Posts Tagged ‘electron configuration’
This is a reading aid (cheat sheet!) for Chapter 4: Arrangement of Eelectrons in Atoms from Modern Chemistry by Holt, Rinehart, and Winston, a 10th grade Honors Chemistry text book in California.
This reading aid could be useful for any student. It can also be used by parents and teachers looking for alternative ways of explaining atomic model and basic quantum mechanics.
This chapter and preceding chapters tries to explain concepts that Niels Bohr, Pauli, Heisenberg, and others contributed to understanding of atoms in the early 1900s.
When we were in school we didn’t have so many distractions and activities. We would read , re-read as many times as needed, including coming up with analogies, until we can memorize the concepts. Not so with iTunes / Facebook generation kids. They are too busy with their iPhone or with their social network or with after school sports and activities. They don’t have time to read at least once let alone re-read. They are likely to think and talk about Bohr, Pauli and others mentioned in the chapter:
Oh, these dudes of a century ago probably didn’t have anything to do. They invented some stuff. Must have been geeks, crazy, retards. …
Fun aside; kids are likely to find it challenging to read the chapter as the concepts discussed are abstract. Atoms and its tenants – nucleus, protons and electrons are invisible to all of us and talking about them to kids is obviously going to be a challenge.
So, in this blog an unconventional approach will be used to teach the following concepts:
- Bohr’s atomic model
- Heisenberg uncertainty principle
- Hund’s rule
- Aufbau’s principle
- Pauli ‘s Exclusion Principle
- Schrödinger Equation
There is a quiz at the end to help prepare students for their class room unit test.
The unconventional approach uses analogies from our real life that the kids can easily relate to. One could even use characters and concepts from popular movies/shows that kids are familiar with such as Wizard of Oz, Harry Potter . You will see one example where Tempur-Pedic mattress is used in explaining Hund’s rule.
Use of analogies and stories is not necessarily to make kids understand atomic model. It is too complex for their age and even for grown ups. Instead, the objective is to help them memorize the concepts more easily.
First, let us start with recap of some basic concepts that students may have come to memorize already about atom:
Atom and its Tenants
- Atom is the basic building block of matter (like Lego blocks is used to build car, truck, home, etc)
- Each atom contains three types of tenants: protons, neutrons and electrons
- Center of the atom is called nucleus (analogy: biological cell with nucleus)
- Protons and neutrons huddle inside the nucleus
- The number of electrons in an atom is equal to the its atomic number(e.g, Helium has two electrons since its atomic number is two)
Unlike protons and neutrons, which stay inside nucleus as a group of well behaving kids, electrons don’t stick to one place. They are like toddlers. They keep running around inside the atom in spaces called orbitals (also called shells).
Since electron, like a toddler, keeps jumping and moving around, we can’t say, with 100% certainty, where it is or how fast it is moving at any given time. That is what Heisenberg (1901-1976), a German scientist, said almost a century ago: it is difficult to know at the same time where the toddler is and what the toddler is up to because the toddler is unpredictable! The toddler (electron) could be in one place with probability of 1/3, and, at the same time, in another place with probably of ½ and so on. I am sure your mother knew this when you were toddler and you would soon when you have your own kids.
Thus we have the famous Heisenberg’s uncertainty principle for electrons (the toddlers)
it is impossible to know both the exact position and the exact velocity of an electron at the same time
Nobel Prize for ‘No’ answer
Question posed to Heisenberg and team: Can you determine position and speed of electron at the same time?
Their answer: No
Their reward for answering: Nobel Prize in 1932!
So the next time your teacher asks you a science or math question and your response is going to be ‘Don’t know’ then know that you could be in line for Nobel prize!
Joke apart, it is very important to learn to say ‘No’, and how to say it. Saying ‘No’ or ‘Dont know’ requires courage and, more importantly, knowing what you don’t know. Heisenberg and team didn’t just say ‘Don’t know’. They produced scientific explanation of why they Can’t simultaneously determine position and momentum of the quantum particles such as electrons.
Electron’s house and sleeping rules
Like toddlers, electrons wander around inside their home – the atom. But how is their home (atom) organized?
To understand this, imagine a mystery house with multiple floors, rooms and beds. Each bed is laid with head side on north and foot side facing south. Also, each bed can sleep at most two – one on east side and one west side. The occupants of the house are ones with varying level of mystical powers and spells. Some can only stay on the ground floor. Some have enough power to just get to the first level. Some can get upto 2nd floor. Some can go even higher because they have extra spell powers to take them that high. To help easily remember the energry level of electrons, picture characters fromThe Wizard of Oz , Harry Potter and others that exhibit different types of spell powers with varying intensity.
To avoid pillow or fist fights or spell wars, for getting the popular rooms and beds, we will have a simple rule : ones that are less able (i.e with less energy / spell power) are allowed to take lower level floors. Next, smaller rooms shall be taken first before bigger rooms are taken as smaller rooms requiredless power to take. Neils Bohr and Pauli used the term ‘Aufbau’, a german version of this word roughly means building/construction principle. The rule is now commonly called ‘Aufbau’s rule:
electrons fill orbitals starting at the lowest available (possible) energy states before filling higher states (e.g. 1s before 2s)
Do you really need Scientists like Bohr and Pauli to tell us how to pick bed rooms in the house? Your parents would have told you that the bed room on the ground is for first two weakest (typically young babies or grand parents or sick ones) and is off-limits for more able members. So to memorize Aufbau’s rule, remember rules for taking floors and bed rooms in the mystery house based on the energy /ability of the individuals involved.
Let us now tabulate the rules for each floor:
Summarizing the order of taking bed rooms (equivalent to order of taking the shells by electrons):
3s 3p 4s 3d
4p 5s 4d
5p 6s 4f 5d
6p 7s 5f 6d
7p 6f 7d 7f
The rule for skipping of 3d and others might be hard to remember. In that case use couple of additional analogies listed in the notes section to help memorize easily.
Long time ago, someone named Hund went into a hotel room with some friends. He noticed that the room had more one bed laid from left to right with entry door being close to the left most bed. To take the right most bed one need to walk from entry door all the way to that bed. So needs more energy to reach the beds farther from the left.
In Hund’s time there were no memory foam or Tempur-Pedic beds. So naturally, he surmised that if two sleep on the same bed then each one when they move in the sleep is going to disturb the other. So he felt that they need to spend extra effort /energy to get some sleep.
Given the two observations from the hotel bed room, Hund laid out the following rule for bed occupancy so that least energy/effort is spent in getting a bed to sleep well:
First person entering the room takes the left most bed. Next person entering takes the next bed and son on until all the beds are occupied by one person each. After that, the next person entering the room pairs up with the one sleeping on the left most bed and so on. See below:
Obviously, we don’t need scientists like Hund to tell us rules on how to pick a bed in a room with multiple beds as it is common sense and we can figure that ourseleves easily. But, electrons, as mystical as they are with varying levels of spell power, do need Hund’s rule to help pick orbitals , the space where they live and move around. Here is what Hund said in scientific terms with respect to sub shell occupancy rules for electron:
When there are multiple sub shells (read beds) in a level (read bed room. e.g 2p), electrons take individual orbitals before they are paired
Take a short Quiz to help you memorize concepts from this chapter. You will get to see your score immediately. You can take the quiz as many times as needed.
Story style explanation of emission spectrum and Schroedinger’s equation (the most complex one) will be in a separate post. They are part of the atmoic model chapter. But worth explaining separately.
- Couple of analogies to help memorize why fill room 4s on 4th floor first while room 3d is vacant on 3rd floor:
- Imagine an expensive hotel with beatiful master suite on 3rd floor. The price to take a bed in the master suite on 3rd floor is way more expensive than taking a bed on the 4th floor. So obviously, electrons prefer 4s compared to 3d as they have to spend less money (energy) to take 4s.
- In monopoly game, you may have to skip a rail road or utility to take the next property available since you don’t have enough money.