Name: average atomic mass and isotopes
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 1 min 52 s
Channel: JoVE

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.1

Atomic Mass

كتلة الذرة المفردة صغيرة جدًا،لذلك ليس من العملي قياس كتلة الذرة بالجرام أو الكيلوجرام. كتلة الذرات والجزيئات تقاس بوحدات تسمى دالتون"وتختصرالى Da،أو وحدات الكتلة الذرية،اختصارها amu أو في بعض الأحيان فقط حرف u"كتلة الذرة في وحدة amu"تقريبًا تساوي مجموع عدد البروتونات،وعدد النيوترونات؛ومع ذلك،فإن معظم العناصر لها العديد من النظائر الطبيعية. لكل من هذه النظائر عدد مختلف من النيوترونات،وبالتالي كتلة مختلفة.لكل عنصر،متوسط كتلته يتم حسابها بجمع كتل نظائر هذاالعنصر،كل واحدة مضروبة في وفرة الكسور الطبيعية الخاصة بها. على الارض. الكتلة والوفرة النسبية لمختلف النظائر لعنصر ما يمكن ان تحدد باستخدام مطياف الكتلة.يتم فصل النظائر بالكتلة،والطيف الكتلي للعينة يوضح الوفرة النسبية للنظائر. يتم استخدام هذه القيم لحساب متوسط الكتلة الذرية التي تظهر في الجدول الدوري. الطيف الكتلي لعينة تمثيلية من البورون يظهر أن 19.9٪من هذه العينة بورون-10 بكتلة 10.0129 amu.الباقي هو البورون-11 بكتلة 11.0093 amu. كتلة البورون-10 والبورون-11 يتم ضرب كل منها في وفرة كسورهم،19.9 لكل 100 للبورون-10 و 80.1 لكل 100،من الباقي،للبورون-11. ثم يتم جمع هذه القيم لإيجاد متوسط الكتلة الذرية من البورون.

الكتلة الذرية

الذرات — والبروتونات، والنيوترونات، والإلكترونات التي تتكون منها — صغيرة للغاية. على سبيل المثال، تزن ذرة الكربون أقل من 2 &مرات; 10−23 غ. عندما نصف خصائص الأجسام الدقيقة مثل الذرات، نستخدم وحدات قياس صغيرة مناسبة، مثل وحدة الكتلة الذرية (amu). تم تعريف amu في الأصل على أساس الهيدروجين، أخف عنصر، ثم فيما بعد على أساس الأكسجين. منذ عام 1961، تم تعريفه على أساس أكثر نظائر الكربون وفرة، ذرّات كتلتها الذرية بالضبط 12 وحدة دولية. وبالتالي، فإن وحدة amu هي بالضبط 1 / 12 من كتلة ذرة كربون -12 amu:1 amu = 1.6605 &مرات; 10&مرات;24 غ.تعدّ وحدة الدالتون (Da) ووحدة الكتلة الذرّية الموحّدة (u) كوحدات بديلة مكافئة ل amu.   نظرًا لأن كل بروتون وكل نيوترون يساهمان تقريبًا في كتلة ذرة واحدة، ويساهم كل إلكترون أقل بكثير، فإن الكتلة الذرية لذرة واحدة تساوي تقريبًا عدد كتلتها (مجموع عدد صحيح من البروتونات والنيوترونات في الذرة). على سبيل المثال، العدد الكتلي لذرة نيتروجين واحدة هو 14 (7 بروتونات + 7 نيوترونات). ومع ذلك، فإن متوسط ​​كتل ذرات معظم العناصر ليس أعدادًا صحيحة لأن معظم العناصر توجد بشكل طبيعي كمزيج من نظيرين أو أكثر. النظائر هي ذرات من نفس العنصر لها نفس عدد البروتون ولكن بعدد مختلف من النيوترونات. كتلة العنصر الموضح في الجدول الدوري أو المدرج في جدول الكتل الذريّة هي متوسط ​​الكتلة الموزونة لجميع النظائر الموجودة في عينة والتي تحدث بشكل طبيعي لهذا العنصر. متوسط ​​الكتلة يساوي مجموع كتلة كل نظير’s مضروبًا في وفرة الكسر. <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/807/807_Equations-01.png" style="height: 40px;" /> على سبيل المثال ، يتكون عنصر الكلور (العدد الذري 17) من نظيرين: الكلور 35 والكلور 37 Cl. حوالي 75.78٪ من كل ذرات الكلور هي 35 Cl مع كتلة 34.969 amu ، والباقي 24.22٪ 37 Cl مع كتلة 36.966 amu. يُحسب متوسط ​​الكتلة الذرية للكلور على النحو التالي: <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/807/807_Equations-02.png" style="height: 40px;" /> من المهم أن نفهم أنه لا توجد ذرة كلور تزن بالضبط 35.45 amu ؛ هذه القيمة هي متوسط ​​الكتلة لجميع ذرات الكلور، وتزن ذرات الكلور الفردية إما حوالي 35 amu أو 37 amu. أيضًا، نظرًا لأن الكلور الموجود بشكل طبيعي يحتوي على ذرات كلور 35 أكثر من ذرات الكلور 37 ، فإن متوسط ​​كتلة الكلور الموزونة أقرب إلى 35 amu منه إلى 37 amu. يمكن تحديد التواجد والوفرة الطبيعية للنظائر بشكل تجريبي باستخدام أداة تسمى مطياف الكتلة. يستخدم مطياف الكتلة (MS) على نطاق واسع في الكيمياء والطب الشرعي والطب والعلوم البيئية والعديد من المجالات الأخرى لتحليل المواد في عينة من المواد والمساعدة في تحديدها. في مقياس طيف الكتلة النموذجي، يتم تبخير العينة وتعريضها لشعاع إلكتروني عالي الطاقة يتسبب في أن تصبح ذرات العينة’s أو جزيئاتها مشحونة كهربائيًا ، عادةً عن طريق فقدان إلكترون واحد أو أكثر. ثم تمر هذه الكاتيونات عبر مجال مغناطيسي متغير يحرف مسار كل كاتيون’s إلى الحد الذي يعتمد على كل من كتلته وشحنته. بالمحصّلة، يتم اكتشاف الأيونات، ويتم عمل مخطط للعدد النسبي للأيونات المتولدة مقابل نسب الكتلة إلى الشحن، وبذلك يتشكل طيف الكتلة. يتناسب ارتفاع كل استجابة عمودية أو ذروة في الطيف الكتلي مع جزء من الكاتيونات ذات نسبة محدّدة من الكتلة إلى الشحن. منذ استخدامه لأول مرة أثناء تطوير النظرية الذرية الحديثة، تطورت مطياف الكتلة ليصبح أداة قوية للتحليل الكيميائي في مجموعة واسعة من التطبيقات. النص مقتبس من <a href="https://openstax.org/books/chemistry-2e/pages/2-3-atomic-structure-and-symbolism">Openstax Chemistry 2e, Section 2.3: Atomic Structure and Symbolism.</a>

الذرات - والبروتونات والنيوترونات والإلكترونات التي تتكون منها - صغيرة للغاية. على سبيل المثال ، تزن ذرة الكربون أقل من 2 ...

Education

JoVE Core

الكيمياء

واحد

Atoms and Elements

الذرات والعناصر

KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

average atomic mass and isotopes

Average atomic mass explains why many elements on the periodic table have decimal values. Atoms are extremely small, so scientists use the atomic mass unit, or amu, to measure them. A carbon atom weighs less than 2 × 10−23 g, and the amu is defined from carbon-12. One amu is exactly 1/12 the mass of one carbon-12 atom, which is 1.6605 × 10−24 g. The dalton (Da) and the unified atomic mass unit (u) mean the same thing as the amu.

For a single atom, atomic mass is usually close to its mass number. The mass number is the total of protons and neutrons in the atom. Each proton and neutron contributes about 1 amu, while each electron adds far less. For example, a nitrogen atom has a mass number of 14 because it has 7 protons and 7 neutrons.

Most elements do not have a whole-number atomic mass because they exist as mixtures of isotopes. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The atomic mass listed for an element in the periodic table is a weighted average of the masses of all naturally occurring isotopes. This average is found by multiplying each isotope’s mass by its fractional abundance and then adding the results.

Chlorine is a good example. It has two isotopes, chlorine-35 and chlorine-37. About 75.78% of chlorine atoms are chlorine-35 with a mass of 34.969 amu, and 24.22% are chlorine-37 with a mass of 36.966 amu. The average atomic mass of chlorine is 35.45 amu. No single chlorine atom has exactly that mass. Individual chlorine atoms are closer to 35 amu or 37 amu, depending on the isotope.

Scientists can measure isotopes and their natural abundance with a mass spectrometer. Mass spectrometry, or MS, is used in chemistry, forensics, medicine, environmental science, and other fields to identify substances in a sample. In a mass spectrometer, the sample is vaporized and hit with a high-energy electron beam. This makes the atoms or molecules become charged, usually by losing one or more electrons. The cations then move through a magnetic field, which bends their paths based on mass and charge. The detector records them and creates a mass spectrum, a graph of ion count versus mass-to-charge ratio. Each peak shows how many ions have a given mass-to-charge ratio. Since its early use in modern atomic theory, MS has become a powerful tool for chemical analysis.

Average Atomic Mass and Isotopes

the atomic theory of matter

Dalton’s Laws and Early Atomic Ideas

Dalton’s atomic theory explains matter with atoms, elements, and compounds. It also shows how chemical changes follow simple laws. This idea gave chemistry a microscopic model for what can be seen in the lab. The earliest recorded ideas about matter came from ancient Greek philosophers. Leucippus and Democritus said all matter was made of tiny, finite particles called atomos, meaning “indivisible.” Aristotle and others later argued that matter was made from four elements: fire, earth, air, and...

subatomic particles

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Atomic structure is built from three subatomic particles: protons, neutrons, and electrons. Atoms make up all matter, and these particles explain an atom’s mass and charge. The electron was first identified at the end of the 19th century by J.J. Thomson. He used a cathode ray tube, a sealed glass tube with almost all the air removed and two metal electrodes. When high voltage was applied, a visible beam called a cathode ray formed. The beam bent toward positive charge and away from negative...

elements: chemical symbols and isotopes

Reading Element Symbols and Isotope Notation

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ions and ionic charges

Predicting Ion Charges on the Periodic Table

Ion charges can often be predicted from an element’s position on the periodic table. In chemical reactions, the nucleus stays unchanged, but electrons may be transferred, lost, or shared. When atoms gain or lose electrons during compound formation, they become ions, which are electrically charged particles. A positive ion is called a cation. A cation has more protons than electrons because it lost electrons. A negative ion is called an anion. An anion has more electrons than protons because it...

the periodic table

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The periodic table organizes elements by repeating patterns in their properties. Chemists first noticed that some elements behaved alike, so they grouped them by chemical behavior. Lithium, sodium, and potassium are one example. They are shiny and conduct heat and electricity well. Calcium, strontium, and barium form another group with similar physical and chemical properties. Yet these two groups are not the same. The lithium group is much more reactive, and its elements form oxygen compounds...

atomic mass

Average atomic mass explains why many elements on the periodic table have decimal values. Atoms are extremely small, so scientists use the atomic mass unit, or amu, to measure them. A carbon atom weighs less than 2 × 10−23 g, and the amu is defined from carbon-12. One amu is exactly 1/12 the mass of one carbon-12 atom, which is 1.6605 × 10−24 g. The dalton (Da) and the unified atomic mass unit (u) mean the same thing as the amu. For a single atom, atomic mass is usually close to its mass...

molar mass

Mole, Mass, and Avogadro’s Number

The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2. The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 ×...

key terms and concepts

Learn how isotopes, fractional abundance, and mass spectrometry explain average atomic mass values on the periodic table for high school chemistry.

Average Atomic Mass and Isotopes | JoVE K12

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JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

PS.Chm.4.3

PS.Chm.4.4

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Ohio’s Learning Standards and Model Curriculum

Physical Science

Study Of Matter

Atoms

Structure And Properties Of Matter

Quantifying matter

Physical Geology

Minerals

Atoms and elements

Virginia  Standards of Learning

HIgh School

CH.2

CH.2.a

CH.4

CH.4.a

PS.4

PS.4.a

Texas Essential Knowledge and Skills

High School

CHEM.8

CHEM.8.A

CHEM.11

CHEM.11.E

Alabama Course of Study Science

Matter and Its Interactions

9-12.C.1.1

9-12.PS.3.4.d

Next Generation Science Standards

Earth and Space Science

Earth’s Place in the Universe

HS-ESS1-3

Florida Department of Education

SC.912.P.8

SC.912.P.8.4

Chemistry and Physics

IPC.7

IPC.7.A

Georgia Standards of Excellence

SC1.

Isotopes and Atomic Mass

Scientific Laws and Theories

Scientific Laws

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The Atomic Theory of Matter

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Elements: Chemical Symbols and Isotopes

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Alpha (&#945;) decay is the ...

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Radioactive Decay and Radiometric Dating

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PS.Chm.1.2

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The Pauli Exclusion Principle

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The Energies of Atomic Orbitals

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PS.Chm.1.3

Periodic Classification of the Elements

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The Periodic Table

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Atomic Radii and Effective Nuclear Charge

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Ionic Radii

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Ionization Energy

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Electron Affinity

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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.

PS.Chm.2.2

Types of Chemical Bonds

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Lewis Symbols and the Octet Rule

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The Born-Haber Cycle

Lattice Energy&#xA0;

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	Determine the ...

Resonance

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Formal Charges

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Exceptions to the Octet Rule

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	...

Bond Energies and Bond Lengths

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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

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Surface Tension
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Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Average Atomic Mass and Isotopes

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