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Chapter Seven Atomic Structure atoms neutrons protons (positive charge ) electrons (negative charge) 7-1 Changing Ideas about Atomic Structure 7-2 The Quantum Mechanical Description of Electron in Hydrogen Atoms 7-3 Electron Configuration of Manyelectron Atoms 7-4 The Periodic Table and Periodic Law 7-1.1 The Bohr theory of Hydrogen Atom 1805 dolton proposed atom theory, proved exist of atom 1900 electron were discovered 1911 Ruthrford proposed the atomic nucleus by α-ray scatting 1931 neutron were discovered Ruthrford’s nuclear model Figure 7-1: In classical theory: 1.atoms constructed are not stable; 2.the electron would quickly spiral into the nucleus. 3. Not is the line spectra of atoms Continuous spectrum Atomic Line Spectra Na (H、He、Li、Na、Ba、Hg、Ne light emission) In 1913, Niels Bohr(1885-1962), founded Bohr theory by using the work of Planck and Einstein Quantum of concept no continuum emission Atom a copy of energy absord Least unit quantum The Photoelectric Effect Einstein used the quantum theory to explain the photoelectric effect : Each energy packet called photon, is a quantum of energy E=h v Physicist Albert Einstein (1879 -1955) h Planck’s constant = 6.623×10-34J s. E = hv = h c (波粒二象性) Photons of high frequency radiation have high energies, whereas photons of lower frequency radiation have lower energy. 7-1.1 The Bohr theory of Hydrogen Atom Bohr set down the following two postulates to account for: (1) the stability of the hydrogen atom (that the atom exists and its electron does not continuously radiate energy and spiral into the nucleus) (2) the line spectrum of the atom. Bohr theory of Hydrogen Atom Bohr assumed that: 1.Energy-level postulate an atom looked something like the solar system: 1) a nucleus at the center 2) the electron could have only certain orbits h Ln 2 量子化条件: L P r m v 代表电子运动轨道的角动量(L= p ·r =mv r ) 是电子轨道运动动量, 是轨道半径, 是电子的质量, 是电子的运动速度。 电子在任意轨道做圆周运动的角动量mv r h 必须是 2 的整数倍, n = 1, 2, 3, n=3 n=2 n=1 + Bohr theory of Hydrogen Atom 3) energy levels: an electron in an atom can have only specific energy values, which are called the energy levels of the electron in the atom En = - (Z2/n2) ×2.180 × 10-18J (for H atom) Z : 核电荷数 n : 能级数 1, 2, 3, --- ∞ n值越大,表示电子运动轨道离核越远,能量越高。 2. Transitions(跃迁)between energy levels photons are given off or absorbed when an electron moves from one orbit to another. ground state a lower energy state ( if n = 1, is called ground state ) excited state a high energy state ( if n = 2、3……, is called ground state) Ground state Energy • Orbitof– emitted photon ΔE = Ei - Ef = hv Excited state a higherstate energy •EiGround – level (initial energy level) Ef a lower energy level (final energy level ) In 1885, J.J. Balmer showed that the wavelengths, λ, in the visible spectrum of hydrogen could be reproduced by a simple formula. 1 1 1 --- = 1.097 × 107m-1 ( ---- - -----) λ 22 n2 postulate from level n = 4 to level n = 2 light of wavelength 486 nm (blue green ) is emitted Hydrogen atom spectra Low E Visible lines in H atom Long spectrum are called the Low n High E Short High n BALMER series. 6 5 4 Energy 3 2 1 Ultra Violet Lyman Visible Balmer Infrared Paschen n Bohr’s theory Successful 1.established the concept of atomic energy levels (atomic orbit) 2. explaining the spectrum of hydrogen Unsuccessful 1. atomic orbit is fastness 2. can’t explain minuteness the spectrum of hydrogen atom 7-1.2 De Broglie Waves (Matter Waves) Louis-Victor de Broglie, (1892 -1987, France) In 1929, he was awarded the Nobel Prize for Physics for his research on quantum theory and his discovery of the wave nature of electrons. He showed that the wavelength of moving particles is equal to Planck's constant divided by the momentum. h h mn p Mass: mn >> h , (7-4) is short wave properties ignored Particle: mn <<h, can not ignored wave properties [例7-1] 分别计算m=2.5×10-2kg,v = 300m·s-1的子弹 和me=9.1×10-31kg v =1.5×106 m·s-1的电子的 波长,并加以比较。 解: 按(7-4)式,子弹的波长为: 34 6.626 10 35 23 8 . 8 10 ( m ) 8 . 8 10 ( pm) 2 2.5 10 300 电子的波长为: 6.626 10 34 9.1 10 31 1.5 10 6 500 ( pm) 计算结果表明,子弹的波长很短,完全可以不予考虑。 电子的波粒二象性(Davisson和Germer实验 ) 1927年美国物理学家Davisson C和Germer L根据电子的波长 与X射线波长相近,用电子束代替X射线,用镍晶体薄层 作为光栅进行衍射实验,得到与X射线衍射类似的图像, 证实了电子的波动性。 X-diffracted electron diffracted 7-1.3 The Heisenberg Uncertainty principle 1927 ,He recognized : Heisenberg German physicist (1901-1971) no experimental method can be devised that will measure simultaneously the precise position(x) as well us the momentum (mv) of an object. Uncertainty principle formula h h px 或 x 4 4mν Δp uncertainty of the momentum Δx uncertainty of the position h Planck's constant The more precisely one knows Δp, the less precisely Δx is known, and vice versa. (中文p148_) •Example Suppose Δx=1.0 ×10- 4 m for a substance with mass of 0.01kg h 6.626 10 34 v 4mx 4 3.14 1.0 10 2 1.0 10 4 5.3 10 29 (m s 1 ) In hydrogen atom, for an electron, v =106m/s , suppose Δx=1.0 ×10- 10 m, 电子速度的不准确量 与其速度本身十分接近 h 6.626 10 34 v 31 10 4mx 4 3.14 9.1110 10 5.8 105 (m s 1 ) Quantum or Wave Mechanics Schrodinger applied idea of ebehaving as a wave to the problem of electrons in atoms. 2 2 2 8 2 m 2 2 2 ( E V ) 0 2 x y z h E. Schrodinger 1887-1961 1933 received the Nobel Prize E the total energy V the potential energy m a particle in terms of its mass x y z respect to its position in three dimensions 7-1.4 Schrődinger Equation (wave function) 8 m 2 2 2 ( E V ) 0 2 x y z h 2 2 2 2 Solution to WAVE EQUATION gives set of mathematical expressions called WAVE FUNCTIONS ψ (psi) wave function ψ has an amplitude(振幅) at each position in space (just as for a water wave or a classical electromagnetic wave). 7-2.1 Wave Function, Atomic Orbital and Electron Cloud ψ is a function of distance and two angles. ——— Ψ(r,θ,φ)、 For 1 electron, ψ corresponds to an ORBITAL — the region of space within which an electron is found. ψ does NOT describe the exact location of the electron. 7-2.2 Atomic Orbital ____ Quantum Numbers n the principal quantum number l the angular momentum quantum number m the magnetic quantum number. they will be used to describe atomic orbitals and to label electrons that reside in them. 1. Principal quantum number (n): Shell K L M N... n 1 2 3 4... As n increases, the orbitals extend farther from the nucleus,average position of an electron in these orbitals is farther from the nucleus Energies: K<L<M<N<O< … 1<2< 3< 4< 5 < … 2. Angular momentum quantum number (l ) Within each shell of quantum number n , there are n different kinds of orbital, each with a distinctive shape, denoted by the l quantum number. subshell l s p d f g... 0 1 2 3 4 . . .(n-l) Energies: s<p < d < f < g… 3. Magnetic quantum number (m): A subshell has the same shape, but a different orientation, or direction, in space. m = (2 l + 1) or 0 1 2 3 l... Each orbital of a particular subshell (no matter how it is oriented in space) has the same energy. Example: p orbit have 3 different orientation p x. p y p z About Quantum Numbers —— Orbital An atomic orbital is defined by 3 quantum numbers: n l m Electrons are arranged in shells and subshells of RBITALS . n shell l subshell m designates an orbital within a subshell Table 7-1: The allowed sets of quantum numbers for atomic orbitals 4. Spin quantum number (ms) : ms the spin quantum number refers to a magnetic property of electrons called spin. Values for the spin quantum number are +1/2 and –1/2. A fourth quantum number Note: n. l. m. ms they will be used to describe electrons that reside in them QUANTUM NUMBERS 1. Which of the following is not a valid set(有效的组合) of four quantum numbers to describe an electron in an atom? (1) 1, 0, 0, +½ (2) 2, 1, 1, +½ (3) 2, 0, 0, –½ (4) 1, 1, 0, +½ 2. The energy of an orbital in a many-electron atom depends on (1) the value of n only (2) the value of l only (3) the values of n and l (4) the values of n, l, and m 7-2.3 Sizes and Shapes of Atomic Orbitals n.l .m ( r . . ) Rn.l ( r )Yl .m ( . ) Radial wave function angular wave function n.l .m ( r . . ) Rn.l ( r )Yl .m( . ) .0.30 Spherical coordinates x = r sin cos y = r sin sin z = r cos Shapes of the orbitals Shapes of the orbitals for: (a) an s subshell ? (b) a p subsell (c) a d subshell 如:氢原子的角度部分 【s轨道】 1 Ys ( , ) 4 Ys是一常数与(,)无关,半径为: 【pz轨道】 1 4 3 Ypz ( , ) cos 4 节面:当cos 0时,Y0, 90° 我们下来试做一下函数在Pz平面的图形。 z 0 30° 30 + θ 60° 60 90 x,y - 0 cos 1 30 0.866 60 90 0.5 120 0 - 0.5 150 - 0.866 180 -1 YPz 0.489 0.423 0.244 0 - 0.244 - 0.423 - 0.489 节面:当θ = 90° cosθ= 0 Y=0时 波函数的角度分布图 由图可知,原子轨道的角度分布图有正负之分, 这对于讨论分子的化学键及对称性十分重要。 同样地,可以画出其它原子轨道的角度分布图。 The Probability Function (ψ2) —— Electron Cloud ψ2 is related to the probability per unit volume such that the product of ψ 2 and a small volume (called a volume element) yields the probability of finding the electron within that volume. 1. Electron Cloud The total probability of locating the electron in a given volume (for example, around the nucleus of an atom) is then given by the sum of all the products of ψ2 and the corresponding volume elements. 2pz 2px f orbitals 电子云的径向分布图 n.l .m ( r . . ) Rn.l ( r )Yl .m ( . ) |Ψn,l,m(r,θ,φ) 2 | = 2 R n,l(r) • 2 Y l,m(θ,φ) Probability density Probability 2 P=|Ψ| • dV 几率(dP)=几率密度(|ψ|2)×体积(dV) 电子云的径向分布图 考虑离核距离为r,厚度为dr的薄层球壳内发现电 子的几率. 1s球壳微体积: dV = 4πr2dr D(r) =4πr2dr •R2(r) -----壳层几率(球壳层 内发现电子的几率) Probability 2 P=|Ψ| • dV= 2 =4πr dr 2 •R 2 2 |Ψ| •4πr dr (r) = D(r) • • 2 Y 2 Y l,m(θ,φ) l,m(θ,φ) Radial distribution Angular distribution function diagram function diagram P= |Ψ|2 •4πr2dr 离核越近: r值越小,体积越小,|ψ|2越大,D(r)不是最大, 离核越远: r值越大,体积越大,|ψ|2越小,D(r)亦不是最大, 在ao处: |ψ|2不是最大的, 但体积较大,使D(r)可达最大。 ao=52.9pm处。 当r=2ao时, D(r)=0,出现第一个节面。 当r=4ao时, D(r)又出现最大值,此即2s电子云 当r=2ao时, D(r)=0,出现第一个节面。 当r=4ao时, D(r)又出现最大值,此即2s电子云 电子云的径向分布图 峰数= n-l 7-3 Electron Configuration of Many-electron Atoms 1. An electron configuration describes the arrangement of electrons in the subshells of an atom. 2. The chemical properties of elements are related to these configurations. 3. The four quantum numbers n, l, m, and ms enable us to label completely an electron in any orbital in any atom. Order of filling orbitals Generally, the energy of an orbital depends on the quantum n and l . E1s E2sE 2p E3sE3p E3d E4sE 4p E 4d E4f E5s… 1s 2s2p 3s3p 4s3d4p 5s4d5p 6s4f5d6p 7s… Why? This phenomenon can be explained by shielding effect (screening effect) and penetrating effect. 1. The shielding effect is that it reduces the electrostatic attraction between protons in the nucleus and the electron in outside orbital. 2. The penetrating effect of an electron can decrease the energy of orbital. The penetrating effect D(r) D(r) 1s 2s 3d 3s 3s 图1 l 相同, n不同时的比较 3p r 图2 n 相同, l 不同时的比较 从上图可以看出: (1) l相同,n不同: 1s<2s<3s . n 增大时,电子离核的距离 (主峰)将增加。 (2) n相同,l不同 3s<3p<3d. l 值大,峰个数减少。 l 值小,电子在核附近出现的机会(钻穿峰)较多。 r (3) n,l都不同时,将出现能级交错 : 4s<3d<4p 钻穿效应: 外层电子向内层穿透,导致内层电 子对它的屏蔽作用减弱的效应叫钻穿效应 Question 为什么 2s 价电子比 2p 价电 子受到较小的屏蔽? 2s电子云径向分布 曲线除主峰外,还有 一个距核更近的小峰. 这暗示, 部分电子云 钻至离核更近的空间, 从而部分回避了其他 电子的屏蔽. The electron fill law 1.principle of energy levels lowest Electrons in an atom occupy the lowest possible energy levels, or orbitals. 2.The Pauli exclusion principle: No two electrons in the same atom can have the same set of four quantum numbers. 3.Hund's rule: Every orbital in a subshell is singly occupied (filled) with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin; 1.principle of energy levels lowest All of the electrons in an atom reside in the lowest energy orbitals possible as long as permission of Pauli exclusion principle . The electrons filling order is: 1s, 2s2p, 3s3p, 4s3d4p, 5s4d5p, 6s4f5d6p, 7s5f…… 6p 6s 5p 5s 4p 4s 3s 2s 1s 3p 2p 5d 4d 3d 4f 2. Pauli Exclusion Principle (2n2) The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers: n l m and ms. Thus, for two electrons to occupy the same orbital, one must have ms = + ½ and the other must have ms = – ½. • electrons with the same spin keep as far apart as possible • electrons of opposite spin may occupy the same orbital 3. Hund’s rule(洪特规则) This rule states that for orbitals with the same energy, the lowest energy is attained when the number of electrons with the same spin is maximized. N Ô-×ÓÐòÊýΪ7 按洪特规则的基态电子构型 而不是 是 1s 1s 2s 2p 2s 2p Example Boron(atomic number =5) B Nitrogen (atomic number =7) N 1s22s2 2p1 1s22s2 2p3 Magnesium (atomic number =12) Mg 1s22s2 2p63s2 or [Ne]3s2 Chromium (atomic number =24) Copper (atomic number =29) ? Lanthanum (atomic number =57) According to Hund’s rule and Pauli exclusion principle, we can writing the electron configurations for other elements. Example: chromium (Z = 24) [Ar]4s13d 5 or [Ar]4s23d4 half-filled (s1 p3 d5) Subshells completely empty(s0p0d0) stability completely filled (s2 p6 d10) 电子层结构式要与原子的电子排布式区别开, 以29号元素铜为例: 电 子 排 布 式: 29Cu: 1s2 2s2 2p6 3s2 3p6 4s1 3d10 电子层结构式: 29Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1 (或电子构型式) SPECTROSCOPIC NOTATION for H, atomic number = 1 1 1s value of n no. of electrons value of l 7- 4 The Periodic Table and Periodic Law Then in 1869, Russian chemist Dimitri Mendeleev (1834-1907) proposed arranging elements by atomic weights and properties (Lothar Meyer independently reached similar conclusion but published results after Mendeleev). Mendeleev's periodic table of 1869 contained 17 columns with two partial periods of seven elements each (Li-F & Na-Cl) followed by two nearly complete periods (K-Br & Rb-I). 7- 4 The Periodic Table and Periodic Law The modem Periodic Table consists of 7 horizontal(水平) rows of elements (often referred to as periods or series) and 32 vertical(垂直) columns of elements (referred to as families or groups). 维尔纳长式周期表 IA 1 H 2 1 氢 IIA Li 4 Be 3 IIIA IVA 5 B 6 C 2 锂 铍 11 Na 12 Mg 3 钠 镁 IIIB VA VIA VIIA 7 N 8 O 9 F He 氦 10 Ne 硼 碳 氮 氧 氟 氖 13 IVB VB VIB VIIB 19 K 20 Ca 21 Sc 22 Ti 23 V 24 Cr 25 Mn VIII IB IIB 26 Fe 27 Co 28 Ni 29 Cu 30 Zn Al 14 Si 15 P 16 S 17 Cl 18 Ar 铝 硅 磷 硫 氯 氩 31 Ga 32 Ge 33 As 34 Se 35 Br 36 Kr 4 钾 钙 钪 钛 钒 铬 锰 铁 钴 镍 铜 锌 镓 锗 砷 硒 溴 氪 37 Rb 38 Sr 39 Y 40 Zr 41 Nb 42 Mo 43 Tc 44 Ru 45 Rh 46 Pd 47 Ag 48 Cd 49 In 50 Sn 51 Sb 52 Te 53 I 54 Xe 5 铷 锶 钇 锆 铌 钼 锝 钌 铑 钯 银 镉 铟 锡 锑 碲 碘 氙 55 Cs 56 Ba 57-71 72 Hf 73 Ta 74 W 75Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 6 铯 钡 La-Lu 铪 钽 钨 铼 锇 铱 铂 金 汞 铊 铅 铋 钋 砹 氡 112 87 Fr 88 Ra 89-103 104 Rf 105Db 106Sg 107Bh 108 Hs 109Mt 110 111 118 114 116 7 钫 镭 Ac-Lr 钅卢 钅杜 钅喜 钅波 钅黑 钅麦 Uun Uuu Uub 57 La 58 Ce 59 Pr 60 Nd 61 Pm 62 Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69Tm 70 Yb 71 Lu 镧系 镧 铈 镨 钕 钷 钐 铕 钆 铽 镝 钬 铒 铥 镱 镥 89 Ac 90 Th 91 Pa 92 U 93 Np 94 Pu 95Am 96 Cm 97 Bk 98 Cf 99 Es 100 Fm 101Md 102No 103 Lr 锕系 锕 钍 镤 铀 镎 钚 镅 锔 锫 锎 锿 镄 钔 锘 铹 periods short period long periods First (2 element) second (8 element) (8 element) third fourth 18 elements fifth 18 elements sixth 32 elements seventh 32 elements periods or series The first short period contains two elements hydrogen (H)and helium(He). The second short period contains eight elements, beginning with lithium (Li) and ending with neon (Ne). The third short period also contains eight elements, beginning with sodium (Na)and ending with argon (Ar). The two long periods, The fourth period and the fifth period are two long periods, each of which contains 18 elements. The fourth period includes the elements from potassium (K)through krypton (kr). Within this period are the elements from scandium (Sc)through copper(Cu), which are known as the first transition series. The fifth period is begins with rubidium (Rb)and ends with xenon (Xe). Within this period are the elements yttrium (Y) through silver (Ag),which comprise the second transition series. The sixth period The sixth period, beginning with cesium (Cs)and ending with radon (Rn),contains 32 elements. The third transition series, made up of lanthanum (La)and the elements hafnium (Hf)through gold (Au) The sixth period The third transition series is split: between lanthanum and hafnium is a series of 14 elements, cerium (Ce) through lutetium (Lu),called the first inner transition series, or the lanthanide series or the rare earth elements. The seventh period The seventh period extends from francium through element number 118. However, no elements after element 109 have been characterized. The known elements in this period include a part of the fourth transition series (actinium, and elements 104 through 109). Electronic Structure and the Periodic Law the periodicity with respect to the number of valence electrons; valence electrons that is, electrons in the outermost shell. the Periodic Table is simply an arrangement of atoms that puts elements with the same number of valence electrons in the same group. 表:基态电中性原子的电子组态 “电子仁”或“电子实” 1 氢H 2 氦He 1s2 3 锂Li [He] 2s1 4 铍Be [He] 2s2 5硼B [He] 2s22p1 6 碳C [He] 2s22p2 7 氮N [He] 2s22p3 8 氧O [He] 2s22p4 9 氟F [He] 2s22p5 10氖Ne 1s2 2s22p6 11钠Na [Ne] 3s1 12镁Mg [Ne] 3s2 13铝Al [Ne] 3s23p1 14硅Si [Ne] 3s23p2 1s1 价电子层 价层电子 15磷P [Ne] 3s23p3 16硫S [Ne] 3s23p4 17氯Cl [Ne] 3s23p5 18氩Ar 1s22s22p63s23p6 19钾K [Ar] 4s1 20钙Ca [Ar] 4s2 21钪Sc [Ar] 3d14s2 22钛Ti [Ar] 3d24s2 23钒V [Ar] 3d34s2 24铬Cr* [Ar] 3d54s1 25锰Mn [Ar] 3d54s2 26铁Fe [Ar] 3d64s2 27钴Co [Ar] 3d74s2 28镍Ni [Ar] 3d84s2 不 符 合 构 造 原 理 1-48号元素的核外电子层结构 1 H 1s1 17 Cl [Ne]3s23p5 33 As [Ar]3d104s24p3 2 He 1s2 18 Ar [Ne]3s23p6 34 Se [Ar]3d104s24p4 3 Li [He]2s1 19 K [Ar]4s1 35 Br [Ar]3d104s24p5 4 Be [He]2s2 20 Ca [Ar]4s2 36 Kr [Ar]3d104s24p6 5 B [He]2s22p1 21 Sc [Ar]3d14s2 37 Rb [Kr]5s1 6 C [He]2s22p2 22 Ti [Ar]3d24s2 38 Sr [Kr]5s2 7 N [He]2s22p3 23 V [Ar]3d34s2 39 Y [Kr]4d15s2 8 O [He]2s22p4 24 Cr [Ar]3d54s1 40 Zr [Kr]4d25s2 9 F [He]2s22p5 25 Mn [Ar]3d54s2 41 Nb [Kr]4d45s1 10 Ne [He]2s22p6 26 Fe [Ar]3d64s2 42 Mo [Kr]4d55s1 11 Na [Ne]3s1 27 Co [Ar]3d74s2 43 Tc [Kr]4d55s2 12 Mg [Ne]3s2 28 Ni [Ar]3d84s2 44 Ru [Kr]4d75s1 13 Al [Ne]3s23p1 29 Cu [Ar]3d104s1 45 Rh [Kr]4d85s1 14 Si [Ne]3s23p2 30 Zn [Ar]3d104s2 46 Pd [Kr]4d10 15 P [Ne]3s23p3 31 Ga [Ar]3d104s24p1 47 Ag [Kr]4d105s1 16 S [Ne]3s23p4 32 Ge [Ar]3d104s24p2 48 Cd [Kr]4d105s2 families or groups 1. Elements in any one group have the same number of electrons in their outermost shell 2. The similarity in chemical properties among elements of the same group occurs because they have the same numbers of valence electrons 3. The number of electrons in the valence shell of an atom determines its chemical properties. 4. It is the loss, gain, or sharing of valence electrons that determines how elements react. families or groups 1. A number of groups = electron number of outmost shell = greatest oxidation number Example: 17Cl 15P 2. B number of groups =lose electron number [(n-1)d+ns] (except ⅧB) =greatest oxidation number(but it can be changed ) Example: Cr +2, +3, +6 Mn +2 ,+3,+4,+6,+7 Electronegativity The electronegativity of an atom is a measure of the ability of an atom to draw bonding electrons to itself when chemically combined with another atom In general, electronegativity increases in any row of the periodic table from left to right, and it decreases in going from the top of a column to the bottom. 电负性的应用 1.判断元素的金属性和非金属性 金属性 元素的电负性一般在2.0以下,非金属性性 元素一般在2.0以上。电负性最大的元素是 位于右上方的F,电负性最小的元素是位于 左下方的Fr(Fr是放射性元素) 2.估计化学键的类型 在化合物中,可以根据电负 性的差值大小,估计化学键的类型。 电负性差越大,离子性越强,一般说来,电负性差 大于1.7时,可认为是离子键,小于1.7时为共价键。 原子半径及其周期性变化 原子半径:有三种不同的定义 (1)共价半径:同种元素的两原子以共价单键相连时,两原子 核间距离的一半叫共价半径. (2)金属半径:金属元素的两原子以密堆积方式(金属键)结 合成金属晶体时,两原子核间距离的一半叫金属半径. (3)范氏半径:同种元素的两原子不能以共价键相连,只靠分 子间力作用时,两原子核间距离的一半叫范德华半径. 1、原子半径及其变化规律 2、主族元素原子半径的变化规律 主族元素: (1)同周期 从左到右 逐渐减小.但到希 有气体元素时又有 增大,原因是半径 定义不同. (2)同一族 从上到下 逐渐增大.从左到 右,核电荷增加是 主要因素,但从上 到下,电子层增加 是主要因素. 副族元素原子半径的变化规律 副族元素: (1)同周期 从左到右逐渐减小.但减小幅度不如主 族元素,这是由于最后一个电子是填在(n-1)层d 轨道上对核电荷的抵消作用造成的. (2)同一族 从上到下逐渐增 大。但增加幅度较小,甚 至第五、第六周期基本没 有增加,这是由于镧系收 缩的原因造成的.