From the periodic table, we know that elements that behave very similarly are grouped vertically.
From left to right, the first red group is group I, the orange group is group II (in roman numerals)
Is there a reason why the name of Group I has the number one in it?
This understanding is very important because whenever we try to predict what kind of compound an element forms, the first step is always to note which group that element is in.
Check out the picture below:
Do you observe a trend or pattern about the pink dots?
Besides noting that the number of pink dots (aka electrons) increase from left to right (from 1 to 8), notice the cousins (they are in the same vertical group) Li and Na [Group I], Be and Mg [Group II] so on, all have same number of pink dots.
These pink dots are outermost electrons or called valence electrons.
Please take note: The number of outer electrons give elements in the same group similar properties!
Are you wondering at this point:
Why are the electrons arranged in circles?
This will be explained shortly. One key thing to point out here is that an atom has the same number of electrons [negatively-charged] as protons. [positively-charged]
One thing you must know about electric charge is that it can exert a strong pull (to attract stuff of opposite charge) or a strong push (to push off stuff of same charge)
Having unbalanced positive or negative charges is like turning on a light bulb at night.
Basically, the unbalanced charge is out there really looking to get hooked with charges of the opposite charge.
Just like how moths find the light irresistible, and how the light shines through the night at a lighthouse beckoning boats to come near.
Any atom has a nucleus that has positive charge of +[some number],
any atom with a right mind would grab electrons of -[that same number],
so that at the end the atom is electrically neutral,
Its like an atom is born with the same number of electrons as protons.
Therefore, in an atom, number of protons = number of electrons.
Then, how are the electrons arranged?
We know that electrons are very fast and light little fellows running around an atom,
So imagine yourself running around a track in the stadium, automatically you will space out.
So do electrons space out around the atom.
Notice that shells with more space, just like the outer lanes of the track, can hold more electrons.
Learning point: electrons are arranged in shells
So the two things mentioned above, both the idea that number of protons = number of electrons + electrons arranged in shells,
will help us to understand why is it that…
Sodium with proton number of 11, has a electron configuration of 2.8.1
And why Chlorine with proton number of 17, has electron configuration of 2.8.7
And now finally to the big idea of this page, why reactions occur:
What does it mean by filled outer electron shell?
Notice that the 2nd shell starts off after the 1 complete shell of 2 electrons is filled up.
This why we write electron configurations like 2.8.1 or 2.8.7, because we arrange the electrons in terms of shells.
Furthermore, note that if Group I elements are very reactive, somehow or rather it got to do with it having 1 single electron consistently dangling out.
Recall the Octet rule above?
K’s cousin, Sodium would do the same!
Simply lose an electron, and it gets a full shell of electrons!
How about those with lots of electrons and whom prefer to gain some?
Exciting takeaway: Reactions happen because atoms want to either shed electrons / or gain electrons to get full electron shell!
Learn about ionic bonding!