44

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; ...


39

The choice of whether to write binary hydrides with the hydrogen first or second depends on whether that hydrogen is considered acidic, with water marking the delineation point. By convention, if a binary hydride is more acidic than water, then they are written in the form $\ce{H}_{\text{n}}\ce{X}$. If a binary hydride is less acidic than water, they are ...


38

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 ...


37

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 confusion. ...


33

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 ...


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 ($\ce{CH3COO-}$) ...


30

If you are using MS Word 2007 or newer, use the equation feature. It is designed for math but works okay for chemistry. Go to the insert tab. (For shortcut you can press Alt+= sight together) Click on the equation button on the far right. Type in your equation. Use the buttons in the ribbon to do superscripts and subscripts. Alternatively you can use _ ...


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, ...


24

The results of measurements and other numerical values of quantities are often given with an associated standard uncertainty. A numerical value and the associated uncertainty may be expressed as shown in the question: $$\begin{align} y&=21(1)\ \mathrm{cm^{-1}}\\[6pt] &=a(b)\ \mathrm{cm^{-1}} \end{align}$$ where $y$ is the estimate of the measurand (...


23

The notation refers to the valency (number of valence electrons involved in bonds, i.e., either 3 or 5) and coordination number (number of substituents attached) in organophosphorus compounds. For example, it could refer to the tautomeric forms of phosphonate esters:1 So the λ3σ3 refers to a P with 3 bonding valence electrons (the lone pair doesn't count) ...


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.


21

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 ...


21

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 ...


19

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 ...


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

Even though your question has already been answered (and this is not an alternative answer), but if you're open to it, switching from Word to LaTeX with the chemmacros package (PDF) will benefit you greatly in the long run.


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 ...


17

You not only can, but also must treat symbols for units by the ordinary rules of algebra, since unit symbols are mathematical entities and not abbreviations. The value of a quantity is expressed as the product of a number and a unit. That number is called the numerical value of the quantity expressed in this unit. This relation may be expressed in the form ...


16

In the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), single italic lower-case Latin letters are used in some stereodescriptors (such as ‘c’, ‘m’, ‘p’, ‘r’, ‘s’, and ‘t’), in descriptors for fusion sites (e.g. dibenzo[c,g]phenanthrene), in locants (‘o’, ‘m’, and ‘p’ in place of ortho, meta, ...


15

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 ...


15

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 ...


15

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

Yes, you are correct: “lit.” refers to the literature value. Usually one would also expect the corresponding reference, but it looks like Merck doesn't attach any to the provided data directly in place. From The ACS Style Guide, section Reporting Analytical Data [1, p. 274] (emphasis mine): Melting and Boiling Points mp 175.5 °C (lit.25 mp 175–176 °C)...


14

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 ...


14

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.


14

The curly brackets denote "activity of" the species therein. See the Wikipedia section: Basic definitions and properties of Equilibrium constant


14

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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