Sciblr - Blog Posts

Shapes of Molecules

his post is more information than trying to explain something - the truth is, you just need to learn shapes of molecules like you do with anything. I’ve got a physical chemistry mock tomorrow that I’m dreading since I’ve done zero revision. The fact that I run a study blog yet don’t revise myself is odd, but what else can I do? Oh, wait … revise. So here it is, my last minute revision for myself and you too - I present, shapes of molecules!

VSEPR stands for valence shell electron pair repulsion theory. If you’ve ever seen a moly-mod or a diagram of a molecule in 3D space, you may wonder how they decided it was that shape. Well, VSEPR answers all.

The theory essentially states that electron pairs are arranged to minimise repulsions between themselves - which makes sense, since electrons carry the same charge and therefore try to repel each other. Of course, there are different types of electron pairs, lone and bonding. The strongest repulsions happen between lone pair - lone pair followed by lone pair - bonding pair and finally, bonding pair - bonding pair have the least repulsion. 

Since the repulsion governs the shape of the molecule, to work out a molecule’s shape you must look at dot and cross diagrams or electron configurations to see how a molecule is bonded. There are many methods to do this, but the bottom line is that you must work out how many bonding pairs of electrons and how many lone pairs are involved.

The easiest shape to learn is linear. This has two bonding pairs and no lone pairs at an angle of 180 degrees, since that is the furthest the two can get away from each other. Examples of linear molecules include carbon dioxide and beryllium chloride.

Next up is trigonal planar. This has three bonding pairs and no lone pairs, each at the angle of 120 degrees. Trigonal means three and planar means on one plane, this should help you in identifying the molecules since after a fourth pair of electrons, the shape becomes 3D. Examples of trigonal planar molecules include boron trifluoride and sulfur trioxide.

What if you were to have two bonding pairs and two lone pairs? Well, then you’d have a bent molecule. Water is a good example of a bent molecule. Since it has two lone pairs that repel the other two bonding pairs more than they repel each other, the bond angle is 104.5. I’d be careful though, as in many textbooks it shows a bent molecule to have one lone pair and a different bond angle.

Another variation of the bent molecule I’ve seen is the one with two bonding pairs and one lone pair. It is deemed as bent with a bond angle of 109 or sometimes less than 120 degrees.

Tetrahedral molecules have four bonding pairs and no lone pairs. The bond angle is 109.5 degrees. Examples of this include the ammonium ion, methane and the phosphate ion. A good thing to note here is how these molecules are drawn. To demonstrate the 3D shape, where the molecule moves onto a plane, it is represented with a dashed line and triangular line along with a regular straight line. 

Trigonal pyramidal, sometimes just called pyramidal, is where there are three bonding pairs and a lone pair. Bond angles are roughly 107 degrees due to the repulsion from the lone pairs. An example of a trigonal pyramidal molecule is ammonia, which has a lone pair on the nitrogen.

Having five bonding pairs gives a trigonal bipyramidal structure. I guess the three bonding pairs on the trigonal plane accounts for that part of the name, where the rest comes from the position of the remaining two. These molecules have no lone pairs and have a bond angle of 90 degrees between the vertical elements and 120 degrees around the plane. Diagrams below are much clearer than my description! Examples of this include phosphorus pentachloride.

Six bonding pairs is an octahedral structure. I know this is confusing because octahedral should mean 8 but it’s one of those things we get over, like the fact sulfur isn’t spelt with a ph anymore. It’s actually to do with connecting the planes to form an octahedral shape.There are no lone pairs and each bond angle is a nice 90 degrees. Common examples include sulfur hexafluoride.

Square planar shapes occur when there are six bonding pairs and two lone pairs. All bond angles are 90 degrees! They take up this shape to minimise repulsions between electrons - examples include xenon tetrafluoride.

The final one to know is T-shape. This has three bonding pairs and two lone pairs. These molecules have bond angles of (less than) 90 degrees, usually a halogen trifluoride like chlorine trifluoride.

There are plenty more variations and things you could know about molecular geometry, but the truth is, there won’t be an extensive section on it. It’s a small part of a big topic!

I’m not going to do a summary today since I’d just be repeating the same information (I tried to keep it concise for you guys) so instead I’ll just leave you with, 

Happy studying!

physics feels so inaccessible.

like what do you mean the only information on this topic is a power point presentation from ten years ago with only half of the information on it?

or you tell me there's three ways to download the software i need for the calculations, but only one of the ways actually works and you don't even tell me how to do it!!!

never mind the sheer amount of prerequisites. i never struggled with math too much, but i also never took the opportunity to skip a level in math. when i was starting out, you can't do anything without trig. so then i went and learned trig on my own, but then i needed matrices. so i went and i learned matrices and vectors on my own, but now i need calculus. and holy shit is there a lot of stuff within calculus.

half the important papers are hidden behind paywalls and the diagrams are so confusing they take me forever to figure out. maybe i'm just inexperienced, but isn't the whole point of diagrams for the information to be more accessible?????

i might be wrong since i'm young and inexperienced, but it seems as if there's this tone of exclusivity in physics. why is it so hard to find mentors, and when i do, they have such trouble believing in me? i might be young, but i can still understand and help with something. why would you ignore all my emails and just tell me to take the easy way out? i'm in it for the long run.

Thor's Helmet taken by Chris DeCosta and Martin Pugh on February 28 2019

NGC 2359, also referred to as Thor's Helmet, is an emission nebula in the constellation Canis Major. At the heart of this nebula is a Wolf-Rayet star WR7, which is in this phase briefly before a supernova occurs.

The bubble appearance of this nebula is due to the strong stellar winds coming from WR7. These winds contribute to forming a complex structure, with a huge mass of ionized material. The high energy radiation coming from the star ionizes hydrogen to produce red light and doubly ionizes oxygen to produce blue light.

The gas absorbs and then reemits this light, leading to the name of "emission nebula".

tonight’s setup 😊 hopefully I go to bed at a reasonable time. I still have TWO lab reports to finish though.

I thought my math homework was going to be chill because it was just two problems but tell me why I open it and it’s part a-p 😭

Star Trails taken by Rob on February 24 2024

Star trails are photographs taken over long exposures, where the rotation of the Earth causes the stars to appear as arcs in the sky instead of points. The Earth rotates around its axis every 23 hours and 56 minutes.

Typically, star trails are focused on Polaris in the northern hemisphere, but I found this photo unique because it opted for a different composition. It also really highlights how dense the sky is with stars.

It begs the question, why isn't the sky infinitely bright with infinite stars? This is actually the observation that helped cosmologists find theories for the age of the universe. For a young universe, not enough time has passed for the light from incredibly distant stars to reach us, leading to the dark sky we see when we look up at night.

Dolphin Head Nebula taken by Ben Brown on February 23 2024

The Dolphin Head Nebula, Sh 2-308, is an emission nebula caused by the Wolf-Rayet star EZ Canis Majoris. WR stars have completed fusion of hydrogen and are now fusing heavier elements such as helium and carbon. They have unique emission spectrums for this reason, with no hydrogen emission lines.

The temperature of WR stars is much higher than typical stars, reaching 20,000 K to 210,000 K. WR stars are some of the most luminous stars due to their high temperatures, but most of their output is in the ultraviolet spectrum, meaning we can't see it with the naked eye.

This UV radiation ionizes the gas around it, leading to the emission nebula you can see in the photograph.

my new favorite poem ✨

The Large Magellanic Cloud taken by Rory Broesder on Februrary 18 2023

The LMC is a satellite galaxy to the Milky Way, set to collide in 2.4 billion years. It is an easily observable object from the southern hemisphere and is the fourth brightest galaxy in the local group. Within this galaxy is the Tarantula Nebula, a very active star forming region.

It was once a barred spiral galaxy before it was disrupted through tidal interactions with the Milky Way galaxy and Small Magellanic Cloud. In fact, there is a bridge of hot gas showing the connection between the LMC and SMC which is also an active star forming region.

The LMC is one of around sixty other satellite galaxies orbiting the Milky Way.

😮 what…..

i went to the local library to pick up some books today :) i'm literally so dumb because i was in the wrong row for the nonfiction section (looking for spacefarers) and i did not even stop to question why all the books around me were on the culinary arts.

i heard the master of djinn is a really good book and would be good for people that liked arcane. IF I MAKE IT THROUGH THE BOOK, maybe i'll write a review. hopefully i can because it looks really interesting...

i have a lot of random stuff to do these next two days since i wasn't productive for the first three days of break :(

study for computer science midterm

calculus unit 4 problem sets

magnetostatics FRQ

read literature for research project

update astrophysics notes

work on cosmology simulation

Flame Nebula taken by Hubble Space Telescope

This nebula is an emission nebula— a star forming region in the Orion constellation. The nebula is filled with young stars; however, dense gas obscures the majority of the cluster.

In this cluster, it contains at least one O-type star that emits light, exciting the gas around it.

O-type stars are huge blue stars that are easily seen from Earth, even from farther distances. They have extremely high surface temperatures, causing them to lose energy at a much faster rate than other stars. These massive stars live for much smaller lifetimes, before resulting in supernova explosions and eventually forming a neutron star or black hole.