How do you find core and valence electrons?
Refer to the explanation.
For the main group (representative) elements, the valence electrons are the outermost (highest energy)
You can determine the number of valence electrons in the atoms of the main group elements by the group number of the element. Across a period, elements in group 1/IA have one valence electron, elements in group 2/IIA have two valence electrons, elements in group 13/IIIA have three valence electrons, and so on, ending with group 18/VIIIA, which have eight valence electrons, which is the maximum number of valence electrons.
You can also find the core and valence electrons by determining or looking up the electron configurations of the main group elements. The atomic number is the number of protons in the nuclei of the atoms of an element. A neutral atom has the same number of electrons as protons.
We can look at period 2 as an example.
For all of the preceding elements, the valence electrons are the outermost (highest energy)
For the transition metals, determining valence electrons is tricky because they can use inner electrons as valence electrons.