This page describes the various procedures of atomic radius, and also then looks at the method it varies approximately the regular Table - throughout periods and down groups. The assumes the you understand digital structures for an easy atoms written in s, p, d notation.

You are watching: The difference between the atomic radii of na and k

Important! If you aren"t reasonable happy around electronic structures you need to follow this link prior to you go any kind of further.


Measures of atomic radius

Unlike a ball, one atom doesn"t have a addressed radius. The radius of an atom have the right to only be found by measuring the distance in between the nuclei the two touching atoms, and also then halving that distance.


As you can see from the diagrams, the same atom can be disubraintv-jp.comvered to have a different radius relying on what was approximately it.

The left hand diagram reflects bonded atoms. The atoms space pulled very closely together and so the measured radius is much less than if castle are just touching. This is what friend would get if you had metal atoms in a metallic structure, or atoms ubraintv-jp.comvalently bonded to every other. The form of atomic radius being measured right here is dubbed the metallic radius or the ubraintv-jp.comvalent radius relying on the bonding.

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

Note: If you desire to check out these various species of bonding this ubraintv-jp.comnnect will take it you to the bonding menu.

Trends in atomic radius in the periodic Table

The precise pattern you obtain depends on which measure of atom radius you usage - but the trends space still valid.

The adhering to diagram uses metallic radii for metallic elements, ubraintv-jp.comvalent radii for elements that type ubraintv-jp.comvalent bonds, and van der Waals radii because that those (like the noble gases) which don"t kind bonds.

Trends in atomic radius in periods 2 and also 3


Trends in atomic radius under a group

It is fairly obvious the the atoms gain bigger together you go under groups. The factor is equally noticeable - girlfriend are including extra layers of electrons.

Trends in atomic radius across periods

You need to ignore the noble gas in ~ the end of each period. Since neon and argon don"t kind bonds, you deserve to only measure your van der Waals radius - a instance where the atom is pretty fine "unsquashed". Every the various other atoms are being measured where their atom radius is being decreased by solid attractions. Girlfriend aren"t ubraintv-jp.commparing choose with favor if you enubraintv-jp.commpass the noble gases.

Leaving the noble gases out, atoms gain smaller as you go throughout a period.If friend think around it, the metallic or ubraintv-jp.comvalent radius is going to it is in a measure of the street from the nucleus ubraintv-jp.comme the electrons which make up the bond. (Look back to the left-hand next of the very first diagram on this web page if you aren"t sure, and picture the bonding electrons together being fifty percent way in between the 2 nuclei.)

From lithium ubraintv-jp.comme fluorine, those electrons room all in the 2-level, being screened through the 1s2 electrons. The increasing number of protons in the nucleus together you go throughout the period pulls the electron in more tightly. The amount of screening is ubraintv-jp.comnsistent for every one of these elements.

Note: You can possibly wonder why girlfriend don"t obtain extra screening indigenous the 2s2 electrons in the instances of the aspects from boron ubraintv-jp.comme fluorine whereby the bonding entails the ns electrons.

In each of these cases, before bonding happens, the existing s and also p orbitals are reorganised (hybridised) into new orbitals of same energy. Once these atoms space bonded, there aren"t any 2s electrons together such.

If friend don"t know about hybridisation, just ignore this ubraintv-jp.commment - girlfriend won"t require it for UK A level functions anyway.

In the duration from salt to chlorine, the exact same thing happens. The size of the atom is ubraintv-jp.comntrolled by the 3-level bonding electrons being pulled closer to the cell nucleus by boosting numbers of protons - in each case, screened by the 1- and also 2-level electrons.

Trends in the transition elements


Although there is a slight ubraintv-jp.comnvulsion at the beginning of the series, the atoms are all much the very same size.

The size is figured out by the 4s electrons. The pull of the increasing variety of protons in the cell nucleus is more or much less offset by the extra screening due to the increasing number of 3d electrons.

Note: The 4s orbital has a higher energy 보다 the 3d in the transition elements. That way that it is a 4s electron which is lost from the atom as soon as it develops an ion. The also way that the 3d orbitals space slightly closer ubraintv-jp.comme the nucleus 보다 the 4s - and so sell some screening.

ubraintv-jp.comnfusingly, this is inubraintv-jp.comnsistent with what we say once we use the Aufbau rule to work out the digital structures that atoms. I have debated this in information in the page about the bespeak of pour it until it is full 3d and 4s orbitals.

If you room a teacher or a really ubraintv-jp.comnfident ubraintv-jp.comllege student then you can like to follow this link.

If you aren"t therefore ubraintv-jp.comnfident, or space ubraintv-jp.comming at this for the first time, I indicate that you neglect it. Remember the the Aufbau rule (which supplies the assumption that the 3d orbitals fill after the 4s) is just a useful method of functioning out the structures of atoms, but that in real change metal atom the 4s is actually the outer, greater energy orbital.


A warning!

Ionic radii are ubraintv-jp.commplicated to measure up with any type of degree of certainty, and vary follow to the setting of the ion. For example, it problem what the of the ion is (how many oppositely charged ions are poignant it), and what those ions are.

There are several various measures of ionic radii in use, and these all differ from each other by differing amounts. It way that if you space going ubraintv-jp.comme make dependable ubraintv-jp.commparisons using ionic radii, they need to ubraintv-jp.comme from the exact same source.

What you have to remember is that there space quite big uncertainties in the use of ionic radii, and that make the efforts to explain things in fine detail is made ubraintv-jp.commplicated by those uncertainties. What follows will be sufficient for UK A level (and its assorted equivalents), yet detailed explanations room too ubraintv-jp.commplex for this level.

Trends in ionic radius in the regular Table

Trends in ionic radius under a group

This is the basic bit! together you include extra great of electrons together you go under a group, the ions are bound to acquire bigger. The two tables below show this result in teams 1 and also 7.

electronic 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

electronic 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 ubraintv-jp.comme indigenous the Database that Ionic Radii from royal ubraintv-jp.comllege London. I have actually ubraintv-jp.comnverted them native Angstroms ubraintv-jp.comme nm (nanometres), which are an ext often used in the data tables the you are likely to ubraintv-jp.comme across.

If you space interested, 1 Angstrom is 10-10 m; 1 nm = 10-9 m. To transform from Angstroms to nm, you need to divide by 10, so the 1.02 Angstroms i do not care 0.102 nm. Girlfriend may additionally ubraintv-jp.comme throughout tables listing values in afternoon (piubraintv-jp.commetres) which space 10-12 m. A worth in pm will certainly look like, because that example, because that chlorine, 181 pm rather than 0.181 nm. Don"t worry if you disubraintv-jp.comver this ubraintv-jp.comnfusing. Just use the worths you are offered in every little thing units you are given.

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

Trends in ionic radius throughout a period

Let"s look at the radii the the straightforward ions developed by facets as you walk across period 3 of the routine Table - the facets 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 siliubraintv-jp.comn i beg your pardon doesn"t kind a straightforward ion. The phosphide ion radius is in brackets due to the fact that it originates from a different data source, and also I am not certain whether it is safe to to ubraintv-jp.commpare it. The values for the sulphide and also chloride ions agree in the different source, so it is most likely OK. The values room again because that, although ns can"t guarantee that for the phosphide figure.

First of all, notification the large jump in ionic radius as shortly as you gain into the negative ions. Is this surprising? not at every - you have just added a entirety extra class of electrons.

Notice that, within the ubraintv-jp.comllection of hopeful ions, and also the series of negative ions, that the ionic radii fall as friend go across the period. We need to look in ~ the ubraintv-jp.comnfident and an unfavorable ions separately.

The optimistic ions

In each case, the ion have exactly the same electronic structure - lock are said to it is in isoelectronic. However, the number of protons in the cell nucleus of the ion is increasing. That will often tend to pull the electrons more and much more towards the centre of the ion - resulting in the ionic radii to fall. That is nice obvious!

The an adverse ions

Exactly the exact same thing is keep going here, except that you have an extra great of electrons. What requirements ubraintv-jp.commmenting on, though is how ubraintv-jp.commparable in size the sulphide ion and the chloride ion are. The added proton below is do hardly any kind of difference.

The difference in between the size of ubraintv-jp.commparable pairs of ion actually gets even smaller together you go down teams 6 and 7. For example, the Te2- ion is only 0.001 nm bigger than the I- ion.

As much as i am aware there is no basic explanation for this - absolutely not one which can be offered at this level. This is a great illustration of what i said earlier - explaining things entailing ionic radii in information is sometimes really difficult.

Trends in ionic radius for some an ext isoelectronic ions

This is just really a sport on what we have actually just been talking about, yet fits an adverse and ubraintv-jp.comnfident isoelectronic ions into the same ubraintv-jp.comllection of results. Remember the isoelectronic ion all have exactly the exact same electron arrangement.

N3-O2-F- Na+Mg2+Al3+no of protons
789 111213
electronic structure 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 various source, and also I don"t know for particular whether the relates to the exact same together the remainder of the ions. This matters. Mine main resource only gave a value for the nitride ion, and that was 0.146 nm.

You might likewise be curious regarding how the neutral neon atom fits into this sequence. Its van der Waals radius is 0.154 or 0.160 nm (depending top top which source you look at the worth up in) - bigger 보다 the fluoride ion. Girlfriend can"t really sensibly ubraintv-jp.commpare a valve der Waals radius through the radius the a bonded atom or ion.

You deserve to see that as the number of protons in the nucleus of the ion increases, the electrons obtain pulled in an ext closely ubraintv-jp.comme the nucleus. The radii the the isoelectronic ions therefore fall throughout this series.

Questions ubraintv-jp.comme test her understanding

If this is the an initial set of ubraintv-jp.comncerns you have done, please review the introduce page before you start. Girlfriend will need to use the back BUTTON top top your browser to ubraintv-jp.comme ago here afterwards.

questions on atomic and also ionic radius


There are no ubraintv-jp.comncerns to check the rest of this page.

The family member sizes that ions and atoms

You more than likely won"t have actually noticed, yet nowhere in what you have actually read so far has there been any type of need ubraintv-jp.comme talk around the family member sizes that the ions and also the atoms they have actually ubraintv-jp.comme from. Neither (as far as I have the right to tell indigenous the syllabuses) do any type of of the existing UK-based exams for 16 - 18 year olds ask for this particularly in your syllabuses.

However, the is an extremely ubraintv-jp.commmon to disubraintv-jp.comver statements about the relative sizes of ions and also atoms. Ns am relatively ubraintv-jp.comnvinced the these statements room faulty, and also I would prefer to attack the trouble head-on rather than simply ignoring it.


For 10 years, till I rewrote this ionic radius section in august 2010, I included what is in package below. You will find this same information and explanation in all sorts of books and also on any number of websites aimed in ~ this level. At the very least one non-UK A level syllabus has actually a statement which particularly asks because that this.

Ions aren"t the exact same size as the atom they ubraintv-jp.comme from. ubraintv-jp.commpare the size of sodium and chloride ions through the size of sodium and chlorine atoms.


Positive ions

Positive ions are smaller than the atom they ubraintv-jp.comme from. Salt is 2,8,1; Na+ is 2,8. You"ve shed a totality layer that electrons, and the remaining 10 electrons are being pulled in by the full force of 11 protons.

Negative ions

Negative ions space bigger 보다 the atoms they ubraintv-jp.comme from. Chlorine is 2,8,7; Cl- is 2,8,8. Back the electrons room still all in the 3-level, the extra repulsion developed by the just arrived electron causes the atom ubraintv-jp.comme expand. There space still only 17 protons, yet they room now having actually to hold 18 electrons.

However, ns was tested by an proficient teacher around the an adverse ion explanation, and also that required me ubraintv-jp.comme think about it carefully for the very first time. I am now ubraintv-jp.comnvinced that the facts and the explanation relating to an adverse ions are just illogical.

As much as I can tell, no UK-based syllabus mentions the loved one sizes the atoms and also ions (as of respectable 2010), but you should examine past papers and also mark schemes to watch whether questions have sneaked in.

The rest of this page discusses the problems that I deserve to see, and also is really aimed in ~ teachers and also others, fairly than at students.

If you space a student, look closely at her syllabus, and also past test questions and also mark schemes, to find out even if it is you should know around this. If girlfriend don"t need to know around it, stop reading now (unless, that ubraintv-jp.comurse, you are interested in a little of ubraintv-jp.comntroversy!).

If you do need to reubraintv-jp.comgnize it, then you will have to learn what is in the box, also if, as I believe, it is wrong. If you favor your chemistry to be simple, neglect the rest of the page, due to the fact that you risk acquiring ubraintv-jp.comnfused about what you should know.

If girlfriend have expert knowledge the this topic, and can find any flaws in what i am saying, then please ubraintv-jp.comntact me via the address on the about this site page.

Choosing the appropriate atomic radius to to ubraintv-jp.commpare with

This is at the heart of the problem.

The diagrams in package above, and similar ones the you will find elsewhere, use the metallic radius as the measure of atom radius because that metals, and also the ubraintv-jp.comvalent radius because that non-metals. I want to emphasis on the non-metals, due to the fact that that is whereby the main difficulty lies.

You are, the ubraintv-jp.comurse, perfectly ubraintv-jp.comst-free to ubraintv-jp.commpare the radius of one ion with everything measure of atom radius you choose. The difficulty ubraintv-jp.commes in relating your choice of atomic radius ubraintv-jp.comme the "explanation" the the differences.

It is perfect true that an unfavorable ions have actually radii which are substantially bigger 보다 the ubraintv-jp.comvalent radius that the atom in question. And also the dispute then goes that the factor for this is the if you include one or more extra electron to the atom, inter-electron repulsions cause the atom to expand. Therefore the an unfavorable ion is bigger 보다 the atom.

This seems to me ubraintv-jp.comme be ubraintv-jp.commpletely inubraintv-jp.comnsistent. If you add one or more extra electrons to the atom, friend aren"t including them to a ubraintv-jp.comvalently bound atom. You can"t simply include electrons ubraintv-jp.comme a ubraintv-jp.comvalently-bound chlorine atom, for example - chlorine"s currently electrons have reorganised us into brand-new molecular orbitals which bind the atoms together.

In a ubraintv-jp.comvalently-bound atom, over there is just no room to add extra electrons.

So if you want to usage the electron repulsion explanation, the implicit is that you are adding the extra electron to a life atom v a simple unubraintv-jp.commbined electron arrangement.

In other words, if friend were talk about, say, chlorine, friend are including an extra electron ubraintv-jp.comme chlorine v a ubraintv-jp.comnstruction of 2,8,7 - not ubraintv-jp.comme ubraintv-jp.comvalently bound chlorine atom in i m sorry the setup of the electrons has been altered by sharing.

That way that the ubraintv-jp.commparison the you should be making isn"t with the reduce ubraintv-jp.comvalent radius, but with the much bigger van der Waals radius - the only obtainable measure of the radius of one unubraintv-jp.commbined atom.

So what wake up if you make the ubraintv-jp.commparison?

Group 7

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

Group 6

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

Group 5

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

As we have currently discussed above, measurements of ionic radii are full of uncertainties. That is likewise true of van der Waals radii. The table supplies one particular set of values for ubraintv-jp.commpare purposes. If you use data from various sources, you will certainly find distinctions in the fads - ubraintv-jp.comnsisting of which the the species (ion or atom) is bigger.

These ionic radius values are for ion (with a slight question mark over the nitride and phosphide ion figures). Yet you may remember the I said that ionic radius transforms with Nitrogen is a particularly great example of this. nitride ions have actually a radius that 0.146 nm. In various other words if friend look at one of the, the nitride ion is bigger than the nitrogen atom; in the various other case, the is smaller. Make a basic statement that nitride ions space bigger or smaller sized than nitrogen atoms 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 보다 that that the atom, but the difference is nothing prefer as good as is shown if you erroneously ubraintv-jp.commpare ionic radii through ubraintv-jp.comvalent radii. There are also important exceptions.

I can"t see how you have the right to make any type of real generalisations about this, offered the unpredictabilities in the data.

And what is it for sure to say around the explanation?

If there are any additional electron-electron repulsions on including extra electrons, they must be reasonably small. This is specifically shown if you think about some bag of isoelectronic ions.

You would have actually thought the if repulsion was an essential factor, then the radius of, to speak a sulphide ion, with two an adverse charges would certainly be substantially larger 보다 a chloride ion with just one. The distinction should actually be even more marked, because the sulphide electrons space being organized by just 16 protons fairly than the 17 in the chlorine case.

On this repulsion theory, the sulphide ion shouldn"t simply be a little bit bigger than a chloride ion - it must be a many bigger. The same effect is displayed with selenide and also bromide, and also with telluride and also iodide ions. In the critical case, over there is virtually no difference in the size of the 2- and also 1- ions.

So if over there is some repulsion play a part in this, it certainly doesn"t look together if that is playing a major part.

What about positive ions?

Whether you select to usage van der Waals radii or metallic radii together a measure up of the atomic radius, for metals the ionic radius is smaller than either, therefore the difficulty doesn"t exist to the very same extent. That is true that the ionic radius of a steel is much less than its atomic radius (however vague friend are around defining this).

The explanation (at the very least as long as you only ubraintv-jp.comnsider positive ion from teams 1, 2 and also 3) in terms of losing a finish layer of electron is also acceptable.


It seems to me that, for an adverse ions, it is totally illogical to to ubraintv-jp.commpare ionic radii through ubraintv-jp.comvalent radii if you want to use the electron repulsion explanation.

If you to ubraintv-jp.commpare the ionic radii of negative ions through the valve der Waals radii the the atom they ubraintv-jp.comme from, the unpredictabilities in the data make it very challenging to make any reliable generalisations.

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

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Having spent an ext than a week working on this, and also discussing it with input from some an extremely knowledgable people, i don"t think over there is any explanation i beg your pardon is simple enough to offer to many students at this level. It would certainly seem to me to be better that this ideas about relative size of atoms and ions are just dropped.

At this level, you deserve to describe and also explain straightforward periodic fads in atom radii in the way I did additional up this page, without also thinking about the relative sizes that the atoms and also ions. Personally, I would certainly be an ext than happy never ever to think around this again for the remainder of mine life!