The Aufbau principle | Atomic structure and properties | AP Chemistry | Khan Academy

Summary

This chemistry lesson focuses on electron configurations, particularly utilizing the Aufbau principle to understand configurations beyond calcium. The instructor explains how to determine electron configurations for elements like neon, argon, and calcium, highlighting the order in which subshells are filled. The video also clarifies misconceptions about the order of filling subshells, especially for elements like scandium. Additionally, it introduces the concept of blocks in the periodic table, categorizing elements into S, P, and D blocks based on the subshells they are filling.

Takeaways

• 🌟 Orbitals and subshells are fundamental to understanding electron configurations in atoms.
• 🔍 The Aufbau principle, derived from the German word for 'building,' is crucial for electron configurations beyond calcium.
• 💡 Neon's electron configuration is 1s²2s²2p⁶, illustrating the filling order of subshells.
• 📚 The periodic table reflects the order of subshell filling, with elements in each period corresponding to a specific shell.
• 🎓 Argon's electron configuration uses noble gas notation, building on neon's configuration with additional electrons in the 3s and 3p subshells.
• 🔧 Calcium's electron configuration (4s²) deviates from filling the 3d subshell first, demonstrating a key exception to the Aufbau principle.
• 🌌 Scandium's electron configuration (4s²3d¹) shows the start of the 3d subshell filling after the 4s subshell, according to the Aufbau principle.
• 🧠 The Aufbau principle is a useful tool for visualizing electron configurations, but it may not always reflect the actual order of electron addition in reality.
• 📊 The periodic table is divided into blocks (S, P, D) based on the subshells being filled, which helps categorize elements and predict their properties.
• 🔬 The D block in the periodic table, including elements like scandium, is named for the perceived filling of d subshells, even though the actual electron filling may differ.

Q & A

What is the origin of the term 'Aufbau principle'?

The term 'Aufbau principle' comes from German and translates to 'building principle' in English.

What is the electron configuration of neon?

The electron configuration of neon is 1s2 2s2 2p6, which corresponds to its atomic number of 10.

How does the Aufbau principle assist in determining electron configurations past calcium?

The Aufbau principle is a method used to predict the order in which atomic subshells are filled with electrons, which is particularly useful for electron configurations beyond calcium.

What is the significance of the order in which subshells are filled as per the periodic table?

The order in which subshells are filled, such as 1s, 2s, 2p, 3s, 3p, etc., corresponds to the order in which elements are arranged in the periodic table.

What is the electron configuration of argon, and how does it relate to neon?

Argon's electron configuration is the same as neon's, which is 1s2 2s2 2p6, but with the addition of two electrons in the 3s subshell and six electrons in the 3p subshell, making it 3s2 3p6.

Why does calcium have its electrons in the 4s subshell instead of the 3d subshell when following the Aufbau principle?

According to the Aufbau principle, calcium's electron configuration is the same as argon's, but with two additional electrons in the 4s subshell instead of the 3d subshell, resulting in the configuration ending in 4s2.

How does the electron configuration of scandium differ from that of calcium?

Scandium, having one more proton than calcium, has an electron configuration that includes two electrons in the 4s subshell (4s2) and one electron in the 3d subshell (3d1), following the Aufbau principle.

What is the significance of the S, P, and D blocks in the periodic table?

The S, P, and D blocks in the periodic table correspond to the subshells that are being filled as you move across the table. The S block includes elements filling their s subshells, the P block includes elements filling their p subshells, and the D block includes elements that appear to be filling their d subshells based on electron configurations, although the actual filling order can be more complex.

Why is the D block named as such, and what does it represent?

The D block is named so because it was historically thought that the elements in this block were filling their d subshells as you move across the periodic table. However, the actual electron configurations may not always follow this pattern, especially beyond calcium.

How does the Aufbau principle simplify the understanding of electron configurations in chemistry?

The Aufbau principle simplifies the understanding of electron configurations by providing a systematic way to predict the order in which subshells are filled, which is crucial for determining the electron configuration of elements in the periodic table.

Outlines

00:00

🔬 Electron Configuration and the Aufbau Principle

This paragraph introduces the concept of electron configuration and the Aufbau principle, which is a method for determining the order in which atomic orbitals are filled with electrons. The instructor uses neon and argon as examples to demonstrate how the Aufbau principle works, explaining how electrons fill subshells in order of increasing energy levels. The instructor also highlights the electron configuration of calcium, which is an exception to the Aufbau principle, as its electrons fill the 4s subshell before the 3d subshell. This sets the stage for a deeper understanding of electron configurations in more complex atoms.

05:02

📚 Electron Configuration Beyond Calcium

The second paragraph delves into the complexities of electron configuration beyond calcium, using scandium as an example. It explains how the Aufbau principle can be applied to predict electron configurations, but also notes that the actual process of electron addition can differ from this simplified model. The paragraph also discusses the blocks of the periodic table, explaining how elements in the S, P, and D blocks are associated with the filling of s, p, and d subshells, respectively. The instructor emphasizes the importance of understanding these patterns for predicting electron configurations and categorizing elements within the periodic table.

Keywords

💡Orbitals

Orbitals are regions around the nucleus of an atom where electrons are most likely to be found. They are characterized by their shape and energy levels. In the video, orbitals are introduced as the basis for understanding electron configurations in different shells and subshells of an atom.

💡Subshells

Subshells are subdivisions of an electron shell and are denoted by the letters s, p, d, and f. Each subshell can hold a specific number of electrons and has a distinct energy level. The video discusses how electrons fill these subshells in a systematic way, starting with 1s, then 2s, and so on.

💡Aufbau Principle

The Aufbau Principle, derived from the German word 'Aufbau' meaning 'building up,' is a principle in chemistry that describes the step-by-step filling of atomic orbitals with electrons. The video uses this principle to explain the electron configuration of elements beyond calcium, emphasizing its utility in predicting the order in which subshells are filled.

💡Electron Configuration

Electron configuration refers to the distribution of electrons in an atom's orbitals. The video script uses the term to describe the process of filling orbitals with electrons according to the Aufbau principle, providing examples like the electron configuration of neon (1s² 2s² 2p⁶) and argon (1s² 2s² 2p⁶ 3s² 3p⁶).

💡Periodic Table

The periodic table is a tabular arrangement of chemical elements by atomic number, electron configurations, and recurring chemical properties. In the video, the periodic table is used as a reference to demonstrate how electron configurations correspond to the filling of shells and subshells in elements.

💡Noble Gas Configuration

Noble gas configuration refers to the electron configuration of a noble gas, which is often used as a reference point for building the electron configurations of other elements. The video mentions using noble gas configurations to simplify the process of determining the electron configuration of elements like argon.

💡S Block

The S block in the periodic table consists of elements that are filling their s subshells. The video explains that elements in the first two columns of the periodic table are part of the S block, including helium, which fills the 1s subshell.

💡P Block

The P block in the periodic table includes elements that are filling their p subshells. The video script illustrates this by discussing how elements in a specific region of the periodic table are associated with the P block due to their electron configurations.

💡D Block

The D block elements are those in the middle of the periodic table, which are associated with the filling of d subshells. The video script explains the historical naming of this block, even though the actual filling order of electrons may not strictly follow the d subshells in every case.

💡Scandium

Scandium is a chemical element with the atomic number 21, and its electron configuration is used in the video as an example to demonstrate the application of the Aufbau principle. The video explains that scandium's electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹, highlighting the principle's utility in predicting electron distribution.

Highlights

Introduction to orbitals and their various subshells in an atom.

Explanation of the Aufbau principle and its relevance in electron configuration.

Electron configuration of neon: 1s2, 2s2, 2p6.

Use of noble gas notation to simplify electron configuration for elements like argon.

Electron configuration of argon: [Ne] 3s2, 3p6.

Electron configuration of calcium: [Ar] 4s2, highlighting the Aufbau principle's prediction.

Surprising fact that the 4s subshell is filled before the 3d subshell for potassium and calcium.

Electron configuration of scandium: [Ar] 4s2, 3d1, demonstrating the filling of the 3d subshell after the 4s.

Clarification that, although 4s is taught to be filled before 3d, in practice, the first added electron may go to 3d.

Introduction to the periodic table's 'S block' for elements filling their S subshell.

Helium as part of the S block, despite its placement, because it fills the 1s subshell.

Identification of the 'P block,' where elements fill or complete their P subshells.

D block elements like scandium are identified by filling their D subshells.

Explanation of how the Aufbau principle leads to the classification of elements into S, P, and D blocks.

Recognition that the electron filling order becomes more complex beyond calcium, challenging the basic Aufbau model.

Transcripts

- [Instructor] In other videos we introduced ourselves

to the idea of orbitals

and these are various orbitals in their various subshells

that you could find in various shells of an atom.

And in this video we're gonna get a little bit more practice

with electron configuration.

In particular, we're going to expose ourselves

to the idea of the Aufbau principle.

Now Aufbau comes from German.

It means the building principle.

It's a very useful way

of thinking about electron configurations past calcium.

Let's just a get little bit warmed up.

What is the electron configuration of neon?

Pause this video and think about it

and as a hint I will give you the periodic table here.

All right, well neon has an atomic number of 10

and if we're talking about a neutral neon atom

it's gonna have 10 electrons.

And so, the first two will that fill that first shell.

So we have 1s2 and then, the next two are going to fill

the 2s subshell in your second shell.

So then you're gonna have 2s2.

And then we have six more electrons to get to 10

and that's now going to fill your 2p subshell, so 2p6.

And so what's the order of the subshells

that we just filled?

Well, first we filled 1s, then we filled 2s,

then we filled 2p

and you can also see that in the periodic table of elements.

In this first row, you're filling that first shell.

In this second row or this second period,

you are filling that second shell.

Now what's going to happen if we were to go to say, argon?

So if you're going to go to argon,

what will that electron configuration look like?

Pause the video and think about that.

Well, we can use the noble gas configuration

or the noble gas notation.

We could say, all right we're going to be

building off of neon.

So we're gonna have the electron configuration of neon,

but then we're going to add electrons into our third shell.

So from neon we would then add two electrons

into the 3s subshell, 3s2.

And then, to get to 18 electrons,

we're at 12 right now,

we're gonna have six more that are going to be

in the 3p subshell, so 3p6.

So, on this diagram over here

we went from 2s to 2p to fill up neon

and then as we went to argon

we go to 3s to 3p.

Now what would be the electron configuration of calcium?

Pause the video and think about it.

All right, well calcium has 20 protons.

So a neutral calcium would have 20 electrons.

So two more electrons than argon.

So we can build off of argon

and where are those electrons going to go?

And this is where the Aufbau principle is interesting.

There is indeed a 3d subshell,

but in the case of calcium

instead of those two electrons being in the 3d subshell,

they end up in the 4s subshell.

So calcium's electron configuration is the same as argon

and instead of it being 3d2 here on top of that

it goes straight to 4s2.

And so that's why I was drawing this diagram like this

and you'll often see that

in an introductory chemistry class.

You fill 1s first, no surprises.

You're filling in that first shell.

Then you fill 2s.

Then you fill 2p and you filled your second shell.

Then you go to 3s, once again no surprises.

Then you go to 3p.

Now this is the surprise

and why this Aufbau diagram is useful.

For electron configuration purposes,

if you're thinking about potassium or calcium,

the extra electrons are now going to go in the 4s subshell.

So now let's think about what the electron configuration

of the scandium would be.

Pause this video and think about that.

Well scandium has one more proton than calcium.

It has 21 protons and if it is neutral,

it's also gonna have one more electron

relative to a neutral calcium atom.

And so, it could have a similar electron configuration.

So we could base it off of argon.

We have two electrons in the 4s subshell,

so I'll write 4s2

and the Aufbau principle would describe that

and the Aufbau principle, this little diagram,

would say, all right that other electron is going to be

in the 3d subshell, so you do 3d1.

And this is indeed an accurate electron configuration

for scandium.

Now if the Aufbau principle makes you think

that you're filling 4s first

and then you are starting to fill 3d,

if you were actually building up a scandium atom,

and that's actually taught in most chemistry books

and in most classes,

but actually if people start with the scandium nucleus

that has 18 electrons.

So that would have a positive charge,

when they add that first electron

it actually does not go to 4s.

It goes to 3d.

So this electron actually gets added first.

If you're actually thinking about building.

But I don't want to confuse you too much.

In this video we're just thinking about

the electron configuration

and for that Aufbau can be very useful.

Now for electron configuration purposes,

3d, you then go to 4p, and then you then go to 5s,

and that's why you might see this type of a diagram,

once again, in your traditional first year chemistry books.

So the big takeaway here is the Aufbau principle

that you'll learn, this type of diagram,

it's useful for electron configuration

and it might be useful to think about it

as you're building these atoms electron by electron,

but if you really want the precise accurate truth

once you get beyond calcium

it gets a little bit more complicated.

Now one other thing that I want you to appreciate

based on what we just learned is patterns

in the periodic table of elements.

So for which elements are we building out our S subshell?

Well, you could see that for all of these elements

right over here,

these first two columns,

we're building out our S subshell.

Now it looks like something is missing there.

Is there something else that builds out our S subshell?

Well, from that point of view we could actually think

of helium as being right over here

'cause helium, we're building out that 1s subshell

and because of that all of these elements right over here,

we say that they are in the S block.

Now, which elements are building out their P subshells?

Well, all of these elements right over here

are building out their P subshells

or have it fully built out.

And because of that, all of these elements,

we call these the P block.

And these elements in the middle right over here,

scandium is one of them,

they are called the D block.

Now one reason why folks might have called it the D block

is if you really imagine the Aufbau principle

as building up atoms,

it might be tempting to say,

oh well we're building in the fourth row here,

we're building the 3d subshell

or in the fifth row here we're building the 4d subshell.

Now we now know that that actually isn't true,

but from electron configuration point of view

it can appear that way

and so that's why it is called the D block

and I will leave you there.