Name: atomic mass and mole calculations
Uploaded: 2023-06-23T15:38:52.000Z
Duration: 1 min 25 s
Channel: JoVE

משקלו האטומי של יסוד, המכונה לעתים מסה אטומית, שווה בקירוב לסכום מספר הפרוטונים והנייטרונים. זה מבוטא ביחידות מסה אטומית, מקוצרות כמו אמו, או דלתונים, והוא מיוצג ליד הסמל הכימי שלה על הטבלה המחזורית.
אבל, לרוב היסודות יש כמה איזוטופים הנבדלים זה מזה במספר הנייטרונים וכך במסות שלהם.
עבור יסודות כאלה, המשקל האטומי הוא המסה הממוצעת של כל האיזוטופים של יסוד, המשוקללת על פי השפע היחסי שלהם או אחוז מכל איזוטופ שנמצא בטבע.
לדוגמה, לכלור יש שני איזוטופים טבעיים. המסה של כלור-35 היא 34.97 אמו ושל כלור-37 היא 36.97 אמו.
כאשר מכפילים אותם בשפע היחסי שלהם, 75.77% עבור כלור-35 ו-24.23% עבור כלור-37, ומוסיפים, מתקבל המשקל האטומי הסופי של 35.45 אמו.
מכיוון שכלור-35 נפוץ יותר בטבע מאשר כלור-37, המשקל האטומי של כלור קרוב ל-35 אמו.

משקל אטומי

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לפרוטונים ולנייטרונים יש מסה זהה בערך, בערך 1.6721510 גרם. מדענים מגדירים באופן שרירותי את כמות המסה הזו כיחידת מסה אטומית אחת (אמו) או דלתון אחד. המסה של אלקטרונים קטנה בהרבה מהפרוטונים, ומשקלם רק 9.111028 גרם, או בערך 1/1800 מיחידת מסה אטומית. כתוצאה מכך, הם אינם תורמים הרבה למסה האטומית הכוללת של היסוד. המשמעות היא שכאשר בוחנים מסה אטומית, נהוג להתעלם מהמסה של אלקטרונים כלשהם ולחשב את מסת האטום על סמך מספר הפרוטונים והנייטרונים בלבד.

עם זאת, מכיוון שרוב היסודות המופיעים באופן טבעי מורכבים מאיזוטופים, חשוב לקחת בחשבון את המסה והשפע הטבעי של איזוטופים בעת קביעת המסה האטומית של יסודות כאלה. המסה האטומית במקרים כאלה מחושבת על ידי סיכום המסות של כל האיזוטופים של היסודות, כל אחד מוכפל בשפע השבר הטבעי שלהם.

כימאים משתמשים לעתים קרובות ביחידה מולה כדי לקבוע את מספר האטומים של תרכובת המשתתפים בתגובה כימית. שומה של יסוד היא המשקל האטומי שלו בגרמים, בעוד שמול של תרכובת היא סכום המשקל האטומי של מרכיביה, הנקרא משקל מולקולרי. דוגמה המשמשת לעתים קרובות היא חישוב שומה של גלוקוז עם הנוסחה הכימית C_6H_12O_6. המשקל האטומי של פחמן (C) הוא 12.011 גרם, ויש שישה פחמנים בגלוקוז למשקל אטומי כולל של 72.066 גרם. אם עושים את אותם חישובים עבור מימן (H) וחמצן (O), המשקל המולקולרי של גלוקוז שווה ל-180.156 גרם.

טקסט זה מותאם חלקית מ <a href="https://openstax.org/books/biology-ap-courses/pages/2-1-atoms-isotopes-ions-and-molecules-the-building-blocks?query=atomic%20mass&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-idp140338797859808">Openstax, Biology for AP® courses. Section 2.1 Atoms, Isotopes, Ions, and Molecules: The Building Blocks</a> and <a href="https://openstax.org/books/anatomy-and-physiology-2e/pages/2-4-inorganic-compounds-essential-to-human-functioning?query=mole&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id1218107">Openstax, Anatomy and Physiology 2e, Section 2.4 Inorganic compounds Essential to Human Functioning</a>
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לפרוטונים ולנייטרונים יש בערך אותה מסה, בערך 1.67 × 10-24 גרם. מדענים מגדירים באופן שרירותי כמות זו של מסה כמסה אטומית אחת ...

Education

JoVE Core

Anatomy and Physiology

אנטומיה ופיזיולוגיה

Unit 1

יחידה 1

Fundamentals of Chemistry

יסודות הכימיה

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

Atomic Weight

atomic mass and mole calculations

Atomic mass and mole calculations help explain how much matter is in atoms and compounds. Protons and neutrons have about the same mass, about 1.67 × 10 -24 grams. Scientists define that mass as one atomic mass unit, or amu, also called one Dalton.

Electrons are much lighter than protons. They weigh only 9.11 × 10 -28 grams, or about 1/1800 of an atomic mass unit. Because of that small mass, electrons do not add much to an element’s overall atomic mass. When chemists calculate atomic mass, they usually ignore electrons and use only the protons and neutrons.

Many naturally occurring elements are made of isotopes, which are atoms of the same element with different masses. For these elements, atomic mass must include each isotope’s mass and its natural fractional abundance. Chemists find this by adding the masses of all the isotopes after multiplying each one by how common it is.

Chemists also use the mole to count atoms and compounds in reactions. A mole of an element is its atomic weight in grams. A mole of a compound is the sum of the atomic weights of its parts, and that total is called the molecular weight. Glucose, C6H12O6, is a common example. Carbon has an atomic weight of 12.011 grams, and six carbons in glucose give 72.066 grams. Doing the same for hydrogen and oxygen gives glucose a molecular weight of 180.156 grams.

Atomic Mass and Mole Calculations

what is matter?

Matter, Mass, and Weight Basics

Matter is the substance that makes up the universe, from a grain of sand to a star. Scientists define matter as anything that occupies space and has mass. Mass is the amount of matter in an object, while weight depends on gravity. Mass stays the same whether an object is on Earth or in the zero-gravity environment of outer space. Weight changes when gravity changes. An object weighs more where gravity is stronger and less where gravity is weaker. A piece of cheese that weighs a pound on Earth...

the periodic table and organismal elements

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Organismal Elements in the Periodic Table

The periodic table helps identify the elements that make up living organisms. Biology groups these as organismal elements because they are the chemical building blocks of life. These elements are found in different amounts in the body. Some are present in large quantities, while others are needed only in small amounts. The video connects the periodic table to the elements that organisms actually use. Understanding organismal elements helps students link chemistry to biology. It shows how...

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From Atomos to the Modern Atom

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chemical symbols

Chemical Symbols and Element Names

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Atomic Identity, Mass, and Isotopes

Atomic identity depends on the number of protons in the nucleus, and mass depends on protons and neutrons together. The number of protons is the atomic number, or Z. It defines the element. For example, any atom with six protons is carbon, no matter how many neutrons or electrons it has. In a neutral atom, the number of protons equals the number of electrons. Because of that, the atomic number also tells you the number of electrons in the atom. The total number of protons and neutrons is...

atomic weight

Atomic mass and mole calculations help explain how much matter is in atoms and compounds. Protons and neutrons have about the same mass, about 1.67 × 10 -24 grams. Scientists define that mass as one atomic mass unit, or amu, also called one Dalton. Electrons are much lighter than protons. They weigh only 9.11 × 10 -28 grams, or about 1/1800 of an atomic mass unit. Because of that small mass, electrons do not add much to an element’s overall atomic mass. When chemists calculate atomic mass,...

isotopes and radioisotopes

Radioisotopes in Medicine and Imaging

Radioisotopes are unstable isotopes that can be used to study the body and treat disease. An isotope is an atom of the same element that has a different mass because it contains a different number of neutrons. It still has the same number of protons, so it remains chemically identical. English chemist Frederick Soddy explained this idea in the early 1900s and later received the Nobel Prize in Chemistry in 1921. Some isotopes have more neutrons than usual and are called heavy isotopes. These...

electron behavior

Electron Shells and Discovery

Electrons are negatively charged subatomic particles that stay near the positively charged nucleus of an atom. They occupy spaces linked to energy levels called shells. They are also arranged into subshells and orbitals within each shell. An electron’s shell shows its general energy level. Electrons closer to the nucleus have less energy, while electrons farther away have more energy. The outer shells have more space, so they can hold more electrons than the inner shells. Subshells describe...

introduction to chemical bonds

How Atoms Bond to Reach Stability

Atoms bond to become more stable, and their outermost electrons play the biggest role in that process. The inner electron shell can hold two electrons. The next two shells can each hold eight electrons. This is called the octet rule. It states that, except for the innermost shell, atoms are most stable when their valence shell, or outer shell, has eight electrons. When atoms do not have enough electrons to fill that outer shell, they form chemical bonds with other atoms. A chemical bond is the...

ionic bonds

Ionic Bonds and Charge Transfer

Ionic bonds form when atoms transfer electrons and become charged particles. A positive ion and a negative ion attract each other because opposite charges pull together. This type of bonding helps explain how some substances are held together. In many cases, an atom loses one or more electrons, while another atom gains them. The atom that loses electrons becomes a cation, which is a positively charged ion. The atom that gains electrons becomes an anion, which is a negatively charged ion. The...

covalent bonds

Polar and Nonpolar Covalent Bonds

Covalent bonds form when atoms share electrons to fill their valence shells. The way those shared electrons are pulled depends on electronegativity, which is an atom's tendency to attract electrons in a bond. That difference also helps explain why some molecules are polar and others are nonpolar. Atoms with similar electronegativities form nonpolar covalent bonds. In these bonds, electrons are shared equally, so there is no charge across the molecule. When two atoms have different...

hydrogen bonds

Hydrogen Bonding and DNA Stability

Hydrogen bonding helps explain why some molecules stick together more strongly than others. A hydrogen bond forms when a weakly positive hydrogen atom, already bonded to an electronegative atom, is attracted to another electronegative atom on a nearby polar molecule. Common examples include water, hydrogen fluoride, and ammonia. This attraction happens because hydrogen has a much lower electronegativity than atoms such as fluorine, oxygen, and nitrogen. The electronegativity difference, along...

ions, molecules, and compounds

Atomic Charges, Bonds, and Compounds

Atoms can gain or lose electrons during chemical reactions and become ions. An ion is an atom with an electrical charge. This charge is positive or negative, and it often helps the atom reach a full valence shell. Some atoms are more likely to donate electrons, while others are more likely to accept them. Potassium is one example. It has atomic number 19 and one electron in its valence shell, so it is likely to donate that electron and become a slightly positive ion called a cation. Fluorine...

states of matter

Solids, Liquids, Gases, and Plasma

Solids, liquids, gases, and plasma are the main forms of matter described here. Solids are rigid and keep a definite shape. Liquids flow and take the shape of their container, although gravity gives the surface a flat or slightly curved top. Solids and liquids also have volumes that stay nearly the same even when pressure changes. Gases behave differently from solids and liquids. A gas takes both the shape and the volume of its container. In matter with this behavior, the particles are far...

colloids and suspensions

Suspensions, Sedimentation, and Colloids

Suspensions and colloids are two common kinds of mixtures seen in everyday life. Mud and water, flour and water, and tempera paint are suspensions. These mixtures are heterogeneous, which means their particles are not spread out evenly. The particles are relatively large and can be seen with the naked eye or a magnifying glass. Suspension particles make mixtures cloudy, and they settle out after mixing. The settling of particles in a suspension is called sedimentation. A blood test can measure...

introduction to chemical reactions

Chemical Reaction Basics and Equilibrium

Chemical reactions start with reactants and end with products. Reactants are the substances that enter a reaction, and products are the new substances that form. For example, sodium and chloride ions are the reactants in the formation of table salt. Chemical reactions follow the law of conservation of mass. Matter is not created or destroyed during the reaction. That means the atoms and elements in the reactants are still present in the products, and nothing new appears in the products that...

types of chemical reactions: anabolic and catabolic

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Anabolism, Catabolism, and ATP Use

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types of chemical reactions: exchange and reversible

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Electron Transfer and Chemical Equilibrium

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Chemical Energy and Reaction Types

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factors influencing the rate of chemical reactions

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Chemical Reaction Rates and Catalysts

Chemical reaction rates depend on how often particles collide and whether those collisions have enough energy to start a reaction. That starting energy is called activation energy. Factors that affect reaction rate either lower that energy or make successful collisions more likely. Concentration and pressure can speed reactions up because they put more particles in the same space. When more particles are present, they are more likely to bump into one another. Chemists can also increase...

acids, bases and neutralization reactions

How pH Affects Cells and Digestion

Acids, bases, and pH help control how cells and body systems work. In biology, pH can strongly affect enzymes, proteins, and nucleic acids. Enzymes work best within a certain pH range, and pH changes can alter their structure or denature them, which changes how they function. Acids release hydrogen ions, or H+, in solution. A hydrogen ion is a proton, so it is very reactive in chemical reactions. Strong acids release all of their H+ in water and ionize completely. Hydrochloric acid (HCl) is a...

key terms and concepts

Learn how protons, neutrons, isotopes, and the mole are used to calculate atomic mass and molecular weight with a glucose example for high school chemistry.

Atomic Mass and Mole Calculations | JoVE K12

anatomy and physiology

JoVE Core Anatomy and Physiology showcases the anatomical and physiological details of the human body in comprehensive, clear, and engaging video animations. This collection of lessons focuses on the structures, components, functions, and requirements of the human body. It is complemented by scientist-in-action videos that demonstrate the application of this knowledge in real-world medical and research settings.

Atomic Mass and Mole Calculations

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