How do valence e- determine an element's reactivity?
Your valence electrons refers to the electrons sitting on the outermost shell. Say your atom has 10 protons. So, since the number of protons is equal to the number of electrons, then your atom has 10 electrons as well. Therefore, you would have 2 electrons in your first shell and 8 electrons in your shell. This is your outermost shell.
Now, the maximum number of electrons in your outermost shell is 8 electrons. The elements on the periodic table who all have a full outermost shell or a shell with 8 electrons are called the halogens. They are iodine, bromine, fluoride, chloride and astatine. They are the most stable elements out of the periodic table.
Well, all elements want to become more stable by having a full outermost shell consisting of 8 electrons. So for elements such as sodium, lithium and potassium, they are extremely reactive. This is because, for example, sodium has 11 electrons. So 2 electrons in the first shell, 8 electrons in the next shell and 1 electron in the final shell.
To become more stable or have 8 electrons in their outermost shell, they can either lose that one pesky electron in the third shell or they could gain 7 more electrons. Now elements are just like us humans. They are extremely lazy! So, they decide to ditch that 1 electron in their third shell. When they release that electron, they also release a large amount of energy. Hence, that's why all elements in the first group are considered extremely reactive.
Just to get the gist of the rest of the periodic table, the elements become more stable as the electrons in the outermost shell increases. That's because they go from ditching their electrons to accepting 1,2 or 3 more electrons from another elements to have a full outermost shell.