Average atomic mass | Atoms, isotopes, and ions | AP Chemistry | Khan Academy

Summary

Highlights

• Understand the role of atomic mass units in measuring extremely small masses. 🎯
• Discover how protons, neutrons, and electrons contribute to atomic mass. ⚖️
• Learn about isotopes and their influence on atomic mass through weighted averages. 🧬
• Grasp why periodic tables display average atomic mass and not individual isotope masses. 📚
• Find out the importance of relative atomic mass in understanding chemical compositions. 🧠

Key Takeaways

• Atomic concepts bring the minute world of chemistry into perspective! 🧪
• Atoms are primarily weighed using atomic mass units (amu or u). ⚖️
• Protons and neutrons take the mass cake in atoms, not electrons! 🍰
• Isotopes introduce fun versions of elements with different neutron numbers. 🚀
• Periodic tables showcase average atomic masses, a neat weighted average trick! 📊

Overview

Chapters

• 00:00 - 00:30: Introduction to Chemistry and Atomic Scales The chapter introduces chemistry as a field of science that helps us understand processes occurring at extremely small scales, specifically the atomic and subatomic levels. By examining these scales, we can gain insights into the universe and the world we live in, and even make predictions about them.
• 00:30 - 01:00: Units of Measurement at Atomic Scale The chapter 'Units of Measurement at Atomic Scale' discusses the necessity of developing specific units of measurement for operating at atomic scales to create meaningful tools for human use. It emphasizes the challenge of measuring mass at such minute levels and introduces the historical solution adopted by the chemistry community.
• 01:00 - 02:00: Understanding Atomic Mass Unit (AMU) The chapter introduces the concept of an Atomic Mass Unit (AMU), explaining its historical notation as 'amu' and its modern version as the Unified Atomic Mass Unit denoted by 'u'. It further explores the connection between these units and the larger mass units like grams and kilograms that are commonly used.
• 02:00 - 03:00: Definition and Scale of Unified Atomic Mass Unit The unified atomic mass unit (u) is defined as 1.660540 x 10^-27 kilograms. The chapter discusses reactions to this small size, emphasizing the scale and implications of using such a diminutive measurement unit in scientific contexts.
• 03:00 - 04:00: Mass of Subatomic Particles and Comparison The chapter explores the incredibly small mass of subatomic particles and attempts to quantify it using mathematical expressions. It discusses the difficulty of conceptualizing such small numbers, given their numerous zeros and tiny values, such as 0.000...01660540. The chapter also touches on the rationale behind defining these numbers in particular ways, specifically mentioning how certain definitions can simplify calculations and improve understandability.
• 04:00 - 05:00: Protons, Neutrons, and Atomic Mass The chapter discusses the mass of atomic particles, specifically protons and neutrons, in terms of unified atomic mass units. It highlights that both protons and neutrons have a mass of approximately one atomic mass unit, although the proton is slightly more, at about 1.007 atomic mass units.
• 05:00 - 06:00: Periodic Table: Atomic Number and Element Definition The chapter titled 'Periodic Table: Atomic Number and Element Definition' discusses the basic components of an atom, specifically focusing on the mass of these components measured in atomic mass units (amu). It describes the mass of protons and neutrons as approximately 1 amu, with neutrons having a slightly greater mass than protons (1.008 amu). In contrast, the mass of an electron is significantly smaller, nearly 1/2000 of that of a proton or neutron. The chapter emphasizes that the mass of an atom is primarily determined by its protons and neutrons.
• 06:00 - 07:00: Understanding Isotopes and Hydrogen Isotopes This chapter provides an overview of isotopes, focusing specifically on hydrogen isotopes. It starts by explaining the basic concept of isotopes, which are atoms of the same element with different numbers of neutrons. Thus, understanding the number of protons and neutrons in an atom's nucleus is crucial for determining its atomic mass. The chapter also touches on how this information is represented in the periodic table of elements using the example of hydrogen, where 'H' is the chemical symbol and the atomic number is displayed prominently at the top of the element's box. Further exploration of the periodic table is mentioned as a topic for future discussion.
• 07:00 - 08:00: Calculating Average Atomic Mass This chapter introduces the concept of atomic number and its significance in identifying elements. It explains that the number of protons in an atom's nucleus defines the element. Examples include hydrogen, with one proton, calcium, with 20 protons, and krypton, with 36 protons. The chapter sets the stage for discussing atomic mass by presenting these fundamental principles of element identification.
• 08:00 - 09:00: Understanding Atomic Weight vs. Atomic Mass This chapter explains the concept of atomic weight versus atomic mass, using hydrogen as an example. It illustrates that while all hydrogen atoms have one proton, they can have different numbers of neutrons. The most common form of hydrogen, which makes up approximately 99.98% of hydrogen in the universe, has no neutrons, but there are other forms with one or two neutrons.
• 09:00 - 10:00: Relative Atomic Mass and Its Significance The chapter discusses the concept of isotopes, particularly in hydrogen. It explains that while isotopes of an element have the same number of protons, they differ in the number of neutrons. The common form of hydrogen is described, having one proton, zero neutrons, and one electron if it's neutral. The chapter likely elaborates on how this affects its atomic mass.

Average atomic mass | Atoms, isotopes, and ions | AP Chemistry | Khan Academy Transcription

• 00:00 - 00:30 the thing that i've always found amazing about chemistry it's an entire field of science that we as human beings have developed to actually understand what is happening at an almost unimaginably small scale in particular we're going to be thinking about the atomic and even the subatomic scale and by looking at that scale we can then begin to understand the universe in which we live in the scale in which we live in and even be able to make predictions about
• 00:30 - 01:00 what will happen and make things that are useful for for human beings so if we're going to operate at this small of a scale and we're going to appreciate in a few seconds how small of a scale it is we're going to have to have some units of measurement and this video is going to focus on mass how do we measure mass at such a small scale well to do that the chemistry community has historically used something called
• 01:00 - 01:30 an atomic mass unit i'll write it here atomic atomic mass unit and it's historically denoted as amu and more recently the more modern version of this is the unified atomic mass unit that is denoted by just a u instead of an amu so how does a unified atomic mass unit connect to our units of mass that we might use on a larger scale like say grams or kilograms
• 01:30 - 02:00 well the unified atomic mass unit is defined as 1.6605 four zero times ten to the negative twenty seven kilograms so when you see something like this you might have a few reactions your first reaction which would be an appropriate reaction is that wow 10 to the negative 27 power is very small to appreciate it
• 02:00 - 02:30 you could write it out it would be zero point and then 26 zeros and then you would have one six six zero five four zero so very very very small really unimaginably small we can only try to abstract it with things like mathematics the other thing you might appreciate is like this feels like a bit of a a hairy number here one point six zero why did they define it that way and the answer to your question is this definition makes it a lot cleaner when
• 02:30 - 03:00 we think about the mass of whether it's an atom or the constituents of an atom like a proton or a neutron roughly speaking the mass of a proton is approximately one unified atomic mass unit the mass of a neutron is approximately one unified atomic mass unit it actually turns out that a proton is a little bit more than one it's about 1.007
• 03:00 - 03:30 atomic mass units but it's approximately 1 and the neutron is actually a little bit more than even a proton it's 1.008 approximately unified atomic mass units now an electron's mass is actually far smaller than either of these it's actually almost 1 2 000 of a proton or neutron and so you can imagine an atom which is made up of protons and usually neutrons and electrons as well the mass is mainly going to be the protons and
• 03:30 - 04:00 neutrons in the nucleus and so if you know the number of protons and neutrons in the nucleus you're going to have a pretty good sense of its atomic mass and you can see that indicated on a periodic table of elements which we have here and we will study the periodic table of elements in a lot more detail in other videos but you can see a couple of interesting elements one you have the abbreviation of a given element h represents hydrogen the number on top on this periodic table that's the atomic number and that tells
• 04:00 - 04:30 you how many protons it has and an element is defined by the number of protons so any atom that has exactly one proton in its nucleus is going to be hydrogen by definition any atom that has exactly 20 protons in its nucleus is going to be calcium by definition any atom that has exactly 36 protons in its nucleus is going to be krypton by definition so what would you expect the mass of a
• 04:30 - 05:00 hydrogen atom to be pause this video and think about it well we know that all hydrogen atoms by definition have one proton but it actually turns out there's different versions of hydrogen that can have different numbers of neutrons most of the hydrogen in the universe actually has zero neutrons zero neutrons there are versions that have one or two neutrons but most 99.98 percent roughly of hydrogen in the
• 05:00 - 05:30 universe has one proton zero neutrons and if it's a neutral hydrogen it's going to have one electron and when we talk about versions of a given element there's a fancy word for it they're called isotopes and the different isotopes they'll all have the same number of protons because they're talking about the same element but they'll have different numbers of neutrons and so if this is the most common form of hydrogen what do you think its mass is going to be well its mass is going to be essentially
• 05:30 - 06:00 the mass of a proton plus an electron and roughly speaking it's going to be the mass of a proton because the mass of a proton is going to be so much larger than the mass of an electron and so you would expect that its mass is approximately one unified atomic mass unit now if you were to precisely look at the mass of a proton and a electron if you add them together you actually get something that's a little bit closer to 1.008 and you actually see that right over here on the periodic table of elements
• 06:00 - 06:30 now this number although it is pretty close to the mass of the version of hydrogen that i just described it's actually a weighted average of the various versions of hydrogen it's just close to this version because this version represents most of the hydrogen that we actually see around us if for example you had two versions of an element some hypothetical element and let's say that eighty percent of the
• 06:30 - 07:00 element that we see is version one and version one has a mass of let's call it five atomic mass units and then version two it's the remainder twenty percent of what we observe of that element it has an atomic mass of six atomic mass units you would get a weighted average here of 5.2 unified atomic mass units and that's actually how these numbers are calculated they are not just the mass of
• 07:00 - 07:30 one type of that element their weighted average mass of the various isotopes of the various types and so this number on a periodic table of elements is known as the average atomic mass average average atomic atomic mass now in older chemistry books and this is actually the case when i first learned chemistry they call this number atomic weight and i've always complained about it because it's really talking about
• 07:30 - 08:00 mass not weight if you don't know the difference you will learn that at some point in the future it's really talking about average atomic mass now i'll give you one little detail that might be useful to you sometimes you'll hear something called relative atomic mass it actually turns out this periodic table of elements because it does not write a little u after each of these numbers it's essentially these numbers are unitless so it's really talking about relative atomic mass so it's saying hey on average for example the mass of a
• 08:00 - 08:30 carbon atom is going to be roughly 12 times that of on average the mass of a hydrogen atom if they put the units here then that would actually truly be average atomic mass but for our purposes as we go into chemistry you can look at these numbers and say okay if oxygen has a relative atomic mass of 16 its average atomic mass is going to be 16 unified atomic mass units and as we will see in the
• 08:30 - 09:00 future this understanding of average atomic mass will prove to be very very useful