Atomic radius[1]（Atomic Radius）It is one of the parameters describing the atomic size.
According to different scales and measurement methods, atomic radius has different definitions. Common atomic radii are orbital radius, van der Waals radius (also known as normal radius), covalent radius, metal radius, etc.The atomic radius of the same atom obtained by different definitions may vary greatly, so when comparing the relative sizes of different atoms, the data sources used must be consistent.[2]
The atomic radius is mainly affected by the number of electron layers and the number of nuclear charges.In general, the more the number of electron layers, the smaller the number of nuclear charges, and the larger the atomic radius.This also makes the atomic radiusperiodic table of ele mentsThere is an obvious law of periodic recurrence.
The atomic radius has a great influence on the chemical properties of elements, so the study of atomic radius has extremely important significance and value in the development of chemistry.
There are three factors affecting the atomic radius: firstNuclear charge numberThe larger the number of nuclear charges, the greater the attraction of electrons outside the nucleus (making electrons shrink towards the protonucleus), the smaller the atomic radius;When the number of electron layers is the same, the atomic radius decreases with the increase of the number of nuclear charges;The second is the number of extranuclear electrons. Because the electron movement needs to occupy a certain space, the more electrons there are, the larger the atomic radius is, that is, the more electrons there are, the larger the atomic radius is;The third is the number of electronic layers. The more electronic layers, the larger the atomic radius, that is, the larger the number of electronic layers, the larger the atomic radius.The atomic radius decreases with the increase of nuclear charge, while the atomic radius increases with the increase of the number of electrons and the number of electron layers.
The atomic radius is determined by the above contradictory factors.Nuclear charge numberIncreasing makes the atomic radius smaller, whileNumber of electronsThe atomic radius increases with the increase of electron layers.When the interaction between these two contradictory factors reaches equilibrium, the atom has a certain radius[3]。
It is not difficult for us to understand the change rule of different atomic radius sizes as long as we compare the considerable interaction between the above contradictory factors.
Atomic radius
IThe law of atomic radius in the same period。
For example, compare the radii of sodium and magnesium.
From sodium to magnesium, the nuclear charge increases by one, each electron outside the nucleus increases a certain force, and the atom tends to shrink, while the electron outside the nucleus also increases by one, so the atomic radius tends to increase because the electron moves to occupy a certain space.Experiments show that the atomic radius of sodium is larger than that of magnesium, which means that the increasing nuclear charge has a smaller effect on the atomic radius than the increasing electron has a larger effect on the atomic radius.Therefore, the atoms of elements in the same period gradually decrease from left to right (except for rare gases).
IIComparison of atomic radius of adjacent periodic elements。
The experimental results show that the atomic radius of potassium is greater than that of sodium, which means that from sodium to potassium, the increase of eight electrons and one electron layer on the atomic radius is greater than the decrease of the atomic radius by the increase of eight nuclear charges.So, the same asMain family elementsThe atomic radius increases gradually from top to bottom.The nuclear charge from neon to sodium increases by 1, and the extranuclear electrons andElectronic layerIt is inferred that the radius of sodium is greater than that of neon, that is, the increasing effect of an added electron and an electron layer on the atomic radius is greater than the decreasing effect of an increased nuclear charge on the atomic radius.It is worth noting that the atomic radius is not necessarily large if there are more electronic layers, such as: lithium atomic radius>aluminum atomic radius.This is because when the nuclear charge increases to more than eight, the shrinking effect of the nucleus radius is stronger than the increasing effect of adding an electron layer on the radius.
IIIComparison of radius of an atom and its anion or cation。
For example, compare the radius of chlorine atom and chlorine ion.
Both have the same nuclear charge, but the chloride ion has one more electron, which needs to occupy a certain space for movement, so the radius of chloride ion is greater than that of chlorine atom.
The radii of atoms and their cations are exactly the opposite.For example: sodium ion radius<sodium atom radius.
IVComparison of particle radii with the same electronic layer structure but different nuclear charges。
For example, compare the radii of sodium ion, magnesium ion, oxygen ion and fluorine ion.
Because the structure of the electron layer outside the nucleus is the same, it is obvious that the more nuclear charges, the greater the attraction of the electron outside the nucleus, the smaller the particle radius.Therefore, the particle radius is: magnesium ion<sodium ion<fluoride ion<oxygen ion.
Chemical terminology
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Atomic radius
Usually refers to the two adjacent types measured by experimental methodatomHalf the distance between nuclei.Theoretically, there is no strictly fixed motion orbit of the extranuclear electrons, so the size of the atom has no strict boundary, and it is impossible to accurately measure the radius of a single atom. Therefore, the atomic radius data commonly used is only of relative and approximate significance.According to different methods of measurement, there are three atomic radii:
(1) Covalent radius: half of the distance between two nuclei when two atoms are bound by covalent bonds (atoms can be the same or different).In fact, the distance between nuclei is covalentKey length。
（2）Metal radius: Half the distance between two adjacent metal atoms in a metal crystal.
(3) Normal radius:Van der WaalsHalf the distance between two identical nuclei in adjacent different molecules that attract each other.
Atomic radius
The atomic radius is related to the following three aspects: the number of electron layers, the number of protons in the nucleus, and the number of electrons.(IntranuclearProton number=Number of nuclear charges)
1. The more electron layers, the larger the atomic radius (applicable to the same main family)
2. For elements of the same period, the more the number of outermost electrons, the smaller the atomic radius.
3. The larger the number of electrons, the larger the atomic radius.
Relationship between atomic radius and atomic number of elements
Compare the atomic radius of the same period with the number of protons in the nucleus.
Compare the elements of the same family by the number of electron layers.
If the period and family of the two elements are different, the number of electron layers is mainly considered, andNumber of outermost electronsGenerally, it doesn't matter.
If the atom is assumed to be a sphere, the diameter of the standard atom is about 10 to 10 meters.
2 Supplementary special note: refers to half of the effective range of atomic interaction, that is, half of the distance between adjacent atomic nuclei.The atomic radius is about 10 ^ (- 10) m.
classification
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There is no exact size of atoms. The so-called effective size refers to the distance between atoms in the chemical movement, that is, the balance distance between attraction and repulsion.
According to the different interaction forces, there are several effective radii: van der Waals radius, metal radius, ionic radius and covalent radius.
Vander Waals force radius
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Interaction force
The intermolecular interactions (van der Waals forces) mainly include the interactions between charged groups, dipoles, induced dipoles, hydrogen bonding, hydrophobic group interactions, stacking interactions, and non bond electron repulsion.
Orientation force
Electric attraction between inherent dipoles.
When polar molecules are close to each other, the molecules are arranged in a certain orientation in space, so that the system is in a more stable state, because the inherent dipoles of the molecules repel each other and attract each other.This inherent force between the dipoles is called the orientation force.Its essence is electrostatic force.
Induction force
The electric attraction between the induced dipole and the inherent dipole.
Induced force
Dispersion force
The intermolecular interaction force caused by the generation of instantaneous dipole is called dispersion force.
Summary: The orientation force, induction force and dispersion force are collectively referred to as van der Waals force.There are orientation force, induction force and dispersion force between polar molecules, induction force and dispersion force between polar molecules and non-polar molecules, and dispersion force between non-polar molecules and non-polar molecules.
Hydrogen bond
X-H... Y is used to indicate hydrogen bond, where X-HσThe electron cloud of the bond is biased towards the highly electronegative X atom, resulting in the presence of a small screened positively charged hydrogen nucleus, which is strongly attracted by another highly electronegative Y atom.
Some characteristics of hydrogen bond:
1) Directivity: because the volume of H atom is small, in order to reduce the repulsion between X and Y, they should be as far away as possible, and the bond angle is close to 180 °, which is the directivity of hydrogen bond
2) Saturation: Due to the small size of the hydrogen atom, it is difficult for another larger atom to approach it after it contacts the larger X and Y, so the coordination number of hydrogen in the hydrogen bond is generally 2, which is the saturation of the hydrogen bond.
Vander Waals radius
The intermolecular interaction energy E-r curve will have the lowest point.The distance corresponding to this lowest point is the balance distance.In other words, when the molecules are close to each other and the attraction and repulsion reach equilibrium, the energy of the system is the lowest.At this time, certain contact distance is maintained between molecules.The distance between atoms in contact with adjacent molecules is the sum of the van der Waals radii of the two atoms.The van der Waals radius is larger than the covalent radius, and the range of variation is also large, that is, poor conservation[4]。
Metal atomic radius
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Divide the nearest contact distance between atoms by 2 to get the radius of metal atoms.
Schematic diagram of metal radius
The change of metal atomic radius in the periodic table has certain regularity[5]
1. Except for a few exceptions, the atomic radius of metal elements in the same period gradually decreases from left to right, and then slowly increases.
2. Lanthanide shrinkage effect
Affected by the contraction effect of lanthanum, the radius of the second long period is larger than that of the first long period, but the radius of the third long period is very similar to that of the second long period. The radius of Zr is very similar to that of Hf, Nb and Ta, Mo and W, making it extremely difficult to separate. The atomic radius and chemical properties of Ru, Rh, Pd, Os, Ir and Pt are similar, so they are generally called platinum group elements.
The radius of metal atoms is related to the coordination number. The coordination number is high and the radius is large.
Covalent radius
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When two electrons of the same element are connected by covalent bond, 1/2 of the distance between their nuclei is called the covalent radius of the atom (such as H2, O2)
Schematic diagram of covalent radius (d/2)
The following two cases should be considered when calculating bond length using atomic covalent radius[6]
(1) The calculated bond length between heteronuclear atoms is often slightly larger than the experimental value.
Polarity correction of covalent bond length: Shomaker Stevenson formula.
(2) The same chemical bond has its particularity for different molecules, and the bond length is also different.For example, as for C-C bond, the length of C-C bond also changes when the composition of s orbital in the bond changes due to the change of hybrid form,
When there are delocalized π bonds or other complex bonds, the covalent single bond radius cannot be used to calculate the bond length.On the contrary, we can understand the properties of the bond according to the bond length.
Ionic radius
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In ionic crystals, the nuclear spacing between adjacent ions is equal to the sum of the radii of two ions[7]。
Determination of ionic radius
Lande ion radius
In 1920, Lande thought that the cell parameters of MgS and MgS, MgSe and MnSe were almost the same after comparing the cell parameters of the compounds with NaCl type structure in the following table (the figures in the brackets in the table are the later more accurate measured values), which means that the negative ions and negative ions have contacted in the crystal.He used simple geometric relations to deduce the ionic radius of S2 - and Se2 -.
Wasastjerna ion radius
Wasastjerna divided the size of ions in 1925 according to the method that the molar refractive index of ions is proportional to their volume.Eight positive ions and eight negative ion radii were obtained, including F - (133pm) and O2 - (132 Pm).
Goldschmidt ion radius
Goldscbmidt used the ion radius data of F - and O2 - of Wasastjerna in 1927 to derive the radius of more than 80 ions (Goldscbmidt ion radius) according to the data of the contact distance between ions in the ion crystal measured experimentally.
Pauling ion radius
In 1927, Pauling used a semi empirical method to deduce a large number of ion radii based on the distance data between nuclei of five crystals (NaF, KCl, RbBr, CsI and Li2O).Because the size of an ion is determined by the distribution of its outermost electronsEffective nuclear chargeInversely.
Shannon ion radius
Shanon et al. deduced the ion radius according to the contact distance under different conditions according to the coordination number of ions, the geometric configuration of coordination polyhedron, the spin of ions and other conditions.One set starts with O2 - radius of 140pm, and the other set starts with O2 - radius of 132pm to get two sets of effective ion radii.
The size of ion radius has the following trend
1) For the same main group and the same charged ions, the radius increases from top to bottom.
Li+<Na+<K+<Rb+<Cs+；F-<Cl-<Br-<I-
2) With the increase of the number of positive charges, the ionic radius of positive ions with the same number of electrons outside the nucleus of elements in the same period decreases significantly.
3) For the ions of various valence states outside the nucleus of the same element, the more the number of electrons, the larger the ion radius.
4) With the increase of negative electricity price, the radius of negative ion pairs with the same number of extranuclear electrons slightly increases, but the increase value is small.
5) The radii of the trivalent positive ions of lanthanide elements decrease from La3+to Lu3+, which is caused by the shrinkage effect of lanthanide.
