Sorry I haven’t had time to post as per my schedule.
Lab reports, Literature Review and other work all came in at once!
But back to it for this week. I have 80% of a post prepared on chemical bonding but then realized there are one or two more key concepts we need to cover before we get there.
So today, the exciting world of electron shells. This will be important to understand some of the types of bonding and other principles in the next few posts even though it is hard to relate it to everyday life as I hope to do with other posts on here.
As we have seen in Chemistry Basics 2 the atom consists of electrons, neutrons and protons.
Remember that the protons are positively charged and the electrons negatively charged.
Electron shells are a convenient way to describe the distribution of an atom’s electrons.
The shells can be thought of as an orbit of sorts that the electrons follow in a path around the nucleus. Higher shell numbers represent a greater distance from the nucleus of the atom.
Using this image below from the last post we can see that both Hydrogen and Helium have one electron shell with Hydrogen having one electron in its shell and Helium having two.
If we go further along the table to atoms with many more electrons we can get a better idea of this shell model.
An example to use is Fluorine (try to find it here as a practice try to figure out its atomic number and therefore the number of electrons it has before looking at the image or scrolling further)
As we can see Fluorine has 9 electrons from the image above.
The shell system look a bit more complicated than those of Hydrogen and Helium.
To see why we need to look at a new concept, Electronic Configuration.
Electrons want to conserve energy (as we all do) and as such will occupy the lowest energy state possible. It takes more energy for them to occupy higher energy shells and so they fill these from the lowest energy shell first to the highest.
Each of these shells must be full before the next one can start to be filled. This is known as the Aufbau principle.
The first shell can hold 2 electrons (as we see with Helium)
The second shell can hold 8 electrons (see Fluorine above which doesn’t have a full outer shell) and each of these need to be filled with electrons before the next shell can be.
The third shell can hold 18 electrons
The fourth shell can hold 32 electrons
The fifth can hold 50 electrons
The sixth can hold 72 electrons
Obviously not all of these shells are relevant to all atoms but the more electrons an atom has the more of its shells will be filled.
An easy way to tell how many electrons a shell can hold is to use the formula 2x(n)^2
This mean 2 multiplied by n(squared) where n is the shells number
For example 2 x 3^2 = 18 electrons for the third shell!
The outermost shell of any atom is known as the Valence shell.
As in the fluorine example we can see that it has 7 electrons in it’s valence shell. Valence shells are important because they are often involved in bonding between atoms which we will cover later.
The shells and the electrons are denoted by the shell number (1st number) and the number of electrons in it (2nd number)
For Hydrogen this is 1s1
For Helium this is 1s2
For Fluorine this is 1s2, 2s2, 2p5
The letters denote the shape of the orbital in the shell (this is a matter for another post) but we can see from the examples above that as the 1s orbital is filled the 2s orbital follows.
Fluorine's valence shell isn’t as full as it could be (remember we said the second shell can hold 8 electrons?).
If we move across on the periodic table by one more element we can see this shell fill completely in Neon which has a total of 10 electrons in total as we can see from its electronic configuration 1s2, 2s2, 2p6
Taking the total of the second shell we get 8 total electrons meaning the shell is now full.
The next element, Sodium (Na) has 11 electrons in total according to the periodic table and so will have the full second shell with the next electron existing in the 3rd shell so we have 1s2, 2s2, 2p6, 3s1
Going higher we find more complicated arrangements which we don't need to cover yet but these are the basics so far.
This can be a bit of a hard topic to grasp so if there are any questions leave them below. I can provide good links to articles and videos if they are too difficult to answer with a comment back!
*A hydrogen and a helium atom. Source; getdrawings
** Image credit; Greg Robson, CC BY-SA 2.0 UK via Wikimedia commons