Name: periodic trends in atomic size
Uploaded: 2020-09-03T13:38:28.000Z
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Atomic Radii and Effective Nuclear Charge

تملأ الإلكترونات حجم الذرة أو مداراتها. ومع ذلك،لا تصف المدارات مساحة محددة،بل احتمالات إحصائية للمكان،الذي يمكن العثور فيه على إلكترون. فكيف يتم تحديد الحجم الذري،وماذا يؤثر عليه؟يمكن وصف نصف القطر الذري بطريقتين.نصف القطر الذري غير المترابط،أو نصف قطر فان دير فالس للذرة،و هي نصف المسافة بين النوى المتجاورة في المادة الصلبة الذرية. بالمقابل،نصف القطر الذري المترابط،أو نصف القطر التساهمي،يميز بين المعادن واللافلزات. في المعادن،يتم وصف نصف القطر للذرات في تركيبها البلوري على شكل نصف المسافة بين مركزي اثنين من الذرات المتجاورة.في الجزيئات اللافلزية ثنائية الذرة،نصف القطر يوصف بأنه نصف المسافة بين مركزي الذرات المترابطة. الجدول الدوري يصور الاختلافات في نصف القطر التساهمي،والذي يطلق عليه غالبًا نصف القطر الذري،يتأثر بعاملين عدد مستويات الطاقة الرئيسية لإلكترونات التكافؤ،والشحنة النووية الفعالة. الآتجاه في نصف القطر الذري لعناصر المجموعة الرئيسية أسفل الأعمدة،يصور هنا.بتحريك مجموعة للأسفل،الرقم الكمي الأساسي،n،يزيد بواحد لكل عنصر. وهكذا،عندما تبتعد الإلكترونات الخارجية عن النواة،يزيد نصف القطر الذري أسفل المجموعة. على سبيل المثال،بالانتقال بالمجموعة 1 إلى أسفل،فإنّنصف القطر الذري يزيد من الليثيوم إلى السيزيوم.يتم توضيح هذا الاتجاه من خلال الجدول الدوري بأكمله. علاوة على ذلك،تكشف المعادلة أن نصف القطر الذري يصل للحد الأقصى لكل معدن قلوي،ويسقط إلى الحد الأدنى مع كل غاز نبيل خلال الفترة. تناقص نصف القطر الذري خلال فترة يمكن تفسيرها بالشحنة النووية الفعالة.لنتذكر مفهوم الشحنة النووية الفعالة. في أي ذرة متعددة الإلكترونات،تحمي الكترونات الغلاف الداخلي جزئياًإلكترونات الغلاف الخارجي من سحب النواة. وبالتالي،فإن الشحنة النووية الفعالة،الشحنة التي يشعر بها إلكترون خارجي،تكون أقل من الشحنة النووية الفعلية.الإلكترونات في نفس غلاف التكافؤ لا تحمي بعضها البعض بشكل فعال. خلال هذه الفترة،الشحنة النووية تزيد بينما عدد إلكترونات الغلاف الداخلي يبقى ثابتا. وهكذا،مع زيادة الشحنة النووية الفعالة بشكل مطرد،تصبح حماية الإلكترونات الخارجية أقل،وهذا يؤدي إلى انخفاض في نصف القطر الذري.نصف قطر معظم العناصر الانتقالية،مع ذلك،يبقى ثابتًا تقريبًا على مدى الصف. وذلك لأن عدد الإلكترونات في مستوى الطاقة الأساسي الخارجي يكاد يكون ثابتًا.

نصف القطر الذرّي والشحن النووي الفعّال

تعرض العناصر الموجودة في مجموعات الجدول الدوري سلوكاً كيميائياً مشابهاً. يحدث هذا التشابه لأن أعضاء المجموعة لديهم نفس عدد الإلكترونات وتوزيعها في أصداف التكافؤ الخاصة بهم. بالمرور من اليسار إلى اليمين، تتم إضافة البروتون إلى النواة والإلكترون إلى غلاف التكافؤ مع كل عنصر متتالي. عند النزول إلى العناصر في مجموعة ما، يظل عدد الإلكترونات في غلاف التكافؤ ثابتاً، ولكن العدد الكمي الرئيسي يزداد بمقدار واحد في كل مرة. يتيح لنا فهم البنية الإلكترونية للعناصر فحص بعض الخصائص التي تحكم سلوكها الكيميائي. وتتباين هذه الخصائص بشكل دوري مع تغير الهيكل الإلكتروني للعناصر.  <h4>الاختلاف في نصف القطر الذري </h4>إن الصورة الميكانيكية الكمّية تجعل من الصعب تحديد حجم ذرة معين. ومع ذلك، هناك العديد من الطرق العملية لتحديد نصف قطر الذرات، وبالتالي تحديد أحجامها النسبية التي تعطي قيماً متشابهة تقريباً. نصف القطر الذري للمعادن هو نصف المسافة بين مركزي ذرتين متجاورتين. وهو نصف المسافة بين مراكز الذرات المترابطة للعناصر الموجودة كجزيئات ثنائية الذرة. وبالانتقال الدوري من اليسار إلى اليمين، يكون نصف قطر كل عنصر ذري أصغر من نصف قطر العنصر الذي يسبقه. قد يبدو هذا غير بديهي لأنه يعني ضمناً أن الذرات التي تحتوي على المزيد من الإلكترونات لها نصف قطر ذري أصغر. وبوسعنا أن نفسر هذا استناداً إلى مفهوم الشحن النووي الفعّال. في أي ذرة متعددة الإلكترونات، تقوم إلكترونات الذرات الداخلية بحمايتها بشكل جزئي من قوة سحب النواة.  وبالتالي، فإن الشحنة النووية الفعلية، التي يشعر بها الإلكترون، أقل من الشحنة النووية الفعلية (Z) ويمكن تقديرها بما يلي: Zeff = Z – σ              حيث يعتبر Zeff هو الشحن النووي الفعال، و Z هي الشحنة النووية الفعلية، بينما يمثل نظام مجموعة الثمانية قيمة ثابت التدرع ، حيث يكون ثابت الدرع أكبر من صفر ولكنه أصغر من Z.في كل مرة نتحرك فيها من عنصر إلى آخر دوريّاً، تزيد Z بمقدار عنصر واحد، ولكن التدرع يزداد قليلاً فقط. وهكذا فإن Zeff يزداد مع انتقالنا من اليسار إلى اليمين دوريّاً. إن قوة الجذب (الشحنة النووية الأعلى فعالية) التي واجهتها الإلكترونات على الجانب الأيمن من الجدول الدوري تقربها إلى النواة، مما يجعل الإشعاع الذري أصغر. تعمل الإلكترونات الأساسية على حماية الإلكترونات بكفاءة في المستوى الخارجي الرئيسي من الشحنة النووية، ولكن معظم الإلكترونات الخارجية لا تحمي بعضها البعض من الشحنة النووية بشكل فعال. وكلما كان الشحن النووي الفاعل أكبر، كلما كانت قوة حمل النواة على الإلكترونات الخارجية أكبر، وكلما قل نصف القطر الذري. مع ذلك، يظل شعاع بعض عناصر الانتقال ثابتًا تقريبًا عبر كل صف. ويرجع هذا إلى أن عدد الإلكترونات في أعلى مستوى طاقة رئيسي ثابت تقريبًا، وهي تواجه شحنة نووية فعالة بشكل ثابت تقريبًا. في كل فترة، ينخفض الاتجاه في نصف القطر الذري بزيادة Z؛  وفي كل مجموعة، يزداد نصف القطر الذري مع زيادة Z. المسح الضوئي لأسفل مجموعة ما، نرى أن الرقم الكمي الرئيسيn يزداد بمقدار واحد لكل عنصر. وبالتالي، تتم إضافة الإلكترونات إلى منطقة فضاء تكون بعيدة بشكل متزايد عن النواة. وبالتالي، يجب أن يزداد حجم الذرة (ونصف قطرها الذري) مع زيادة مسافة الإلكترونات الأبعد عن النواة. ويتبين هذا الاتجاه بالنسبة للنديان الذرية من الهالوجينات في الجدول أدناه. 
 
 <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;">Atomic Radii of the Halogen Group Elements</td></tr><tr><td style="color: black; border: 1px solid black; text-align: center;">Atom</td><td style="color: black; border: 1px solid black; text-align: center;">Atomic radius (pm)</td><td style="color: black; border: 1px solid black; text-align: center;">Nuclear charge, 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> هذا النص مقتبس من <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>

تظهر العناصر الموجودة في مجموعات الجدول الدوري سلوكا كيميائيا مماثلا. يحدث هذا التشابه لأن أعضاء المجموعة لديهم نفس الرقم ...

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Periodic Properties of the Elements

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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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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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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 Pauli Exclusion Principle

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The Energies of Atomic Orbitals

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The Aufbau Principle and Hund's Rule

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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 ...

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Periodic Classification of the Elements

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The Periodic Table

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Molar Mass

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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 ...

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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).



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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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The Born-Haber Cycle

Lattice Energy&#xA0;

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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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