parentheses are used. If it were mol L-1 s-2, this would mean moles per litre per second per second. This is really ...


16

The use of dots in inorganic chemistry Let's take the example of copper sulfate penta-hydrate: $\ce{CuSO4. 5H2O}$. The dot here is used essentially as an expression of ignorance to indicate that, though the parts of the molecule separated by the dot are bonded to one another in some fashion, the exact structural details of that interaction are not fully ...


16

As stated in this answer, these are irrep (irreducible representation) labels for molecular symmetry point groups. In the context of chemistry, point groups are usually introduced when learning about structural symmetry (atoms and bonds). This is a broad topic with many technical points, too many for a single answer, so I won't cover the basics but hopefully ...


15

The common confusion here is that two very different things have the same name. $\ce O$ is a free oxygen atom and $\ce{O2}$ is two oxygen atoms chemically bound to form an oxygen molecule. There is no common analogy for $\ce C$, but $\ce{N2}$ is called nitrogen, $\ce{H2}$ is hydrogen and $\ce{Cl2}$ is chlorine, each having the same name as that of their ...


15

According to Nomenclature for Chromatography (IUPAC Recommendations 1993) [1, pp. 843, 845] (also listed in IUPAC Gold Book), retardation factor is denoted as follows: Column chromatography: $R$ (capital $R$ in italics): 3.7.13 Retardation Factor ($R$) The fraction of the sample component in the mobile phase at equilibrium; it is related to the ...


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