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

K12 Standards

north-carolina-k-12-science-standard-course-of-study

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

원자의 크기는 전자나 그 궤도에 의해 결정됩니다. 그러나, 궤도는 한정된 공간을 묘사하는 것이 아니라, 전자가 발견될 수 있는 통계적 확률을 나타냅니다. 그렇다면 원자의 크기는 어떻게 정의되며, 그에 영향을 주는 것은 무엇입니까?

원자 반지름은 두 가지 방법으로 설명할 수 있습니다. 비결합 원자의 반지름 또는 원자의 반데르발스 반지름은 고체 원자에서 인접 핵 사이 거리의 2분의 1입니다. 반대로 결합 원자의 반지름 또는 공유 반지름은 금속과 비금속에서 차이가 납니다.

금속 원자의 반지름은 결정 구조에서 두 개의 이웃한 원자의 중심 사이의 거리의 2분의 1로 설명됩니다. 비금속성, 이원자 분자에서 반지름은 결합된 원자의 중심 사이의 거리의 2분의 1로 설명됩니다. 주기율표는 흔히 원자 반지름이라고 불리는 공유 반지름의 변화를 보여주고 있는데, 이는 원자가 전자의 주 에너지 준위 수와 유효 핵 전하라는 두 가지 요인의 영향을 받습니다.

주요 그룹 원소에 대한 열 아래쪽의 원자 반지름의 경향을 여기에서 보여줍니다. 그룹 아래로 내려가면서 주 양자수 n은 매 원소에 대해 하나씩 증가합니다. 그러므로 외부 전자가 핵으로부터 멀어질수록 원자 반지름이 그룹 아래로 갈수록 증가합니다.

예를 들어 그룹 1로 내려가면서 원자 반지름은 리튬에서 세슘까지 증가합니다. 이런 경향은 전체 주기율표에 나타납니다. 또한, 원자 반지름이 각 알칼리 금속에 대해 최대이며, 그 주기에서 각 비활성 기체로 가면서 최소로 떨어진다는 것을 보여줍니다.

같은 주기를 따라 점점 감소하는 원자 반지름은 유효 핵 전하로 설명할 수 있습니다. 유효 핵 전하의 개념을 상기해 봅시다. 모든 다전자 원자에서, 속껍질 전자는 핵의 당김으로부터 외부 껍질 전자를 부분적으로 가리게 됩니다.

따라서 외부 전자가 느끼는 전하, 즉 유효 핵전하는 실제 핵전하보다 작습니다. 같은 원자가 껍질 속의 전자는 서로를 효과적으로 가리지 않습니다. 주어진 주기에서 속껍질 전자의 수는 일정하게 유지되는 반면에 핵 전하는 증가합니다.

결국 유효 핵전하가 끊임없이 증가함에 따라 외부전자의 가리움이 줄어들게 되며 이는 원자 반지름의 감소로 이어집니다. 그러나 대부분의 전이 원소의 반지름은 각 행에서 대략적으로 일정하게 유지됩니다. 이것은 가장 바깥쪽에 있는 주 에너지 준위의 전자 수가 거의 일정하기 때문입니다.

원자 반경과 유효 핵 전하

주기율표의 그룹에 있는 원소들은 유사한 화학적 거동을 나타냅니다. 이러한 유사성은 그룹의 구성원이 원자가 껍질에 동일한 수와 전자 분포를 갖기 때문에 발생합니다.

왼쪽에서 오른쪽으로 주기를 거치면서 각 연속 원소와 함께 핵에 양성자가 추가되고 원자가 껍질에 전자가 추가됩니다. 한 족의 원소를 아래로 내려가면 원자가 껍질의 전자 수는 일정하게 유지되지만 주양자수는 매번 1씩 증가합니다. 원소의 전자 구조를 이해하면 원소의 화학적 거동을 지배하는 일부 특성을 조사할 수 있습니다. 이러한 특성은 요소의 전자 구조가 변경됨에 따라 주기적으로 달라집니다.

원자 반경의 변화

양자역학적 그림은 원자의 명확한 크기를 확립하는 것을 어렵게 만듭니다. 그러나 원자의 반경을 정의하고 대략 유사한 값을 제공하는 상대적인 크기를 결정하는 몇 가지 실용적인 방법이 있습니다.

금속의 원자 반경은 인접한 두 원자의 중심 사이 거리의 1/2입니다. 이원자 분자로 존재하는 원소의 결합 원자 중심 사이 거리의 1/2입니다.

왼쪽에서 오른쪽으로 기간을 이동하면 일반적으로 각 요소는 이전 요소보다 원자 반경이 더 작습니다. 이는 더 많은 전자를 가진 원자가 더 작은 원자 반경을 갖는다는 것을 의미하기 때문에 직관에 어긋나는 것처럼 보일 수 있습니다. 이는 유효핵전하의 개념으로 설명할 수 있습니다. 다중 전자 원자에서 내부 껍질 전자는 핵의 인력으로부터 외부 껍질 전자를 부분적으로 보호합니다. 따라서 유효 핵전하, 즉 전자가 느끼는 전하는 실제 핵전하(Z)보다 작으며 다음과 같이 추정할 수 있습니다.

Z_eff = Z – σ

여기서 Z_eff 는 유효 핵전하, Z는 실제 핵전하, σ는 차폐 상수입니다. 여기서 차폐 상수는 0보다 크고 Z보다 작습니다.

일정 기간 동안 한 요소에서 다음 요소로 이동할 때마다 Z는 1씩 증가하지만 차폐는 약간만 증가합니다. 따라서 Z_eff 는 일정 기간 동안 왼쪽에서 오른쪽으로 이동할 때 증가합니다. 주기율표의 오른쪽에 있는 전자가 받는 더 강한 인력(더 높은 유효 핵전하)은 전자를 핵에 더 가깝게 끌어당겨 원자 반경을 더 작게 만듭니다.

핵심 전자는 핵 전하로부터 가장 바깥쪽 주 준위에 있는 전자를 효율적으로 보호하지만, 가장 바깥쪽 전자는 핵 전하로부터 서로를 효율적으로 보호하지 못합니다. 유효 핵전하가 클수록 외부 전자에 대한 핵의 유지력이 강해지고 원자 반경이 작아집니다.

그러나 일부 전환 요소의 반경은 각 행에서 거의 일정하게 유지됩니다. 이는 최외각 주에너지 준위의 전자 수가 거의 일정하고 대략 일정한 유효 핵전하를 경험하기 때문입니다.

각 기간 내에서 Z가 증가함에 따라 원자 반경의 추세는 감소합니다. 각 그룹 내에서 Z가 증가함에 따라 원자 반경이 증가하는 경향이 있습니다.

그룹을 스캔하면 주양자수 n이 각 원소에 대해 1씩 증가합니다. 따라서 전자는 핵에서 점점 더 멀어지는 공간 영역에 추가되고 있습니다. 결과적으로, 핵으로부터 가장 바깥쪽 전자의 거리가 증가함에 따라 원자의 크기(및 원자 반경)도 증가해야 합니다. 이러한 경향은 아래 표에 할로겐의 원자 반경에 대해 설명되어 있습니다.

<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;">할로겐족 원소의 원자 반경</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">원자</td>
			<td style="color: black; border: 1px solid black; text-align: center;">원자 반경(pm)</td>
			<td style="color: black; border: 1px solid black; text-align: center;">핵 전하, 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>

이 문서는 에서 발췌되었습니다 <a href="https://openstax.org/books/chemistry-2e/pages/6-5-periodic-variations-in-element-properties">Openstax Chemistry 2e, Section 6.5: Periodic Variations in Element Properties.</a>

주기율표 그룹의 원소는 유사한 화학적 거동을 나타냅니다. 이러한 유사성은 그룹의 구성원이 같은 번호를 가지고 있기 때문에 발생합니다 ...

Education

교육

JoVE Core

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하나

Periodic Properties of the Elements

원소의 주기적 성질

KEY TERMS AND CONCEPTS

주요 용어 및 개념

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

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Periodic Table Trends: Atomic Radii, Ionization, and Electronegativity

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In science, a law is defined as a concise, verbal or mathematical, statement that summarizes a vast number of experimental observations. ...

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Chemistry is an empirical science. Scientists often pose questions to understand the chemistry in everyday life and seek answers to these questions. To ...

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Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

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According to its composition, the matter can be classified into two broad categories &mdash; pure substances and ...

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The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
Alpha (&#945;) decay is the ...

Nuclear Stability

Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10&#8722;15 meters, a nucleus is ...

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. ...

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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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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

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

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).



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.

Types of Chemical Bonds

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

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

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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