What is Atoms: Definition and 876 Discussions

An atom is the smallest unit of ordinary matter that forms a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small, typically around 100 picometers across. They are so small that accurately predicting their behavior using classical physics—as if they were tennis balls, for example—is not possible due to quantum effects.
Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. More than 99.94% of an atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, then the atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively – such atoms are called ions.
The electrons of an atom are attracted to the protons in an atomic nucleus by the electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by the nuclear force. This force is usually stronger than the electromagnetic force that repels the positively charged protons from one another. Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force. In this case, the nucleus splits and leaves behind different elements. This is a form of nuclear decay.
The number of protons in the nucleus is the atomic number and it defines to which chemical element the atom belongs. For example, any atom that contains 29 protons is copper. The number of neutrons defines the isotope of the element. Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature. Chemistry is the discipline that studies these changes.

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  1. Kenneth Boon Faker

    B Why does bonding occur between atoms?

    If each electron cloud repels other electrons and other atoms, then why can two or more atoms form molecules? Is it to do with negative and positive charges?
  2. Kenneth Boon Faker

    Why do cars crash? (Instead of passing through each other)

    If everything is made of atoms, and if atoms have so much empty space inside them - and if the building blocks of atoms are made up of wave-like particles that aren't solid (strictly speaking) - then why do cars crash and billiard balls bounce off each other, as opposed to passing through each...
  3. Jimtaff

    I Do we make atoms of Oganneson etc or are they nuclei?

    When the heaviest elements such as element 118 (Oganesson) are created, the production is described as a a collision of an ion projectile (eg calcium-48) with a target atom (eg californium 249). Does this collision result in an atom of oganesson before the decay to 116 (Livermorium) or is it...
  4. J

    I Composite traps with two-level atoms

    Let's say we have a system of spatially separated traps and two-level atoms where the levels are denoted as ##|a_i\rangle## and as ##|b_i\rangle##, and the subscript ##i## labels the trap number. The traps are initially loaded with ##N_i## atoms in the state ##|a\rangle##. If the two levels are...
  5. I

    What to say: "one mole of C atoms" or "one mole of C molecules"?

    As we know, Please point out if I am wrong here one molecule of CO = one mole of CO molecules = 6.223 X 10^23 number of CO molecules If I split one molecule of CO, we get one C and one O. Here are my questions, What to say these ''one C and one O" one mole of C atoms and one mole of O atoms or...
  6. doglover9754

    What is electricity made out of?

    For starters, it’s like 11 pm and I just had to ask this question so please bear with me if what I’m asking is confusing. So my question is what is electricity made out of. Atom wise. I know I probably could’ve Googled this but I figured why not ask this for everyone’s benefit. I figured hey...
  7. jerromyjon

    How many bonds can there be between two atoms?

    Are there any elements or molecules that exchange greater than 4 electrons per bond? I just thought of something off the wall and have no idea what to even google.
  8. A

    Can we see electrons in atoms?

    I have just seen a video about the most powerful microscope. The best image of that microscope represents a sheet of carbon atoms . Can we now see electrons in these atoms or we need a more powerful one? https://www.physicsforums.com/insights/can-see-atom/
  9. G

    If atoms are neutral, how do they share electrons

    Homework Statement If atoms are neutral, then how do they share electrons with each other? In the ionic bonding the metal requires very little energy do release its valence electrons, but once they are free, I don't see a force of attraction from the non metal nucleus because it is balanced...
  10. M

    I Examples of atoms that have more than 8 electrons in shell

    Hi So we are about to learn about the octet rule. However once you get to higher shells you can have more than eight in the "shells" or orbit? But i have not been able to find a good example of how that works or happens. Does it have a specific name? I assume we are talking about say several...
  11. heavybuilder

    I Force That Binds Atoms: H2, Electromagnetic Force?

    So I now understand the forces within an atom though my question is, what force(s) keeps atoms together? So let's say 2 hydrogen atoms are present. What force(s) keeps them together to make it H2? My thought it would be electromagnetic force. But hydrogen is neutral so they wouldn't attract to...
  12. P

    Calculation of no. of spectral lines for group of similar atoms

    Homework Statement The maximum no of spectral lines for a single atom during it's electron's transition is given by [∆n(∆n+1)]/2 . But I don't seem to arrive at the answer when a group of atoms are present . The question was - What is the maximum number of spectral lines possible for Balmer...
  13. Pushoam

    No. of choosing 4 atoms out of 10 atoms

    Homework Statement Homework EquationsThe Attempt at a Solution A) There is only one way to have 10 quanta of energy i.e. to have all atoms in higher states. B) I have to decide no. of ways of assigning one units of energy to any 4 atoms. It's equivalent to choosing 4 atoms out of 10 atoms...
  14. R

    B How does force transfer through an object microscopically?

    I heard that you can never really touch anything. I also heard from an article that the reason why your butt doesn't fall through your chair is due to forces. Here is a short excerpt: "Cracking like lightning through the void, all the specks of electrons and the specks of nuclei are constantly...
  15. D

    B Understanding the probability cloud?

    I understand that the concept of a atom resembling a solar system is disregarded as being untrue because this is based on the Bohr model which doesn't represent how an electron would actually appear in its probability cloud. However, would it be possible that if there were an observer on the...
  16. B

    Calculate mass or number of atoms based on MBq?

    Trying to work out how many atoms of Technetium-99m there are in a 10 MBq dose? Basically trying to work out if the actual weight of the Technetium-99m will affect the "flight" properties of an aerosol labeled with Technegas. Or would it be negligible? Basically will using Technegas to...
  17. K

    I Why is gold heavier than lead?

    Not only are lead nuclei heavier than gold ones (207 vs 200), but the atoms are smaller (ionic radius 119pm vs 137pm). There is every reason for Pb to be heavier. Why is it lighter? (11.34 vs 19.31 g/cm^3) They both have a FCC crystal structure, so the packing should be equally efficient...
  18. F

    Understand From Atoms to Chess: Get Resources & Advice Here

    Hi! :smile: There is a wonderful video which very briefly explains the progression from atoms to ending up programming a game of chess. I would like to understand, very schematically but with substance, what is going on when I program something in C/C++ backwards and forwards, from the...
  19. M

    B Momentum in Atoms: Understanding its Role in Potential Energy

    Is momentum in the atoms part of the potential energy of the system? I read the momentum is a Hamiltonian.. and the potential is also a Hamiltonian.. so are the momentum the potential itself? In the atoms, the potential could be the electromagnetic attraction between electrons and nucleus.. how...
  20. T

    B Decay of Excited Atoms: Probability & Nonlinearity

    I learned that the probability of radioactive decay for an atom is always the same. However, is the decay of and excited atom or electron non linear(decay probability varies with time)?
  21. Arman777

    I Emitting Photon and Energy Change

    I know that my title is not very suitable for this question, but I couldn't find right words to describe the situation.I read a book called '101 Quantum Questions ' and there writer states that when electron absorbs energy and goes higher energy level, we don't know when and where it will come...
  22. Ivan Bevanda E

    Will two isolated atoms form a bond by themselves?

    For example: If I isolated (in a perfect vacuum), one Magnesium atom and one Oxygen atom, and they are close to each other, will they make a bond by themselves, with no outside energy? Does this work with any combinable pair of atoms, and what other pairs of atoms would make a bond by...
  23. B

    I Can an Atom Become a Black Hole?

    If you shrunk an atom down to a certain point would it become a black hole like all matter in the universe can? How big would it be?
  24. Sveral

    I Can resonance be used to split a molecule of CO2?

    Hello, interested, if the posibility of splitting atoms by using resonance is possible, if so, wheather or not I can use it for something else, thanks in advance.
  25. A

    A Crystal Channeling: Explained

    Can someone explain this paragraph especially the bold part in simpler language: "If it is not in a major crystal direction or plane ("random direction", Fig. 2), it is much more likely to undergo large-angle scattering and hence its final mean penetration depth is likely to be shorter." full...
  26. Xilus

    I Equation for modeling atomic spectra of all atoms

    I've seen the equation I think is just for hydrogen. is this just for hydrogen? of course this doesn't return the atomic spectra, it returns the energy. So using E=h*v and Planck's constant. a simple factor of 1/h would return the frequency. right? Energy is directly proportional to frequency...
  27. E

    B Exploring the Effects of Electron Loss on Atom Properties | Learn More!

    hello everybody :) Let's assume that we could steal all electrons of an atom. What would happen? How would the atom change its properties? Could we measure different values?
  28. WhiteWolf98

    Activity of 1 g of carbon if 1 in 10^12 atoms are carbon-14

    Homework Statement Carbon-14 decays by β emission and has a half-life of 5570 years. What is the decay constant of carbon-14? What is the activity of 1 g of carbon if 1 in 1012 atoms are carbon-14? After what time will the activity per gram have fallen to 3 Bq? Homework Equations λt½ = ln(2)...
  29. P

    Achieve Nuclear Fusion By Splitting Atoms

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  30. O

    Question about nuclear fission

    I was watching an explanation (found here: youtu.be/yTkojROg-t8 ) on nuclear fission. In the video, he described the process of fission to happen one a random neutron smashes into a uranium nucleus. This causes the necleus to split into krpyton and barium, taking part of the nucleon and...
  31. E

    I Why can't atoms accept energy from static electric fields?

    Though I am definitely not an expert in any way in QM, I understand that on a basic level quantum systems can be solved using schrodinger's equation. For a hydrogen atom, the wave function of the electron is found by using the hydrogen proton as an external potential and proceeding from there...
  32. Ags Ivana

    Periodic table - certain no. of atoms in the 1st 4 rows

    Homework Statement Explain why the first four rows of periodic table have 2, 8, 8 and 18 atoms respectively Homework Equations I have a feeling this has something to do with the central field approximation OR the s, p, d, f orbitals and how many electrons can go in each OR something else The...
  33. D

    Why does the potential energy get lower as atoms get closer?

    Hello. I'm new to this forum and to Physics and Chemistry in general and I have a question that's making me go crazy: why does the potential energy decrease as two atoms (say, hydrogen atoms) get closer to form a molecule? I'm talking about this graphic: I've read that it's related to the...
  34. CassiopeiaA

    I Exchange of photons inside atoms

    I was reading Feynman Diagrams and stumbled upon this query: If the electrons and protons interact by exchange of photons, does the electron inside an atoms also interact with the nucleus with a similar kind of exchange?
  35. Elvis 123456789

    Photons emitted by Hydrogen and helium atoms

    Homework Statement An excited hydrogen atom can emit photons of various wavelengths. a) What is the maximum wavelength of the Balmer series (in nm) (5pt) b) What is the minimum wavelength of the Balmer series (in nm) (5pt) c) Corresponding to part b), what is the kinetic energy of the recoil...
  36. moenste

    Nuclear equation of the decay, decay constant, # of atoms

    Homework Statement A sample of iodine contains 1 atom of the radioactive isotope iodine 131 (131I) for every 5 * 107 atoms of the stable isotope iodine-127. Iodine has a proton number of 52 and the radioactive isotope decays into xenon 131 (131Xe) with the emission of a single negatively...
  37. L

    B Do atoms give off radiation that determines the colour?

    Do atoms give off radiation that determines the colour? I want to understand this, please give me explanation.
  38. D

    How electrons and protons and atoms make a whole object

    heisenbergs uncertainity says that we can never predict the position and velocity of a particle exactly simultaneously. that is we can never be sure where an electron or a proton would be seconds from now. its all about probablity. then how can these particles together form something like we...
  39. T

    How to gain percentage of atoms that are ionized in balloon?

    Homework Statement [/B] You have two inflated balloons. You rub them against your hair and hang them as shown in the figure. The length of the strings is 50 cm and the angle between them is 50°. For each balloon, what is the percentage of atoms that are ionized? Provide a rough estimate. You...
  40. D

    B Can Elements Die? Nuclear Fusion & Element Creation

    How could an element and all its atoms die? With nuclear fusion they combine atoms, so are they also making elements?...is it possible to make an element shooting the right number of p+,n, & e- and then contain it/save it for later?
  41. wolram

    B Positions of atoms at super cold temp

    I thought this was an interesting paper, good enough to share https://www.sciencedaily.com/releases/2016/09/160915162930.htm September 15, 2016 Source: Massachusetts Institute of Technology Summary: If you bottle up a gas and try to image its atoms using today's most powerful microscopes, you...
  42. J

    B Do Atoms in Neutron Stars Touch Each Other?

    I have heard that in neutron stars, the atoms do not repel nor do they attract. In a sense, could this mean that these atoms nucleuses could touch each other, or could the atoms touch in general?
  43. T

    Magnetic Mirror for Neutral Atoms

    Homework Statement Consider an infinite sheet of magnetized tape in the x-z plane with a nonuniform periodic magnetization M = cos(2πx/λ), where λ/2 is the distance between the north and south poles of the magnetization along the x-axis. The region outside the tape is a vacuum with no currents...
  44. V

    Entropy in system of non-degenerate atoms

    Hi all, 1. Homework Statement A system of non-degenerate multi-level atoms are all in their lowest energy state. Calculate the entropy of the system. Homework Equations S = kbTln(Ω) S = Q / T dU = TdS - pdV The Attempt at a Solution I'm not sure how to proceed. I know that Ω is the...
  45. shina

    B Electrons: Orbits, Collisions, & What Happens When They Collide

    Electrons moving at descrete orbits or shells ever collide or not. And what will happen if they collide with each other?
  46. shina

    B Atom Nucleus: Orbital Path & Pattern Explained

    An nucleus of an atom is really surrounded by circular orbits or not. And why there is a pattern that orbit k can contain only two electrons, orbit l can contain only 8 electrons and so on. But at some places I observe that the nucleus of an atom is surrounded by cloud like structure containing...
  47. G

    How Do Magnets Really Work? Investigating the Complex Phenomenon of Magnetism

    Hello PF! I am writing a scientific report about magnets, but have really no clue how magnets actually work? Research has yielded a few ideas, specifically from here and here. However, sometimes (in iron, nickel, and cobalt for example) you’ll have one or more un-paired electrons. The...
  48. abrogard

    B How Easy To Pull Two Atoms Apart?

    I was thinking about the forces holding atoms together. Some materials seem very, very strong. But if we take thinner and thinner slices of any material it is easy to imagine that pretty soon no matter what material it is we'll have a slice so thin it would be difficult not to tear it. So...
  49. shina

    Atoms and Cells: Are They Connected?

    Hey I know atom is very small particle. But is it true that cell are made from atoms.
  50. fresh_42

    B Test of the Universality of Free Fall with Atoms

    I have found an article about an experiment that states that gravity on a small scale doesn't experience quantum effects. Of course the headline has been catchy, so I want to ask about the real importance or relevance of the experiment. Or for short: Do the results imply any consequences on...
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