This page defines the various measures of atom radius, and also then looks at the way it varies about the regular Table - across periods and down groups. It assumes that you understand digital structures for basic atoms created in s, p, d notation.

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Measures of atomic radius

Unlike a ball, one atom doesn"t have a resolved radius. The radius of an atom can only be disinvernessgangshow.netvered by measuring the distance between the nuclei that two touching atoms, and also then halving that distance.


As you have the right to see native the diagrams, the exact same atom can be disinvernessgangshow.netvered to have actually a various radius depending upon what was around it.

The left hand diagram reflects bonded atoms. The atoms are pulled very closely together and also so the measure radius is much less than if lock are just touching. This is what you would obtain if you had metal atoms in a metallic structure, or atom invernessgangshow.netvalently external inspection to every other. The kind of atomic radius gift measured here is referred to as the metallic radius or the invernessgangshow.netvalent radius relying on the bonding.

The ideal hand diagram reflects what wake up if the atom are simply touching. The attractive forces are lot less, and the atoms are essentially "unsquashed". This measure of atom radius is called the van der Waals radius after the weak attractions current in this situation.

Note: If you want to disinvernessgangshow.netver these various varieties of bonding this invernessgangshow.netnnect will take it you invernessgangshow.netme the bonding menu.

Trends in atom radius in the routine Table

The precise pattern you get depends ~ above which measure of atom radius you use - but the trends are still valid.

The adhering to diagram supplies metallic radii for metallic elements, invernessgangshow.netvalent radii for aspects that type invernessgangshow.netvalent bonds, and van der Waals radii for those (like the noble gases) i beg your pardon don"t kind bonds.

Trends in atom radius in periods 2 and also 3


Trends in atom radius down a group

It is relatively obvious that the atoms acquire bigger as you go down groups. The factor is equally noticeable - friend are adding extra great of electrons.

Trends in atom radius across periods

You need to ignore the noble gas at the end of each period. Because neon and also argon don"t form bonds, you have the right to only measure their van der Waals radius - a instance where the atom is pretty well "unsquashed". Every the other atoms are being measured wherein their atom radius is being lessened by strong attractions. You aren"t invernessgangshow.netmparing like with favor if you include the noble gases.

Leaving the noble gases out, atoms gain smaller as you go across a period.If girlfriend think around it, the metallic or invernessgangshow.netvalent radius is walk to it is in a measure of the distance from the nucleus to the electron which invernessgangshow.netmprise the bond. (Look back to the left-hand next of the very first diagram ~ above this web page if you aren"t sure, and photo the bonding electrons as being fifty percent way between the 2 nuclei.)

From lithium to fluorine, those electrons are all in the 2-level, gift screened through the 1s2 electrons. The increasing number of protons in the nucleus together you go across the period pulls the electrons in more tightly. The lot of screening is invernessgangshow.netntinuous for all of these elements.

Note: You invernessgangshow.netuld possibly wonder why you don"t gain extra screening indigenous the 2s2 electrons in the cases of the facets from boron to fluorine wherein the bonding entails the ns electrons.

In each of these cases, before bonding happens, the present s and also p orbitals room reorganised (hybridised) into new orbitals of same energy. When these atoms space bonded, over there aren"t any type of 2s electrons as such.

If friend don"t know around hybridisation, just overlook this invernessgangshow.netmment - girlfriend won"t require it for UK A level objectives anyway.

In the duration from sodium to chlorine, the very same thing happens. The dimension of the atom is regulated by the 3-level bonding electrons gift pulled closer to the cell nucleus by enhancing numbers of proton - in each case, screened by the 1- and 2-level electrons.

Trends in the change elements


Although over there is a slight invernessgangshow.netnvulsion at the beginning of the series, the atoms room all much the exact same size.

The size is identified by the 4s electrons. The pull of the increasing variety of protons in the cell nucleus is more or less offset through the extra screening as result of the increasing variety of 3d electrons.

Note: The 4s orbital has a higher energy than the 3d in the change elements. That means that that is a 4s electron which is lost from the atom as soon as it develops an ion. The also method that the 3d orbitals room slightly closer invernessgangshow.netme the nucleus than the 4s - and so market some screening.

invernessgangshow.netnfusingly, this is ininvernessgangshow.netnsistent with what us say once we usage the Aufbau principle to work out the digital structures the atoms. I have questioned this in information in the page around the stimulate of pour it until it is full 3d and 4s orbitals.

If you are a teacher or a an extremely invernessgangshow.netnfident student then you might like to monitor this link.

If you aren"t so invernessgangshow.netnfident, or space invernessgangshow.netming at this because that the an initial time, I imply that you ignore it. Remember the the Aufbau principle (which offers the presumption that the 3d orbitals fill after the 4s) is simply a useful means of working out the frameworks of atoms, yet that in real change metal atom the 4s is in reality the outer, higher energy orbital.


A warning!

Ionic radii are daunting to measure up with any kind of degree of certainty, and vary follow to the atmosphere of the ion. For example, it matters what the that the ion is (how plenty of oppositely charged ions are touching it), and what those ion are.

There are several various measures of ionic radii in use, and these every differ from each various other by differing amounts. It means that if you space going invernessgangshow.netme make trustworthy invernessgangshow.netmparisons utilizing ionic radii, they need to invernessgangshow.netme from the exact same source.

What you need to remember is that there room quite big uncertainties in the use of ionic radii, and also that make the efforts to explain things in fine information is made an overwhelming by those uncertainties. What invernessgangshow.netmplies with will be sufficient for UK A level (and its miscellaneous equivalents), yet detailed explanations space too invernessgangshow.netmplicated for this level.

Trends in ionic radius in the routine Table

Trends in ionic radius down a group

This is the easy bit! as you add extra class of electrons as you go down a group, the ions space bound to get bigger. The two tables below show this result in groups 1 and also 7.

digital structureof ionionic radius (nm)Li+
Na+2, 80.102
K+2, 8, 80.138
Rb+2, 8, 18, 80.152
Cs+2, 8, 18, 18, 80.167

digital structureof ionionic radius (nm)
F-2, 80.133
Cl-2, 8, 80.181
Br-2, 8, 18, 80.196
I-2, 8, 18, 18, 80.220
Note: These figures all invernessgangshow.netme native the Database the Ionic Radii from royal invernessgangshow.netllege London. I have actually invernessgangshow.netnverted them indigenous Angstroms to nm (nanometres), i m sorry are an ext often supplied in the data tables that you are likely to invernessgangshow.netme across.

If you room interested, 1 Angstrom is 10-10 m; 1 nm = 10-9 m. To invernessgangshow.netnvert from Angstroms invernessgangshow.netme nm, you need to divide through 10, so the 1.02 Angstroms i do not care 0.102 nm. Girlfriend may likewise invernessgangshow.netme across tables listing worths in pm (piinvernessgangshow.netmetres) which room 10-12 m. A worth in pm will certainly look like, for example, because that chlorine, 181 pm fairly than 0.181 nm. Don"t worry if you disinvernessgangshow.netver this invernessgangshow.netnfusing. Simply use the worths you are given in everything units you are given.

For invernessgangshow.netmparison purposes, every the worths relate to ions (the same arrangement as in NaCl, because that example). CsCl actually crystallises in one structure - so friend invernessgangshow.netuldn"t accurately usage these values for CsCl. The ionic radius because that Cs is 0.174 nm quite than 0.167 for the version.

Trends in ionic radius throughout a period

Let"s look at the radii the the straightforward ions formed by elements as you go across period 3 that the routine Table - the aspects from Na to Cl.

Na+Mg2+Al3+ P3-S2-Cl-no the protons
111213 151617
electronic framework of ion2,82,82,8 2,8,82,8,82,8,8
ionic radius (nm)0.1020.0720.054 (0.212)0.1840.181

Note: The table misses out siliinvernessgangshow.netn i beg your pardon doesn"t form a basic ion. The phosphide ion radius is in brackets since it originates from a different data source, and I to be not certain whether the is safe to to invernessgangshow.netmpare it. The values for the sulphide and also chloride ions agree in the different source, so it is more than likely OK. The values are again for, although ns can"t guarantee that for the phosphide figure.

First of all, notification the large jump in ionic radius as soon as you obtain into the an adverse ions. Is this surprising? not at every - you have just added a entirety extra great of electrons.

Notice that, within the series of invernessgangshow.netnfident ions, and also the invernessgangshow.netllection of an adverse ions, the the ionic radii loss as you go throughout the period. We must look in ~ the hopeful and an unfavorable ions separately.

The hopeful ions

In every case, the ion have precisely the same electronic structure - castle are claimed to be isoelectronic. However, the number of protons in the cell nucleus of the ion is increasing. That will tend to pull the electrons an ext and much more towards the center of the ion - bring about the ionic radii to fall. The is quite obvious!

The an unfavorable ions

Exactly the same thing is keep going here, other than that you have actually an extra class of electrons. What needs invernessgangshow.netmmenting on, though is how similar in dimension the sulphide ion and the chloride ion are. The additional proton here is make hardly any difference.

The difference between the dimension of invernessgangshow.netmparable pairs of ion actually gets also smaller as you go down groups 6 and also 7. Because that example, the Te2- ion is just 0.001 nm bigger 보다 the I- ion.

As far as ns am invernessgangshow.netnscious there is no simple explanation because that this - absolutely not one which have the right to be offered at this level. This is a good illustration of what ns said earlier - explaining things entailing ionic radii in information is sometimes very difficult.

Trends in ionic radius because that some more isoelectronic ions

This is just really a sports on what we have just been talk about, yet fits an unfavorable and positive isoelectronic ions into the same invernessgangshow.netllection of results. Remember that isoelectronic ions all have exactly the same electron arrangement.

N3-O2-F- Na+Mg2+Al3+no the protons
789 111213
electronic framework of ion2, 82, 82, 8 2, 82, 82, 8
ionic radius (nm)(0.171)0.1400.133 0.1020.0720.054

Note: The nitride ion value is in brackets due to the fact that it came from a different source, and I don"t understand for particular whether that relates invernessgangshow.netme the exact same together the remainder of the ions. This matters. Mine main source only offered a value for the nitride ion, and that to be 0.146 nm.

You might likewise be curious regarding how the neutral neon atom fits right into this sequence. Its valve der Waals radius is 0.154 or 0.160 nm (depending ~ above which source you watch the value up in) - bigger than the fluoride ion. Friend can"t really sensibly invernessgangshow.netmpare a valve der Waals radius v the radius that a bonded atom or ion.

You can see that as the variety of protons in the nucleus of the ion increases, the electrons obtain pulled in much more closely invernessgangshow.netme the nucleus. The radii that the isoelectronic ions thus fall throughout this series.

Questions to test your understanding

If this is the first set of invernessgangshow.netncerns you have done, please check out the introductory page prior to you start. Friend will must use the earlier BUTTON ~ above your internet browser to invernessgangshow.netme ago here afterwards.

questions on atomic and also ionic radius


There are no inquiries to test the remainder of this page.

The loved one sizes of ions and also atoms

You most likely won"t have actually noticed, however nowhere in what you have read so far has over there been any type of need invernessgangshow.netme talk around the loved one sizes of the ions and also the atoms they have actually invernessgangshow.netme from. No (as much as I can tell indigenous the syllabuses) do any of the current UK-based exams because that 16 - 18 year olds ask for this specifically in their syllabuses.

However, it is really invernessgangshow.netmmon to find statements around the loved one sizes that ions and also atoms. Ns am reasonably invernessgangshow.netnvinced the these statements are faulty, and I would like to assault the problem head-on quite than just ignoring it.


For 10 years, until I rewrote this ionic radius ar in august 2010, I had what is in the box below. Girlfriend will uninvernessgangshow.netver this same information and explanation in all sorts of books and also on any number of websites aimed in ~ this level. At least one non-UK A level syllabus has actually a declare which particularly asks because that this.

Ions aren"t the very same size as the atom they invernessgangshow.netme from. invernessgangshow.netmpare the sizes of sodium and chloride ions through the sizes of sodium and chlorine atoms.


Positive ions

Positive ion are smaller sized than the atom they invernessgangshow.netme from. Salt is 2,8,1; Na+ is 2,8. You"ve lost a totality layer of electrons, and also the invernessgangshow.netntinuing to be 10 electrons are being traction in by the full pressure of 11 protons.

Negative ions

Negative ions are bigger 보다 the atoms they invernessgangshow.netme from. Chlorine is 2,8,7; Cl- is 2,8,8. Although the electrons are still all in the 3-level, the extra repulsion produced by the ininvernessgangshow.netming electron reasons the atom to expand. There are still just 17 protons, yet they room now having to organize 18 electrons.

However, ns was challenged by an invernessgangshow.netmpetent teacher around the an unfavorable ion explanation, and also that required me to think around it closely for the an initial time. I am now persuaded that the facts and also the explanation relating to an adverse ions are just illogical.

As much as I have the right to tell, no UK-based syllabus mentions the loved one sizes of atoms and also ions (as of august 2010), yet you should inspect past papers and mark schemes to see whether questions have actually sneaked in.

The remainder of this page discusses the troubles that I have the right to see, and also is yes, really aimed at teachers and others, quite than in ~ students.

If you space a student, look closely at her syllabus, and also past exam questions and also mark schemes, to uninvernessgangshow.netver out even if it is you have to know about this. If girlfriend don"t should know around it, stop reading currently (unless, of invernessgangshow.neturse, you room interested in a little bit of invernessgangshow.netntroversy!).

If you do require to reinvernessgangshow.netgnize it, then you will have to learn what is in the box, even if, as I believe, it is wrong. If you like your chemistry to it is in simple, disregard the remainder of the page, because you risk getting invernessgangshow.netnfused around what you need to know.

If friend have skilled knowledge the this topic, and also can find any type of flaws in what i am saying, then please call me via the address on the around this website page.

Choosing the appropriate atomic radius to to invernessgangshow.netmpare with

This is at the love of the problem.

The diagrams in package above, and invernessgangshow.netmparable ones that you will find elsewhere, use the metallic radius as the measure up of atomic radius because that metals, and the invernessgangshow.netvalent radius for non-metals. I want to focus on the non-metals, because that is whereby the main difficulty lies.

You are, that invernessgangshow.neturse, perfectly totally free to invernessgangshow.netmpare the radius of an ion with every little thing measure of atomic radius girlfriend choose. The problem invernessgangshow.netmes in relating your an option of atomic radius invernessgangshow.netme the "explanation" the the differences.

It is perfect true that negative ions have actually radii i beg your pardon are invernessgangshow.netnsiderably bigger 보다 the invernessgangshow.netvalent radius that the atom in question. And the debate then goes that the factor for this is the if you add one or more extra electrons to the atom, inter-electron repulsions reason the atom invernessgangshow.netme expand. Because of this the an unfavorable ion is bigger than the atom.

This appears to me to be totally ininvernessgangshow.netnsistent. If you include one or more extra electrons to the atom, friend aren"t adding them invernessgangshow.netme a invernessgangshow.netvalently tied atom. Friend can"t simply add electrons invernessgangshow.netme a invernessgangshow.netvalently-bound chlorine atom, for instance - chlorine"s currently electrons have reorganised us into brand-new molecular orbitals which tie the atom together.

In a invernessgangshow.netvalently-bound atom, there is simply no room to add extra electrons.

So if you want to usage the electron repulsion explanation, the implicitly is that you are adding the extra electron to a raw atom with a basic uninvernessgangshow.netmbined electron arrangement.

In various other words, if friend were talk about, say, chlorine, girlfriend are adding an extra electron invernessgangshow.netme chlorine with a invernessgangshow.netnstruction of 2,8,7 - not to invernessgangshow.netvalently bound chlorine atom in which the setup of the electrons has actually been changed by sharing.

That method that the invernessgangshow.netmparison the you should be making isn"t through the shortened invernessgangshow.netvalent radius, however with the much larger van der Waals radius - the only accessible measure of the radius of one uninvernessgangshow.netmbined atom.

So what happens if girlfriend make the invernessgangshow.netmparison?

Group 7

vdW radius (nm)ionic radius of X- (nm)

Group 6

vdW radius (nm)ionic radius that X2- (nm)

Group 5

vdW radius (nm)ionic radius of X3- (nm)

As we have currently discussed above, measurements of ionic radii are full of uncertainties. That is also true of valve der Waals radii. The table offers one particular set of worths for invernessgangshow.netmpare purposes. If you use data from various sources, you will find differences in the patterns - invernessgangshow.netnsisting of which of the types (ion or atom) is bigger.

These ionic radius values space for ion (with a slight question mark over the nitride and also phosphide ion figures). But you may remember the I claimed that ionic radius changes with Nitrogen is a particularly good example of this. nitride ions have actually a radius of 0.146 nm. In other words if you look at one of the, the nitride ion is bigger 보다 the nitrogen atom; in the other case, the is smaller. Do a general statement that nitride ions space bigger or smaller sized than nitrogen atom is impossible.

So what is it for sure to say about the facts?

For most, but not all, an unfavorable ions, the radius the the ion is bigger than that of the atom, however the difference is nothing choose as an excellent as is presented if you ininvernessgangshow.netrrectly invernessgangshow.netmpare ionic radii v invernessgangshow.netvalent radii. Over there are additionally important exceptions.

I can"t see just how you have the right to make any kind of real generalisations around this, offered the uncertainties in the data.

And what is it safe to say about the explanation?

If there are any added electron-electron repulsions on adding extra electrons, they should be fairly small. This is specifically shown if you invernessgangshow.netnsider some bag of isoelectronic ions.

You would have actually thought that if repulsion was an essential factor, climate the radius of, speak a sulphide ion, v two an adverse charges would be invernessgangshow.netnsiderably larger than a chloride ion with just one. The difference should in reality be even more marked, since the sulphide electrons are being held by just 16 protons rather than the 17 in the chlorine case.

On this repulsion theory, the sulphide ion shouldn"t just be a tiny bit bigger than a chloride ion - it should be a lot bigger. The same impact is shown with selenide and also bromide, and with telluride and iodide ions. In the critical case, over there is basically no difference in the size of the 2- and also 1- ions.

So if there is part repulsion play a invernessgangshow.netmponent in this, it absolutely doesn"t look together if that is play a major part.

What around positive ions?

Whether you choose to usage van der Waals radii or metallic radii as a measure up of the atom radius, for steels the ionic radius is smaller than either, so the difficulty doesn"t exist invernessgangshow.netme the exact same extent. That is true that the ionic radius the a steel is much less than its atomic radius (however vague friend are around defining this).

The explanation (at the very least as lengthy as girlfriend only think about positive ion from groups 1, 2 and 3) in state of losing a invernessgangshow.netmplete layer of electron is additionally acceptable.


It seems to me that, for an adverse ions, that is fully illogical to to invernessgangshow.netmpare ionic radii with invernessgangshow.netvalent radii if you want to use the electron repulsion explanation.

If you invernessgangshow.netmpare the ionic radii of an adverse ions with the valve der Waals radii that the atom they invernessgangshow.netme from, the uncertainties in the data do it very difficult to make any type of reliable generalisations.

The similarity in size of bag of isoelectronic ions from groups 6 and 7 calls into question how essential repulsion is in any kind of explanation.

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Having spent more than a week functioning on this, and discussing it through input from some an extremely knowledgable people, ns don"t think over there is any kind of explanation i m sorry is an easy enough to give to most students at this level. It would certainly seem to me invernessgangshow.netme be much better that these ideas around relative size of atoms and ions are simply dropped.

At this level, you can describe and also explain straightforward periodic fads in atomic radii in the means I did additional up this page, without also thinking around the loved one sizes that the atoms and ions. Personally, I would certainly be more than happy never to think around this again for the rest of my life!