Name: periodic trends in atomic size
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 3 min 8 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.2

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

原子的大小由电子或它 们的轨道决定。然而，轨道描述的并非一有限空间，而是可能發现电子的地 点的统计概率。那麽原子大小是如何定义的、其影响又是什麽呢？原子半径可以用两种方式描述。非键原子半径或原子的范德华半径，也就是在原子固体中相邻原子 核之间距离的一半。相反地，键原子半径或共价半径，区分金属和非金属。在金属中，半径描述 原子的晶体结构 其作为两个相邻原子 中心间距的一半。在非金属中，双原子分子的半径，被描述为键原子 中心间距的一半。元素周期表描述了通常被称为原子半径的共价半径的变化。而其受两个因素影响；价电子的主能量数 以及有效核电荷。主族元素原子半径的 趋势显示在此处。向下移动一组，主量子数n 依循每个元素加一。因此，随着外电子离原子核越来越远，原子半径沿着该组向下增加。举例来说，向下移动1组，原子半径 从锂增加到铯。整个周期表证明了这种趋势。此外，该图揭示了原子半径 对于各个硷金属的最大极限，并且在 此期间其每种稀有气体的含量降到最低。整个周期内原子半径的减小 可以用有效的核电何来解释。回顾一下有效电荷的概念。在任何多电子原子中，内壳 电子部分以细胞核的拉力 屏蔽外壳电子。因此，有效的核电荷，外层电子感受到的电荷 比实际核弹来得小。在同价壳中的电子 不会非常有效的互相屏蔽。整个週期中，核弹 增加，而内壳电子数量 保持不变。因此，随着有效核电荷的稳定增长，外电子的屏蔽减少，而这也导致了原子半径的减小。不过，大多数过渡元素的半径，大致上保持恆定。这是因为在最外层主 能级的电子数几 乎是恒定的。

原子半径和有效核电荷

元素周期表中各组的元素表现出相似的化学行为。之所以发生这种相似性，是因为一个基团的化合价壳中的电子数量和分布相同。
 
 在从左到右的整个周期中，质子被添加到原子核，电子被添加到价壳，每个相继的元素。沿着一组中的元素向下，价壳中的电子数量保持恒定，但是主量子数每次增加一个。了解元素的电子结构后，我们可以检查一些控制其化学行为的特性。这些特性会随着元素的电子结构的变化而周期性变化。
 
 <h4>原子半径的变化</ h4>
 
 量子力学图使得难以确定原子的确定大小。但是，有几种实用的方法可以定义原子的半径，从而确定原子的相对大小，从而给出大致相似的值。
 
 金属中的原子半径是两个相邻原子的中心之间的距离的一半。对于以双原子分子形式存在的元素来说，它是键原子中心之间距离的一半。
 
 通常，从左到右移动一个周期，每个元素的原子半径都比其前一个元素的原子半径小。这似乎违反直觉，因为这意味着具有更多电子的原子具有较小的原子半径。这可以根据有效核弹的概念来解释。在任何多电子原子中，内壳电子都部分屏蔽了外壳电子免受原子核的吸引。因此，有效核电荷，即电子感受到的电荷，比实际核电荷（ Z ）小，可以通过以下方式估算：
 
 Z eff = Z ＆ndash; ＆sigma; ＆nbsp; ＆nbsp; ＆nbsp; ＆nbsp; ＆nbsp;＆nbsp;＆nbsp; ＆nbsp; 
 
 其中， Z eff 是有效核电荷， Z 是实际核电荷，＆sigma;是屏蔽常数，其中屏蔽常数大于零但小于Z 。
 
 我们每次跨一个周期从一个元素移动到另一个元素时， Z 都会增加一个，但是屏蔽只会稍微增加。因此， Z eff 在整个周期中从左向右移动时都会增加。元素周期表右侧的电子受到的拉力（有效核电荷更高）使它们更靠近原子核，从而使原子半径更小。
 
 核心电子有效地屏蔽了最外面的主能级中的电子免受核电荷，但是最外面的电子却没有有效地将彼此屏蔽。有效核电荷越大，原子核对外部电子的保持力越强，原子半径越小。
 
 但是，某些过渡元素的半径在每一行中大致保持恒定。这是因为最外层主能级中的电子数量几乎是恒定的，并且它们会经历大致恒定的有效核电荷。
 
 在每个周期内，原子半径的趋势随 Z 的增加而减小；在每个组中，趋势是原子半径随 Z 的增加而增加。
 
 向下扫描一个基团，每个元素的主量子数 n 增加一个。因此，电子被添加到越来越远离原子核的空间区域。因此，随着我们增加最外层电子离原子核的距离，原子的大小（及其原子半径）必须增加。下表说明了卤素原子半径的趋势。
 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody style="border: 1px solid black;">
 <tr>
 <td colspan="3" style="color: black; border: 1px solid black; text-align: center;">卤素族元素的原子半径</td>
 </ tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">原子</td>
 <td style="color: black; border: 1px solid black; text-align: center;">原子半径 (pm)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">核电荷, Z</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">F</td>
 <td style="color: black; border: 1px solid black; text-align: center;">64</td>
 <td style="color: black; border: 1px solid black; text-align: center;">9+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Cl</td>
 <td style="color: black; border: 1px solid black; text-align: center;">99</td>
 <td style="color: black; border: 1px solid black; text-align: center;">17+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Br</td>
 <td style="color: black; border: 1px solid black; text-align: center;">114</td>
 <td style="color: black; border: 1px solid black; text-align: center;">35+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">I</td>
 <td style="color: black; border: 1px solid black; text-align: center;">133</td>
 <td style="color: black; border: 1px solid black; text-align: center;">53+</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">At</td>
 <td style="color: black; border: 1px solid black; text-align: center;">148</td>
 <td style="color: black; border: 1px solid black; text-align: center;">85+</td>
 </tr>
 </tbody>
 </table>
 
  
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties"> Openstax 化学 2e ，第6.5节：元素属性的周期性变化。</a>

元素周期表中各族的元素表现出相似的化学性质。这种相似性源于同族元素具有相同的最外层电子数。

Education

教育

JoVE Core

化学

Periodic Properties of the Elements

元素的周期性质

KEY TERMS AND CONCEPTS

关键术语与概念

periodic trends in atomic size

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one.

Atomic radius is not easy to define with perfect precision because atoms do not have a hard edge. Scientists use practical measurements instead. In metals, atomic radius is one-half the distance between the centers of two neighboring atoms. For elements that form diatomic molecules, it is one-half the distance between the centers of bonded atoms.

Moving from left to right across a period, atomic radius generally decreases. That may seem surprising because atoms on the right have more electrons. The reason is effective nuclear charge, or the charge an electron feels from the nucleus. It can be estimated as Z eff = Z – σ, where Z is the actual nuclear charge and σ is the shielding constant.

As Z increases by one from element to element across a period, shielding rises only a little. This means Z eff also increases from left to right. A stronger pull from the nucleus draws outer electrons closer, so the atomic radius becomes smaller. Core electrons shield outer electrons well, but outermost electrons do not shield one another very effectively. Some transition elements keep nearly the same radius across a row because the number of electrons in the outermost principal energy level changes very little, so the effective nuclear charge stays roughly constant.

The trend within a group is the opposite. Atomic radius increases as Z increases down a group. Each step down adds a principal energy level, so the outermost electrons are farther from the nucleus. That greater distance makes the atom larger. The halogens show this pattern clearly, with atomic radii increasing from fluorine to astatine.

Periodic Trends in Atomic Size

periodic classification of the elements

How Valence Electrons Shape the Periodic Table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine...

ionization energy

Ionization Energy Trends in the Periodic Table

Ionization energy is the energy needed to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy, or IE1, removes the most loosely bound electron and forms a 1+ cation. The second ionization energy, IE2, removes the next electron, and each later ionization energy removes another electron from the atom or ion. Ionization always requires energy, so these processes are endothermic. That means ionization energy values are always positive. In general, larger...

atomic radii and effective nuclear charge

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one. Atomic radius is not easy to define with perfect...

ionic radii

How Ionic Charge Changes Ion Size

Ionic radius describes the size of an ion. It changes when an atom gains or loses electrons. Cations are positive ions with fewer electrons than the parent atom, and they are smaller than the atoms they come from. Aluminum shows this pattern clearly. A neutral aluminum atom has a covalent radius of 118 pm, while Al3+ has an ionic radius of 68 pm. When electrons are removed from the outer valence shell, the remaining core electrons feel a stronger effective nuclear charge, or Zeff. That pull...

electron affinity

Electron Affinity Trends on the Periodic Table

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive. Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities...

alkali metals

Alkali Metals: Reactivity Trends

Alkali metals are Group 1 elements with one valence electron. They are soft, shiny metallic solids that are malleable, ductile, and good conductors of heat and electricity. Their melting points are unusually low for metals and drop as you move down the group. Density generally increases down the group, except for potassium. The trend in atomic size helps explain their behavior. Moving from lithium to cesium, each element has a higher principal quantum number, so the outer electron is farther...

halogens

Halogens: Trends and Reactions

Halogens are Group 17 nonmetals that show clear trends in structure and reactivity. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine is highly unstable and radioactive, so most of its properties are still unknown because it has a very short half-life. Tennessine is a synthetic element that is also predicted to belong to this group. These elements do not usually exist as single atoms. Instead, they form diatomic molecules, which means...

noble gases

Noble Gases: Sources, Uses, and Reactions

Noble gases are the group 18 elements helium, neon, argon, krypton, xenon, and radon. They were called “noble” because they were once thought to be nonreactive. That idea changed in 1962, when Dr. Neil Bartlett at the University of British Columbia showed that this assumption was not fully correct. These gases occur in very small amounts in the atmosphere. Some natural gas contains 1–2% helium by mass. Helium is separated from natural gas by removing the condensable components, which leaves...

key terms and concepts

Learn why atomic radius changes across periods and groups, and how effective nuclear charge and shielding shape the size of atoms in the periodic table.

Periodic Trends in Atomic Size | JoVE K12

chemistry

JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Physical Science

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Pennsylvania State Academic Standards

Chemical Reactions

3.2.9-12.C

Structure and Properties of Matter

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Louisiana Student Standards for Science

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Periodic trends of the elements

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HIgh School

CH.2

CH.2.b

CH.2.c

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Periodic Table Trends

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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Scientific Laws

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Matter: Pure Substances and Mixtures
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The Atomic Theory of Matter

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Subatomic Particles

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Types of Radioactivity

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Nuclear Binding Energy

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Radioactive Decay and Radiometric Dating

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Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

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Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

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The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

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Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

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Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

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Molar Mass

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Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

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Ionic Bonding and Electron Transfer

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The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

trends-in-lattice-energy-ion-size-and-charge

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

Analyze the structure of atoms and isotopes.

Periodic Trends in Atomic Size

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