To determine the number of valence electrons an atom has, you can refer to the periodic table. The valence electrons are the electrons in the outermost energy level, also known as the valence shell, of an atom.
Here's a general guide to finding the number of valence electrons for main group elements:
Group 1 elements (alkali metals): These elements have one valence electron. Examples include hydrogen (H), lithium (Li), sodium (Na), and potassium (K).
Group 2 elements (alkaline earth metals): These elements have two valence electrons. Examples include beryllium (Be), magnesium (Mg), calcium (Ca), and strontium (Sr).
Groups 13-18 elements: For these groups, the number of valence electrons can be determined by looking at the group number. For example:
Group 13 elements (boron group): These elements have three valence electrons. Examples include boron (B), aluminum (Al), and gallium (Ga).
Group 14 elements (carbon group): These elements have four valence electrons. Examples include carbon (C), silicon (Si), and germanium (Ge).
Group 15 elements (nitrogen group): These elements have five valence electrons. Examples include nitrogen (N), phosphorus (P), and arsenic (As).
Group 16 elements (oxygen group or chalcogens): These elements have six valence electrons. Examples include oxygen (O), sulfur (S), and selenium (Se).
Group 17 elements (halogens): These elements have seven valence electrons. Examples include fluorine (F), chlorine (Cl), and bromine (Br).
Group 18 elements (noble gases): These elements have eight valence electrons, except for helium (He), which has two valence electrons. Examples include helium (He), neon (Ne), and argon (Ar).
It's important to note that there are exceptions to the above patterns for transition metals and elements with expanded octets, which can have varying numbers of valence electrons.
By referring to the periodic table and determining the group number of the element, you can generally determine the number of valence electrons an atom has.