What is Molecule: Definition and 460 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. Dario56

    Why Does Raman Activity Require Anisotropic Polarizability?

    It's mentioned that the normal mode of molecule needs to involve the change in molecular polarizability to be Raman active. Explanation is provided in Physical Chemistry textbook by Atkins on the example of the rotational Raman spectra. Only the frequency of the electric field ##(f_i)## occurs...
  2. D

    B Max Speed of Molecules: 1 Electron Stripped?

    I know that we accelerated charged particles very close to the speed of light. How about a full molecule, with at least 2 nuclei and most of the electrons? What is the maximum speed achieved/recorded before the molecule broke apart? I ask this question because I'm afraid that the maximum speed...
  3. hilbert2

    I Pair interaction potential with more than one mininum?

    Pair interactions between atoms and molecules (in gas kinetic theory simulations or other applications) are described by empirical potential energy functions such as the Lennard-Jones potential: ##V_{LJ} (r) = 4\epsilon \left[ \left(\frac{\sigma}{r} \right)^{12} -...
  4. V

    Definition of a non-polar molecule

    I think definition (a) is not correct since the center of charge distribution rather than mass distribution is important here. The correct definition is the one given in (b). I am thinking that a distribution of charge will have a center of charge ##(x_c,y_c,z_c)## for -ve charges according to...
  5. B

    A Electron capture in a molecule

    Hello! If I have a (diatomic for simplicity) molecule containing a nucleus that decays by electron capture, are there any theoretical calculations of how that would behave in practice? For example would the lifetime change? Would the resulting molecule still be bound? For example if I start with...
  6. Z

    Chemistry Creating KF Molecule from Neutral Atoms: A Process of Ionization and Binding

    Let's think now about the energy to create a KF molecule from neutral atoms. First we need to ionize both K and F: K loses an electron and F gains an electron. Then we have to bring the ions together. The first ionization energy of K is 418 kJ/mol and for F is 1681 kJ/mol. The electron...
  7. V

    B Collision time interval of a gas molecule with wall of container

    I have been trying to make sense of the derivation of pressure under Kinetic Theory of Gases chapter, but it's not making sense to me when the impulse momentum equation is used for the collision between a gas molecule and the wall of the container. The book says that for the elastic collision...
  8. Lotto

    B The energy of a molecule has what sign?

    And have all molecules or even atoms negative energies? So when a molecule have energy let's say -70 Ha and the other -75 Ha, does it mean that the second molecule has a lower energy?
  9. S

    I Selection Rules for a Diatomic Molecule?

    Hello! I have a question about selection rules and electronic transitions of a diatomic molecule: I can't find a good explanation about that, and there's so much confusion about the right rules to use. I studied that on Bransden-Joachain's "Physics of atoms and molecules", but there (to whom...
  10. B

    A Trying to reproduce the energy levels of a molecule from a paper

    Hello! This is quite a specific question, so if anyone knows the details I would really appreciate your help (@Twigg ?). I am trying to reproduce figure 1 from this paper (it's for the PV experiment performed on BaF). While I am getting quite close to it, the levels don't fully match (I am...
  11. LCSphysicist

    Diatomic molecule at a constant temperature

    A diatomic molecule ##D_{2}## in ##30K##, in ##t=0##, is in the state ##| \psi (0) \rangle = \frac{1}{\sqrt{26}}(3 | 1,1 \rangle + 4| 7,3 \rangle + | 7,1 \rangle )##, where the kets denote states ##| l,m \rangle##. Use ##\frac{\hbar}{Ic4\pi}=30.4cm^{-1}##. Obtain ##| \psi (t) \rangle ## I...
  12. neilparker62

    Non polar molecule with polar bonds?

    Consider for example Carbon Dioxide. Oxygen is more electronegative than carbon so should obtain the "lion's share" of the paired electrons in the double bonds. But (as I see it anyway) the oxygen atoms on either side of the central carbon "assist" the carbon atom to maintain an even share of...
  13. bbbl67

    What is the most massive molecule ever?

    What is the most massive molecule that's ever been made? How much does it weigh, and how many atoms is it made from?
  14. M

    A Dissociation of a diatomic molecule containing Al

    Hello! Is there any diatomic molecule containing Al (or even a molecular ion), which upon dissociation from a given electronic level, leaves behind Al as a neutral atom? Any reference is appreciated. Thank you!
  15. K

    A Centrifugal distortion of a diatomic molecule

    Hello! Is it possible for the centrifugal distortion constants (D, H and so on) of a diatomic molecules to be negative? It looks like for high enough vibrational distortions they can be negative, but I don't think I've ever seen that in practice so I was wondering if there is something...
  16. Hananism

    The distance traveled by a molecule at a specific temperature

    I found this equation: and kB and T are Boltzmann constant and temperature, respectively.
  17. guyvsdcsniper

    Electric field lines of H2O molecule

    I wanted to post my work so far to see if I am on the right path toward the correct answer so far. I have attached a ss of the actual problem and my work in the attachments
  18. B

    A Power induced shift in rotational transitions for a diatomic molecule

    Hello! I am analyzing some data from some rotational transitions between 2 electronic energy levels in a diatomic molecule. I noticed that for different runs that covered the same regions, the peaks we observe are shifted with respect to each other when the power of the laser driving the...
  19. M

    I Spin-rotation in a molecule

    Hello! Can the spin-rotation constant, ##\gamma##, have a negative value in a diatomic molecule, or does it have to be positive (in my case the state is a ground state ##^2\Sigma_+##, which is about 14000 cm##^{-1}## away from the next electronic level). I don't think I have ever seen negative...
  20. M

    I Rising and lowering operators in a molecule

    Hello! If I have a molecule in Hund case a, I can write its wavefunction (electronic + rotational) as ##|e,\Lambda,\Omega,S>|J,\Sigma,\Lambda,\Omega,M>##. I am not sure what happens if I apply, say, ##S_+## on this wavefunction (assuming I am not applying it to the top of the ladder), where this...
  21. H

    Overlap integral of hydrogen molecule

    Hi! Some help with this problem would be much appreciated. The overlap integral is defined as ##S = \int \phi_A (\mathbf{r}_A) \phi_B (\mathbf{r}_B) \,d\mathbf{r}##. For the two orbitals, I have that $$\phi_A = \frac{1}{\sqrt{\pi}} \Big( \frac{1}{a_0} \Big)^{3/2} e^{-r_A / a_0}$$ for the 1s...
  22. S

    Question about direction of movement of water molecule

    When the boat moves to the left, water molecules below it also move to the left. Is it correct to say the reason is because of Newton's 3rd law? Water molecules provide friction force to the boat so force on boat by water molecules is to the right. By Newton's 3rd law, there will be force on...
  23. M

    I Vibrational cooling of a molecule

    Hello! I understand that many of the non-optical methods used to cool down degrees of freedom in a molecule (e.g. buffer gas cooling, supersonic expansion) are able to cool down translational and rotational, but not vibrational motion. Is this because the gap between vibrational levels is much...
  24. Roby002

    I Feynman's description of the quantum behaviour of an ammonia molecule

    Hi guys! These days I've been reading Feynman's description of the quantum behaviour of an ammonia molecule. He assumes the N up or down as tuo basis states. He then says there's a little probability that the state UP becomes DOWN and viceversa. But for definition basis states are...
  25. B

    I Entangling atoms in a molecule

    Hello! I came across several papers that use entangled states of 2 ions in a trap to perform measurements with a much higher accuracy than classical (non-entanglement) methods. Here is an example of such a paper where they achieve the best measurement to date of an isotope shift. I was wondering...
  26. LCSphysicist

    Wave function for the Helium molecule

    I am having a trouble to understand why the helium's wave function (in which we are ignoring the electric interaction between the electrons, as well the motion and problems that arise in considering the nucleus in the wave function) can be written as the product of the wave function of both...
  27. Arne

    I Z2 symmetry in the hydrogen molecule when mapping to qubits

    Hello! When using a Jordan-Wigner-mapping or parity-mapping to map the hydrogen molecule \mathrm{H}_2 with two electrons and 4 spin-orbitals to 4 qubits, it is possible to reduce the number of qubits down to two [1,2,3]. The reason is apparently that the molecule has a discrete...
  28. D

    Rotational partition function for CO2 molecule

    Hello fellow physicists, I need to calculate the rotational partition function for a CO2 molecule. I'm running into problems because I've found examples were they say this rotational partition function is: ##\zeta^r= \frac T {\sigma \theta_r} = \frac {2IkT} {\sigma \hbar^3}## Where...
  29. LCSphysicist

    Degrees of freedom in a molecule

    I am not understand the solution of the letter b and c given by the author. Since Cv = NfK/2, we have Cv/N = fK/2. Now, the degree of freedom of a N linear molecule is 3(trans) + 2(rot) + x, where x is the degree of freedom due the vibrational motion. I am having trouble to calc x, could you...
  30. N

    Electrical properties of a molecule

    Hello everyone, Is there a straightforward way to determine the electrical properties, such as the dielectric constant (or function), of a molecule (for instance, a metal oxide)? I understand a simple weight-average model wouldn't work for various reasons. Thanks so much!
  31. S

    A What is the difference between aggregation, nucleation and growth?

    Im bit confused about what is exactly the difference between the terms, which process every term describes ?
  32. D

    Could anyone me with identifying a molecule?

    How did you find PF?: Google search Hello everyone. my name is Gary, I'm a British Journalist working for a Mexican TV news station. We are reporting on the appearance of a large crop circle that's been made in the UK today, which resembles some kind of chemical molecule. I've attached a...
  33. S

    A J-aggregation and H-aggregation differences

    Why J-aggregation has red shift and H-aggregation has blue shift what is the physics behind it? And the second question is does J-aggregate can makes with another J-aggregate H-aggregate?? (I'm new in this topic)
  34. LCSphysicist

    Degrees of freedom of a water molecule

    Well, first of all is really good to say that we don't can appeal to quantum mechanics... So, i can see: Three degree of freedoms in translation on a space xyz +3 degree Three normal modes of vibration, but each normal mode has with it a potential and vibrational kinetic energy, so +6 degree...
  35. M

    Is this a more accurate representation of a molecule?

    I see the top version used everywhere. But isn't the bottom version better? Because the top version makes it seem like H2 bonds with the C to its right and the other H2 bonds with the N. Whereas the bottom version shows the bonds correctly. Should I draw it like the top version and stop annoying...
  36. mollwollfumble

    What is the longest molecule?

    What is the longest molecule? Are we talking about of order of magnitude: 1 metre, eg. DNA, carbon nanotube, polyethylene (UHMWPE)? 1 kilometre, eg. silk strand, monofilament nylon? 100 to 1,000 km, eg. steel wire for suspension bridge, copper wire, optical glass fibre? 12,700 km, covalent...
  37. T

    Calculate the bond-dissociation energies and entropy of a molecule

    If we know the molecular structure of a complex chemical (organic), can we calculate the dissociation energy for each and every bond somehow? Also, can we calculate the standard etropy of the same molecule? These information would be needed to calculate Gibbs free energy for reactions of a...
  38. S

    Molecule aggregation in groove in Fused silica

    Image (a) is a view from above of a fused silica sample, the groove depth about one micrometer and the width is about 5 micrometer, the image (b) is a side view of the facet, when I cast a solution with molecules the aggregation is more on the grooves area which is logic, but when I shine a...
  39. K

    I CM frame kinetic energy for a molecule

    Hello! When the kinetic energy of a molecule is written in its CM frame we get the formula attached. The first term is the kinetic energy of the CM frame with respect to the lab frame, the second term is the kinetic energy associated with the distance between the nuclei and the last term is the...
  40. JorgeM

    What is the medium size of a Hydrochloric acid molecule?

    I was just thinking on the fact that there are some comercial gas masks . These ones have a filter, that doesn´´´'t let the Hydrochloric acid gas molecules to go into the mask. This is the way it basicly works, isn't it? So this filther has some little holes that are smaller than these...
  41. Frigus

    Forces between subatomic particles in a molecule

    I was seeing a video about molecular orbital theory..at t=3:56 he counted the forces but I don't why he didn't considered repulsions between electrons. Please can you tell me why he didn't considered repulsions?
  42. B

    Estimating oil molecule diameter

    Can anyone help me in what assumptions I can make and how to justify them for when working out diameter of the molecule in part b) of this question? You can see the method that I have used in my workings but how would I explain it in words? I obtained the diameter to be 0.13 nano metres which...
  43. Elder1994

    Find the electric dipole moment for a water molecule

    The equation that we saw in class is for a continuous charge distribution, I think that for this exercise I need to treat the system as a discrete charge distribution but I'm not sure. Also, I don't know how I can calculate the intensity of the electric field needed to move this charge.
  44. A

    Carbon atom larger than a water molecule? Measuring a molecule's size?

    So there's a website here: https://learn.genetics.utah.edu/content/cells/scale/ That shows the scale of various entities (coffee bean, viruses, a carbon atom, a water molecule etc). It says that the carbon atom is larger than the water molecule. This is based on the 'van der waal radius'...
  45. A

    Why Can a Sound Wave Not Travel Faster than the Average Molecule Speed?

    I am having trouble understanding the following passage in my physics textbook, particularly the bolded sentence: "The speed of sound in a gas is closely related to the rms speed of the molecules of that gas. In a sound wave, the disturbance is passed from one molecule to another by...
  46. S

    Classical hydrogen "molecule" - can it be stable?

    Is it possible to have a configuration of two classical positive charges and two classical negative charges that will remain fairly stable (or at least metastable), if started off in some well-chosen initial condition? For example, we might want to eliminate radiation by having the electrons...
  47. F

    Momentum of intertia for a dihydrogen molecule

    I don't know if the value for distance between protons given in the homework is right (##d = 74.14 pm##). Indeed, on the following link : https://brainly.in/question/7147660 , they take a distance equal to ##d = 4\times10^{-10} m##. In all cases, the same formula is applied ...
  48. Jamister

    I Hydrogen ion molecule

    in the book Molecular Physics of Wolfgang Demtroder, he claims for hydrogen ion molecule ##H_2 ^+## the total angular momentum in absolute value is a constant quantum number. I never saw it any other book and I think it's wrong, because there is no reason for it and he doesn't prove it. here is...
  49. H

    I Preferred orientation of a nitrogen molecule in space?

    Consider an ##N_2## molecule. Chemists say that the triple bond is due to one ##p_x - p_x## overlap, one ##p_y - p_y## overlap and one ##p_z - p_z## overlap. The x-axis (the label is not important; I’m sure you know what I mean) is clear because it’s the longitudinal axis of the molecule. But...
  50. J

    A How to obtain the spectrum for a given atom or simple molecule?

    The Bohr atom gave the answer to the spectrum of the hydrogen atom. But the spectra of stars contains many absorption (and sometimes emission) lines, corresponding to most atoms (up to iron, I believe). And atmospheric absorption is also due to absorption of some molecules, such as water...