What is Molecules: Definition and 560 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. seonjunyoo

    B I have a question regarding temperature and the kinetic energy of molecules

    Does the high temperature increase the kinetic energy of molecules or atoms, or does the high kinetic energy of atoms or molecules increase the temperature ? I'm so curious about this. Which concept is more accurate between the two
  2. K

    A Calculating the interaction potential between 2 molecules

    Hello! I am trying to understand how to calculate the interaction potential between 2 molecules and I am a bit unsure how to proceed. For reference, I am sharing this paper, in particular Fig. 1b. They add some microwave fields which complicates things, but I am interested in reproducing the...
  3. Lotto

    B What does this distance of an atom mean?

    I can look at it as if a vibrational motion of the atoms was a simle harmonic motion. So I can consider one of the two atoms to be at rest and the second one to vibrate. Its deviation can be written as ##x(t)=r(t)-r_0##. When I know that the hydrogen molecule stops exiting when the range of...
  4. lukeskywalker52

    I'm trying to detect PFAS molecules in air samples...

    Hi there, I'm working on a project where my aim is to detect the presence of PFAS (Per- and poly-fluoroalkyl substances) in air samples gathered by a system. I've done a ton of research, but most solutions are unachievable due to price and the available space (the system should be relatively...
  5. C

    B Knudsen Flow: High School Student Q&A

    I am a high school student trying to carry out an experiment about fluid. Thus I am studying Knudsen flow and come up with following questions. 1. How can a Knudsen flow occurs? 2. Can I simply dig a small hole on a board and make Knudsen flow? 3. What the difference between viscous flow...
  6. 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?
  7. L

    B The collision of molecules during thermal expansion

    Greetings everyone. I learned that the distance between molecules in liquid increases while the temperature increases. Hence, its density is decreased. The process is thermal expansion. At the same time, the collision between molecules would be more frequent when the temperature increases...
  8. samy4408

    About the binding of O2 to Hemoglobin molecules

    Hello, I just learned about the binding capacity of Hemoglobin (Hb) and that it is proportional to the partial pressure of O2 in the blood here is the curve : it does have a sigmoidal shape, but here is the problem: in the lecture, it's said that Hb has an increasing affinity to O2 the more...
  9. B

    A AC Stark shift in diatomic molecules

    Hello! I am analyzing data from a diatomic molecules with a rotational constant around ##0.2## cm##^{-1}##, for transitions between rotational levels between the ground ##^2\Sigma_{1/2}## and excited ##^2{\Pi}_{1/2}## electronic state. I was wondering if there is a way to approximate the shift...
  10. tbn032

    Question about the collisions of the molecules in an ideal gas

    (The equation of ideal gas is PV=NRT.if P=1atm,N=1mole,T=0°K,R=gas constant then volume = zero. Hence, the volume of an individual molecule of ideal gas is zero) An individual molecule of ideal gas is assumed to have zero volume. The molecules of ideal gas are assumed to be dimensionless points...
  11. Salmone

    I Two molecules with different polarizability in an EM field

    If I have two separated and non-interacting molecules with different constants polarizabilities ##\alpha_1## and ##\alpha_2## and I send an EM field of frequency ##\omega## first on the molecule no.##1## and then on the molecule no.##2## so that the two molecules will have a dipole moment...
  12. D

    B Wave-particle duality of atoms and molecules

    Hello ladies and gentlemen, On the website of Encyclopedia Brittanicca I read the article about wave - particle duality. The article says that the wave - particle duality is experimentally established for light, electrons and protons. However, i found other internet sources which say that the...
  13. B

    I Isotope measurements in molecules

    Hello! Is there any measurement (can be from different papers/experiments) of a diatomic molecular isotopic chain (i.e. at least 3 isotopes of a given nucleus) where the ground state is ##^{2}\Sigma##?
  14. K

    A Dissociation in diatomic molecules

    Hello! Let's assume we have a molecule in the ground electronic and vibrational state. Let's assume that the first excited electronic state is very far away so it can be neglected for the purpose of the questions. Also we can assume that the shape of the potential is well-behaved i.e. it is a...
  15. B

    I Diatomic molecules containing potassium

    Hello! Are there any diatomic molecules containing potassium that can be produced and are stable (in terms of the chemical bonding) both as positive ions and as neutral molecules? Thank you!
  16. U

    I What happens to photonic molecules?

    My source is below: https://scitechdaily.com/harvard-mit-scientists-create-never-seen-form-matter/ In short, using rubidium atoms, photon particles (as in laser form) are bound together to form a massless molecule.. or acting like a molecule, but with a bonding structure nonetheless. Here is...
  17. agnimusayoti

    Average Separation between molecules in hydrogen at STP (Alonso Finn Problem)

    The relative density data: $${\rho_{H_{2},H_{2}O}}=\frac{\rho_{H_{2}}}{\rho_{H_{2}O}}=8.988\times10^{-5}$$ With avogadro number, thus I can obtain number of molecules per 1 ##m^3## of Hydrogen gas, that is: $$N = \frac{{{\rho_{H_{2},H_{2}O}}}\times{\rho_{H_{2}O}}}{M_{r}}\times{N_{A}} $$ thus, I...
  18. maayan_aloni

    I Exploring Oscillating Electric Fields for Dipolar Molecules

    Hi all! I was wondering, Is it possible, given a specific dipolar molecule, to create the perfect oscillating electric field so as to heat it and not, i.e. the water around it? What I'm basically asking is could there exist a specific microwave just for X and not all dipolar molecules without...
  19. B

    A Question about chiral molecules

    Hello! I started reading a bit about chiral molecules, and I have some questions. For reference, I will use this article for my questions. In Figure 1 in that article they show the energy levels for a typical chiral molecule (my question is for the case in which the left/right enantiomers are...
  20. K

    A Experiments with diatomic molecules

    Hello! I was wondering if there are papers presenting ideas of experiments (ideally searching for new physics, but not only) involving diatomic molecules, that were quite out of reach when they were published but we might have the technology to implement them with the current technology (or in...
  21. B

    Atomic What are the best books for studying polyatomic molecules?

    Hello! What books should I use to study polyatomic molecules (from a physics perspective, more than chemistry)? Is there an equivalent to Brown and Carrington for polyatomic molecules? Thank you!
  22. Huzaifa

    Molecules with an extended π system well described by resonance hybrid

    Homework Statement:: [...] Molecules with an extended π system such as linear polyenes and polyaromatic compounds are well described by resonance hybrids as well as by delocalized orbitals in molecular orbital theory. Relevant Equations:: N/A This is not a Homework question. I was reading...
  23. M

    A Interatomic distance in diatomic molecules

    Hello! I know this question is quite random, but I wasn't sure how else to search for this. Assume we have a diatomic molecule with one large and one small nucleus, for example BaF, such that the electron (assume there is just one electron outside the open shell) is localized around the massive...
  24. L

    Which diagram of water molecules is correct?

    I've seen both types of diagrams when drawing covalent dot and cross diagrams. And some students that I teach said that the electrons should be on the circles. But I do know that this is actually showing the electron overlap and electron field and in reality electrons are not in circular orbits...
  25. K

    A Can Quantum Logic Techniques Be Applied to Complex Molecular Ions?

    Hello! I read several papers about using quantum information techniques in order to do precision spectroscopy on ions. Usually they trap 2 ions in a quadrupole trap and the 2 ions are coupled through their vibrational motion along the trap axis. However most of these results involve atomic ions...
  26. Zahid Iftikhar

    Time of Impact of Gas Molecules on the Inner Wall of a Pressure Vessel

    In derivation of relation for pressure of Gas ,(see pic below) the time for rate of change of momentum of molecules should be the time for which the wall of the vessel and the molecules kept interacting with each other, not the time between successive collisions. As circumscribed below, my...
  27. A

    A The difference between the binding of molecules and superconductors

    Molecules and superconductors bind due to overlaps of the wave functions of the electrons. 1. What is the difference between these two then? 2. Why can't we look at molecules as a macroscopic wave function?
  28. B

    I Molecules when Born-Oppenheimer approximation doesn't work

    Hello! I am trying to do some molecular physics calculations, involving the calculation of the expectation value of certain vector operators (such as the electric dipole moment of the molecule) in given molecular states. In most cases assuming the Born-Oppenheimer (or adiabatic) approximation...
  29. K

    I Electron angular momentum in diatomic molecules

    Hello! I just started reading some molecular physics and I am a bit confused about the electron angular momentum in diatomic molecules. Let's say we have just 2 protons and an electron for simplicity and we are in the Born-Oppenheimer approximation, so we assume that the nuclei are fixed in...
  30. F

    Interpreting Energy diagrams for polyatomic molecules

    Hello, I am trying to correctly interpret the energy diagram below. For example, a diatomic molecule can translate (kinetic energy), rotate (rotational energy), vibrate (vibrational energy). Each different type of energy is quantized and has associated modes (also called states). The...
  31. BillTre

    Software recommendations for Drawing Molecules

    What do people use for drawing molecules (in a digital manner)? I would like to make space filling type models as well as letters representing the atoms connected by lines at different angles. The second kind I could make with ad drawing program, but there's got to be a better way.
  32. T

    Average Kinetic Energy of molecules calculation

    the answer in the solution book is 29K which only comes if I use mass for only one atom. ( They did not show any working ) My attempt: 1/2 x (1.67 x 10^(-27)) x (355)^(2) = 3/2 x 1.38 x 10^(-23) x T T = 29.48820652 K The confusion arises when I tried the following question: Q. Estimate the...
  33. J

    What Are the Possible Connections Between Magnetic Monopoles and Acupoints?

    I don't know if it's called the archetypes. But I need the molecules to have other functions besides its atomic configuration. For example, the trees or other compound being more than the molecules and have higher functions. What is a better word than Archetypes?
  34. N

    A living thing made of different photonic molecules?

    Can an AI one day create a living thing made of different photonic molecules? If it is remotely possible, I would like to know what kind of technological advancement had been made toward that. Thanks.
  35. T

    Relaxation times of molecules during sound propagation

    So i understand completely what the first source is saying i.e. the longer the relaxation time, the more absorption of the ultrasound beam. The first paragraph of the second source however says the opposite- "very long" relaxation times mean the sound wave passage is unaffected? My attempt at a...
  36. cwill53

    Mean Free Path of Air Molecules

    I used the form of the mean free path equation taking advantage of the fact that the Boltzmann constant is equal to the ideal gas constant R divided by Avogadro's number, because I didn't know if I could use the Boltzmann constant in the ##1.381\cdot 10^{-23}J/(molecules\cdot K)## form...
  37. Frigus

    How do molecules in a voltaic cell interact?

    If we put zinc rod in CuSO₄ solution it seems very obvious that they zinc and Cu²⁺ can interact and electrons can be pulled and pushed and one can grab electron and one can loose but in case of voltaic cell it seems magical that they are interacting and I tried to find its answer and I unable to...
  38. Amaterasu21

    At what size is kinetic energy no longer related to temperature?

    Hi all, I've read so many times that "temperature is a measure of the average kinetic energy of the molecules in a substance," or sometimes "particles" to encompass atoms and ions too. But how big can "molecules/particles" be before their kinetic energy is no longer relevant to temperature? If...
  39. N

    How many molecules will hit the cylinder: Was my solution correct?

    This is what I did, the cross-sectional area of the cylinder is ##\pi 10^2 = 100\pi## cm. Further, as the cylinder is moving a distance of 50cm. Thus, the total volume of vacuum, object covers throughout its journey is ##100\pi \times 50 =5000\pi## cm. Assuming that the molecules are uniformly...
  40. S

    A Molecules Aggregating on a Surface that includes a Scratch

    View from above of fused silica chip. In the image is fused silica and the black line is burned fused silica caused by laser, when I cast droplet that contains molecules, never mind which kind of aggregation I tried different kinds of molecules, the aggregation is more preferably on the burned...
  41. Zohar

    Oxidation States of Molecules and Atoms and the Relationship with Charges

    Hey, y'all. I know the oxidation state of a carbon in an ethene is -2 while carbon in Acetylene is -1. As well I know acetylene has more disspating elcetrons due to pai bonds. So how come charges between the acetylene carbon are more negative than in ethene while the carbones oxidations states...
  42. K

    I Born-Oppenheimer approximations in molecules

    Hello! I am a bit confused about Born-Oppenheimer approximations in molecules (mainly diatomic ones). It seems that all the books where I read about it, it is presented as a major breakthrough (at least in the context of molecular physics), but when I look into it in more details, it seems to...
  43. K

    I Vibrational energy in molecules

    Hello! I am not sure how to visualize (ideally in a simple, classical way) the vibrational energy levels of a molecule. For the electronic levels (similar to atoms), I usually think of it as the atom having multiple energy levels (fixed in space, in a probability distribution sense), and when...
  44. K

    I Rotating Molecules: Energies, Angular Momentum & Wavefunctions

    Hello! I am a bit confused about the rotational motion in molecules. Assuming the bond length is constant, the motion can be described as a rigid rotor. In the center of mass frame the energies are given by ##BJ(J+1)## and the wavefunctions are spherical harmonics. However when we measure the...
  45. K

    I Is the Isotope Shift the Same for Different Vibrational Levels in Molecules?

    Hello! Is the isotope shift between 2 (low lying) vibrational levels of the lowest 2 electronic levels (of a diatomic molecule) the same, no matter what the 2 levels are? For example, is the isotope shift associated to the ##0 \to 0## vibrational transition of 2 molecular isotopes (here I mean...
  46. K

    I Transitions in diatomic molecules

    Hello! I started reading some stuff about molecular spectroscopy and I see that the changes in the total angular momentum, J, can be -1, 0 or +1 (corresponding to the P, Q and R branches). Is this because the photon carries a spin of 1? Also is this the most general case (assuming only EM...
  47. Saptarshi Sarkar

    RMS velocity of molecules in a mixture

    I know that for a single monoatomic gas with RMS velocity ##v_{rms}## , $$\frac 1 2mv_{rms}^2 = \frac 3 2k_bT$$ where ##m## is mass of a single molecule, ##k_b## is Boltzmann constant and ##T## is temperature of the gas. For a mixture of gas, I know that the average kinetic energy after mixing...
  48. T

    I Measuring the force between air molecules

    If there was an atractive force between air molecules, how could this be measured? The force should be small. In the range of the force between watermolecules (hydrogen bond).
  49. K

    A Exploring Rocket Exhaust Plume Molecules in the Upper Atmosphere

    I'm pondering the behavior and persistence of rocket exhaust plume molecules far above the atmosphere. For example, the plume from an apogee circularization thrust from GTO (Geosynchronous Transfer Orbit) to GEO (Geosynchronous orbit). CO₂ and H₂O are among the molecular species emitted by a...
  50. darksat

    Infrared energy absorption of H20 molecules as a percentage of energy

    I eventually managed to find that a CO2 molecule can absorb about 8% of infrared energy passing through it that's radiated from Earth but its actually been incredibly hard to find a comparable figure for H2O molecules. Has anyone got a percentage figure for H2O in the form of atmospheric water...