What is Molecular: Definition and 581 Discussions

A molecule is an electrically neutral group of two or more atoms held together by chemical bonds. Molecules are distinguished from ions by their lack of electrical charge.
In quantum physics, organic chemistry, and biochemistry, the distinction from ions is dropped and molecule is often used when referring to polyatomic ions.
In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. This violates the definition that a molecule contain two or more atoms, since the noble gases are individual atoms.A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with two atoms in the oxygen molecule (O2); or it may be heteronuclear, a chemical compound composed of more than one element, as with water (two hydrogen atoms and one oxygen atom; H2O).
Atoms and complexes connected by non-covalent interactions, such as hydrogen bonds or ionic bonds, are typically not considered single molecules.Molecules as components of matter are common. They also make up most of the oceans and atmosphere. Most organic substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids they are made of, the nucleic acids (DNA & RNA), sugars, carbohydrates, fats, and vitamins. The nutrient minerals ordinarily are not molecules, e.g. iron sulfate.
However, the majority of familiar solid substances on Earth are not made of molecules. These include all of the minerals that make up the substance of the Earth, soil, dirt, sand, clay, pebbles, rocks, boulders, bedrock, the molten interior, and the core of the Earth. All of these contain many chemical bonds, but are not made of identifiable molecules.
No typical molecule can be defined for salts nor for covalent crystals, although these are often composed of repeating unit cells that extend either in a plane, e.g. graphene; or three-dimensionally e.g. diamond, quartz, sodium chloride. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with metallic bonding. Thus solid metals are not made of molecules.
In glasses, which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals.

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  1. S

    I What drives osmosis? (Statistics or Dynamics?)

    This letter in PhysRevLett describes molecular dynamics simulations to investigate the underlying factors that drive osmosis. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.97.024501 Copy on Internet Archive ...
  2. P

    I Effective Hamiltonian to Rotational Term Values

    Hello, I am fairly new to the world of molecular spectroscopy, so I apologize for any ignorance on my part. For the last few months, I've been working on a diatomic spectral simulation tool and have reached a point where I want to incorporate more advanced theory to model complex interactions in...
  3. S

    Hexagon Patterns in Chemical/Molecular Makeup

    Are physical atoms really arranged in hexagon patterns, or is that some sort of way to represent them for illustrations in textbooks and in science articles? (i.e. atoms aren't actually arranged in any hexagon pattern)
  4. zachary570

    Chemistry Finding the molecular formula of an unknown hydrocarbon

    Hello, I am having some trouble finishing this problem from my textbook and would like some insight on the problem. We are looking for the molecular formula of a hydrocarbon. So, if we let x = number of C atoms and y = number of H atoms then we need to find ##C_xH_y##. From the problem...
  5. M

    A Nuclear moments in molecular spectroscopy

    Hello! I saw many paper extracting the magnetic dipole and electric quadrupole moments of the nuclei by performing RF/microwave spectroscopy on the rotational levels of a molecule. However, I can't seem to find paper measuring higher order moments. For example, the magnetic octupole moment has...
  6. Lotto

    Where to find molecular geometry for Psi4?

    I suppose there is a database for that, but I can't find it. Could anyone send here a link?
  7. S

    Understanding Molecular Vibration of HF: Equilibrium Distance and Binding Energy

    Starting with a), I have learnt that the potential energy function has a minimum at the equilibrium distance ##a##. So at the equilibrium distance the derivative of the potential energy function should equal zero: ##\begin{align} V'(a)&=-\frac{4A}{a^5}+\frac{B}{a^2}=0 \nonumber \\ \iff...
  8. Lotto

    B Can we use molecular speed of gas formula also for the air?

    Molecular speed of gas is generally $$v=\sqrt{\frac{3RT}{M}},$$where R is gas constant and M is molecular mass. Can we use this formula for the air as well, when the air is a mixture of different gasses?
  9. J

    Chemistry Why Does IF5 Exhibit a Square Pyramidal Shape Despite Exceeding the Octet Rule?

    I thought it was trigonal bipyramidal, with iodine forming 5 bonds with 5 fluorine atoms. Actually, iodine has another lone electron pair, making it square planar. Is there a reason why iodine has another lone pair when it has already exceeded the octet rule? And how would I predict how many...
  10. N

    I Exploring the Mysterious Acceleration Phase of Molecular Behaviour

    Princeton U. has a great intro to molec. behav. w. sound, but neither there or elsewhere have I found much on the initial acceleration phase. Is it assumed that movement over these nano-distances is already happening at c - and then there is just more of it ? How can a little stone you drop on a...
  11. guyvsdcsniper

    Vibrational Levels in Molecular Electronic State

    ##K = \frac{N}{m} = \frac{3eV}{bondlength^2} = \frac{4.806*10^-19 J}{(2*10^-10)^2} = 12.015## Then I know that ##H = \frac{1}{2}mωx^2 ## where ## k = mω^2 ## and also ##H=ħω(n+\frac{1}{2}) ## Therefore, ## \frac{1}{2}kx^2 = ħω(n+\frac{1}{2})## Solving for n, ## n = \frac{1/2kx^2}{ħω} -...
  12. Bernadette

    I Gas molecular attraction in the sense of Newton's law

    Hello What experiment or observation shows that for atoms, molecules or masses of gas, there is attraction in the sense of Newton's law (universal attraction)? Bernadette
  13. Mayhem

    The total molecular Hamiltonian

    Hello. As an assignment, I have to explain the total molecular Hamiltonian. Problem is, I can't find it anywhere in my book (Atkins, Physical Chemistry: Quanta, Matter, and Change, 2nd Edition), even when I access the index for "Hamiltonian -> polyatomic molecules". They do give the electronic...
  14. nomadreid

    Molecular covalent bonds across energy levels

    I am sure this is an elementary question; I'm just trying to clarify some points that were poorly explained to me years ago in secondary school. I know that a full answer would involve solving Schrödinger's equation etc., but keeping this on the level of valence electrons,...) I was confused by...
  15. B

    A Higher order PT terms in molecular parameters

    Hello! Is there a place where I can find the expressions for higher order terms of molecular parameters (in particular the spin rotational parameter, ##\gamma##)? All the papers I find just show them up to second order. Thank you!
  16. B

    A Can we have molecular transitions that change multiplicity?

    Hello! Can we have molecular transitions (not restricted to electric dipole) that change the multiplicity of the electronic level i.e. ##2S+1##. For electric dipole that is strictly forbidden. For magnetic dipole, we have a term in the operator of the form ##S\cdot B## and assuming the B is...
  17. B

    A How to slow down a high energy neutral molecular beam

    Hello! I see that in experiments at facilities like ISOLDE, they produce molecular beams at energies of tens of keV. If I understand it right, they first create the molecule as a positive ion, and using electric fields (and maybe magnetic for mass selection) they take the particle out of the...
  18. P

    A Molecular cloud collapsing and fragmentation

    Good morning, I read on the internet that a molecular cloud contains denser part, I also read that a molecular cloud start to collapse according the Jeans law If it's the full cloud collapsing what is happening to these denser parts ? and after collapsing how the fragmentation occurs ? In fact...
  19. A

    Is the molecular formula for sugar correct?

    Is the following molecular formula for Sugar can be written as H22C12O11 or O11C12H22 or O11H22C12 or C12O11H22 or H22O11C12 instead of C12H22O11? Logically they can be written as mentioned above? If Not, Why? There can be many more examples similar to above.
  20. M

    A How Can Molecular Beams Be Slowed and Neutralized for MOT Trapping?

    Hello! If I have a beam of positively (single) charged molecular ions (they can be either bunched on continuous), and I want to end up with a decelerated (close to stationary) ensemble of neutral molecules (e.g. to trap them in a MOT trap), is there a way to reduce the velocity of the charged...
  21. B

    A Measuring new forces with molecular vibrations

    Hello! I noticed in several papers describing high resolution vibrational measurements in diatomic molecules, such as this one, in the conclusion section, that they mention that we can search for new forces (or deviation from gravity inverse square law) between the 2 nuclei by measuring well...
  22. Dario56

    I Why Does Molecular Potential Energy Curve Have That Specific Shape?

    Molecular potential energy of hydrogen in dependence with atomic distance for bonding orbital is given by picture below. We can see that at large distances force between atoms is attractive and potential energy drops to minimum which corresponds to bond energy and length. This part of the...
  23. R

    Get the mass of an argon atom and its molecular weight from Cv

    I don't see how isolate m or M. That is, how to get rid of <v^2> which is how I wrote the average of v2
  24. B

    I Effective molecular Hamiltonian and Hund cases

    Hello! I am reading some stuff about the effective hamiltonian for a diatomic molecule and I have some questions about relating the parameters of these hamiltonian to experiment and theory. From what I understand, one starts (usually, although not always) with the electronic energy levels, by...
  25. P

    Molecular weight of lead(ii) nitrate Pb(NO_3)_2

    Hello everyone I am trying to solve the following question from the book Analytical Chemistry 7th edition by Gard D. Christian - Page 164 Example 5.17 My problem is when I calculate the molecular weight of Pb(NO_3)_2 I get 331.2 g/mol but in the book they get 283.2 g/mol . Could you explain...
  26. P

    Molecular Weight Surprise: N2 Bigger Than O2 & CO2

    So, doing an experiment with my daughter (making similar size balloons filled with carbon dioxide and air), and verifying that the CO2 balloon fell quite noticeably faster due to higher molecular weight; I later noticed that the CO2 balloon shrank clearly faster than the air balloon. This was...
  27. Helena Wells

    Energy differences of molecular orbitals

    Suppose we have a molecule A-A and a molecule A-B . And we want to compare the differences in energy of the different molecular orbitals for example the 1s antibonding orbital with the 2s bonding orbital.How can we do that?
  28. curiously new

    A Calculating noise of a transducer from molecular vibration

    I'd like approximate the noise floor of an ultrasonic air transducer starting from molecular vibrations. Simply put, if I treat atmospheric air as an ideal gas and I confine each air molecule to exist in a volumetric cube with a square face ##A_\mathrm{face}##, then I approximate the number of...
  29. I

    Chemistry Finding the molecular formula of a hydrocarbon

    Moles are proportional to volume. There is 1000 cm3 of gas (carbon dioxide gas plus water vapour) after the reaction, and 200 cm3 of water vapour after the carbon dioxide has been absorbed. Therefore, there is 800 cm3 of carbon dioxide. Since the stoichiometric ratio of carbon dioxide to the...
  30. I

    Molecular Bio/Genetics YouTube playlist needed for Genomic Data Scienc

    I am from physics background and I need some background in Genetics to learn Genomic Data Science. So, can people here suggest some youtube playlist ? I have not studied biology for a long time , so no idea about biology. Suggestions welcome Thanks
  31. E

    Chemistry Molecular orbital theory book reccomenations

    Hey guys, I decide I need to learn the mathematics of molecular-orbital theory, to build on the qualitative approach of my chemistry coursee. To do this I also first need to study single-electron systems and then many-electron systems, the Born-Oppenheimer approximation, and relevant topics...
  32. F

    Exactly how do pressure waves work at the molecular level?

    I've searched high and low for an answer to my question and I can't find a thing. Let's say I blow into the end of a tube. The opposite end is connected to a reed; when the air hits the reed, the reed vibrates and I can hear a sound. The harder I blow, the louder the reed sounds. I've tested...
  33. M

    I Molecular Ratchet -- Feynman's explanations

    Hi all. Feynman writes: https://www.feynmanlectures.caltech.edu/I_46.html https://en.wikipedia.org/wiki/Brownian_ratchet Does emerging of temperature differences supposedly violate the 2nd law? (yes?) As soon as a temperature difference occurs, e.g. a Carnot process could work...
  34. WMDhamnekar

    MHB Molecular orbital theory question about energy level diagrams

    Hello, The sigma $(\sigma)$ molecular orbitals are symmetrical around the bond-axis while pi $(\pi)$ molecular orbitals are not symmetrical. For example, the linear combination of 1s orbitals centered on two nuclei produces two molecular orbitals which are symmetrical around the bond-axis. Such...
  35. LCSphysicist

    Chemistry Molecular percent of compounds

    mx remets to mass in gram of the compost x nx remets to moles of the compost x nTot remets to the total moles x remets to the additional moles of D2 I considered the molar mass: MH2 = 2u MHD = 3u MD2 = 4u 10 = mH2 + mHD + mD2 10 = nH2*2 + nHD*3 + nD2*4 10 = nTot + 0.9nTot + 0.8nTot nTot =...
  36. Yokoko

    Chemistry Molecular Orbital for Sn2 reaction

    The equation can be seen in the picture. I was assuming it was an Sn2 reaction given the polar aprotic solvent and strong nucleophile. For b), I assumed that the question asked me to draw the orbitals as shown above (as opposed to orbital diagram) because it asked me to identify the HOMO and...
  37. M

    I Fitting rovibrational molecular data

    Hello! I have some data for rovibrational transitions between a ##X^2\Sigma^+## and ##A^2\Pi_{1/2}## and I need to extract the molecular parameters (e.g. B, D, ##\gamma## etc) for the 2 levels. I tried pgopher for a while, using Hund case B and A for the 2 states, respectively. However it...
  38. patric44

    Perturbation treatment of hydrogen molecular ion

    hi guys i am a the third year undergrad student and in this 2nd semester in my collage we should start taking quantum mechanics along with molecular physics , our molecular physics professor choose a book that we are going to take which is " molecular physics by wolfgang Demtroder " when i...
  39. new90

    Calculating Molecular Diameter

    How can I calculate diameter of a molecule
  40. BillTre

    Molecular Mechanism for Bacterial Sliding

    This article from Science Advances (probably not open access, an AAAS publication) has described a complex and interesting molecular mechanism (a nano-bio-molecular machine). A rotary motor in the cell membrane (driving mechanisms unknown) spins a gear that pushes a "tread" along a track imbeded...
  41. B

    Thermodynamics: calculate thermodynamic derivative from data?

    I don't understand how to use output from an NPT molecular dynamics simulation to compute a thermodynamic derivative. I need to compute this (where "d" is a partial derivative, "T" is a subscript that means, "at constant temperature," and "E" is internal energy): -(dE/dV)T I have a simulation...
  42. despues18

    Programs Seeking advice on how to search for a Graduate School in Molecular Biology

    Will finish B.S. in molecular biology in May. Have found asking vague questions about where to go/apply to next are pretty unhelpful. I believe that going through indexes of interesting research is the closest I can get to finding a place to advance myself and be happy in. here is an example...
  43. K

    Best software to fit molecular spectroscopy data

    Hello! I have some data from a molecular spectroscopy experiment, containing vibrational and rotational spectra, and I want to fit the peaks with Voigt profiles (one for each peak) in order to obtain the centers of the peaks. Do you know any software suitable for this kind of fit? I usually use...
  44. S

    Periodic Boundaries in Molecular Dynamics Simulations

    In molecular dynamics people use periodic boundaries to confine particles being simulated. I read here that they are used to simulate large "infinite" particle systems. How can I know that the periodic boundary is simulating actual molecular outcomes for a finite particle system that had a large...
  45. A

    A Visualizing water molecular dynamics in an electromagnetic field

    I am interested in showing a visualization of water molecules in a time-varying electric/magnetic field as part of my PhD work. I would like something like this visualization: , but with an external time-varying field applied. At first, I thought of simply animating water molecules...
  46. D

    Chemistry Molecular Shape of BF3: Trigonal Planar or Pyramidal?

    Answer given is " The molecule could be trigonal pyramidal or T shaped, but could not be trigonal planar." I am confused as the shape of BF3 is trigonal planar...
  47. r12214001

    Chemistry HF MO: Why F Uses 2P for Bonding in HF Molecular Orbital Model

    In figure. Why F use 2P rather than 2S to bonding in HF MO model.
  48. K

    I Molecular Bond in H2+: Explained

    The ## {H_2^+}## molecule consists of two hydrogen nuclei and a single electron.What kind of molecular bond do you think holds this molecular together? Explain.
  49. Q

    Molecular Formula of different Octane rating gasolines?

    Trying to calculate the enthalpy of combustion for gasolines with different octane ratings, 95 and 91 gasoline in particular. The thing is, they are mixtures and not compounds. To calculate the enthalpy of combustion, I am trying to find the molecular formula for each of the gasolines, in order...