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

原子の大きさは 電子や軌道によって 決まります しかし 軌道は限られた 空間を表すものではなく 電子がどこにあるかという 統計的な 確率を表すものです では 原子の大きさは どのように定義され 何が影響しているのでしょうか？原子半径は2つの方法で 記述することができます 非結合性原子半径（原子の ファンデルワールス半径）は 原子の固体中で 隣り合う原子核間の距離の 2分の1の大きさです 逆に 金属と非金属を 区別するのが 結合性原子半径 共有結合半径）です 金属では 結晶構造中の 原子の半径は 隣り合う2つの原子の 中心間の距離の 2分の1と 表されます 非金属の二原子分子では 半径は結合した原子の 中心間の距離の 2分の1と表されます 周期表には 原子半径と呼ばれる 共有結合の変化が 描かれていますが これは原子価電子の 主エネルギー準位の数と 実効核電荷の 2つの要素に影響されます ここでは 主要な族の元素の 原子半径の傾向を示しています 主量子数nは 元素ごとに 1ずつ増加していきます このように 外側の電子が 原子核から遠ざかるほど 原子半径は大きくなります 例えば 1族を 下に移動すると リチウムからセシウムへと 原子半径が大きくなります この傾向は 周期表全体で 示されています さらに このプロットを見ると 各アルカリ金属で 原子半径が最大になり 各希ガスで 最小になることがわかります 原子半径が 周期的に減少していることは 実効核電荷で説明できます 有効核電荷の概念を 思い出してください 多電子原子では 内殻電子が外殻電子を 原子核の引力から 部分的に遮蔽しています このため 外殻電子が 感じる電荷である 実効核電荷は 実際の核電荷よりも 小さくなります 同じ価数殻内の電子は お互いをあまり 効果的に遮蔽しません この周期中 内殻電子の数は 一定であるのに対し 核電荷は 増加します このように 実効核電荷が 着実に増加すると 外殻電子の遮蔽が少なくなり 原子半径が減少します しかし ほとんどの 遷移元素の原子半径は 各列に渡って ほぼ一定に保たれています これは 最も外側の 主エネルギー準位にある 電子の数が ほぼ一定であるためです

原子半径と有効核電荷

周期表の族に属する元素は、似たような化学的挙動を示します。このような類似性は、族のメンバーがその原子価殻内の電子の数と分布が同じであることから生じます。 
 
 1つの周期を左から右に進むと、元素ごとに原子核に陽子１つが、電子価殻に電子１つが追加されます。また、元素族を下っていくと、価電子の数は一定だが、主量子数は1つずつ増えていきます。元素の電子構造を理解することで、元素の化学的挙動を支配するいくつかの特性を調べることができます。これらの特性は、元素の電子構造の変化に伴って周期的に変化します。
 
 <h4>原子半径の変動 </h4>
 
 量子力学的には、原子の大きさを明確にすることは難しいです。しかし、原子の半径を定義し、その相対的な大きさを決定する実用的な方法がいくつかあり、ほぼ同様の値が得られます。 
 
 金属の原子半径は、隣り合う2つの原子の中心間の距離の2分の1です。2原子分子として存在する元素では、結合した原子の中心間の距離の2分の1となります。 
 
 周期を左から右に移動すると、一般的に各元素は前の元素よりも原子半径が小さくなります。このことは、電子数の多い原子ほど原子半径が小さくなることを意味しており、直感的ではないかもしれません。これは、「有効核電荷」という概念で説明できます。電子数の多い原子では、内殻電子が外殻電子を原子核の引力から部分的に遮蔽しているため、電子が感じる電荷である有効核電荷は実際の核電荷（Z）よりも小さく、次のようにして見積もることができます。 
 
 Zeff = Z – σ         
 
 ここで、Zeff は有効な核電荷、 Z は実際の核電荷、 σ は遮蔽定数で、シールド定数はゼロより大きく Z より小さくなります。
 
 1つの周期で1つの元素から次の元素へと移動するたびに、Zは1つ増加しますが、遮蔽はわずかにしか増加しません。したがって、Zeffは、1周期の間に左から右へ移動するにつれて大きくなります。周期表の右側では、電子の引力が強い（有効核電荷が大きい）ため、電子は原子核に近づき、原子半径が小さくなります。 
 
 コア電子は一番外側の主準位にある電子を核電荷から効率的に遮蔽するが、一番外側の電子はお互いに核電荷から効率的に遮蔽しません。有効核電荷が大きいほど、外側の電子に対する核の保持力が強くなり、原子半径は小さくなります。 
 
 しかし、一部の遷移元素の半径は、各列でほぼ一定です。これは、一番外側の主エネルギー準位にある電子の数がほぼ一定であり、有効核電荷がほぼ一定であるためです。 
 
 各周期の中では、Zの増加に伴って原子半径が減少する傾向にあります。各族の中では、Zの増加に伴って原子半径が増加する傾向にあります。 
 
 属を走査していくと、主な量子数であるnは元素ごとに1つずつ増えていきます。このように、電子は原子核からどんどん離れた空間に加えられていきます。したがって、最外周の電子が原子核から離れるほど、原子の大きさ（原子半径）は大きくならざるを得ません。この傾向は、下表のハロゲンの原子半径に示されています。 
 
 <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;">ハロゲン族元素の原子半径 </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;">核電荷、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>
 
  
 
 このテキストは <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">、 Openstax Chemistry 2e の第 6.5 章「元素特性の周期的変化」に基づいています。</a>

周期表のグループ内の元素は、同様の化学的挙動を示します。この類似性は、グループのメンバーが同じ番号を持っているために発生します...

Education

JoVE Core

化学

Periodic Properties of the Elements

元素の周期的な性質

KEY TERMS AND CONCEPTS

主要用語と概念

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 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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Periodic trends of the elements

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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Scientific Laws

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The Scientific Method

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Classifying Matter by State

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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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Radioactivity and Nuclear Equations

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Types of Radioactivity

The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
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Nuclear Stability

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Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

Nuclear Fission

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 ...

Nuclear Fusion

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

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

PS.Chm.1.2

Atomic Orbitals

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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

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The Periodic Table

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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

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Ionic Radii

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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).



This process can be either ...

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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Ionic Bonding and Electron Transfer

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The Born-Haber Cycle

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Trends in Lattice Energy: Ion Size and Charge

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Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

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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