The valence electrons of an atom are the electrons in the outermost energy level or shell. These electrons are involved in the atom's chemical reactions and determine its chemical properties. The number of valence electrons can be determined by looking at the periodic table and examining the group number of the element.
In general, for main group elements (also known as representative elements), the valence electrons correspond to the group number. For example, elements in Group 1 (such as hydrogen, lithium, sodium) have one valence electron, elements in Group 2 (such as beryllium, magnesium, calcium) have two valence electrons, and so on.
However, there are some exceptions to this pattern. For transition metals and inner transition metals, determining the number of valence electrons can be more complex due to the way electrons fill the d and f orbitals. In these cases, it is often useful to refer to the electron configuration of the element to determine the number of valence electrons.
To summarize, the valence electrons are the electrons in the outermost energy level or shell of an atom, and their number is determined by the group number (for main group elements) or the electron configuration (for transition metals and inner transition metals).