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Core electron
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<p>Core electrons are the <a href="/facts/Electron/L0KBo5cW">electrons</a> in an <a href="/facts/Atom/FoQ4kGN5">atom</a> that are not <a href="/facts/Valence_electron/mXPA15VP">valence electrons</a> and do not participate as directly in <a href="/facts/Chemical_bond/ArNUMnIB">chemical bonding</a>. The <a href="/facts/Atomic_nucleus/orpIm5Xl">nucleus</a> and the core electrons of an atom form the atomic core. Core electrons are tightly bound to the nucleus. Therefore, unlike valence electrons, core electrons play a secondary role in chemical bonding and reactions by screening the positive charge of the atomic nucleus from the valence electrons. </p><p>The number of valence electrons of an element can be determined by the <a href="/facts/Group_(periodic_table)/YGBcUabv">periodic table group</a> of the element (see <a href="/facts/Valence_electron/mXPA15VP">valence electron</a>): </p> <ul><li>For <a href="/facts/Main-group_element/2AG3QxJl">main-group elements</a>, the number of valence electrons ranges from 1 to 8 (<i>n</i>s and <i>n</i>p orbitals).</li> <li>For <a href="/facts/Transition_metal/0slGWoLQ">transition metals</a>, the number of valence electrons ranges from 3 to 12 (<i>n</i>s and (<i>n</i>−1)d orbitals).</li> <li>For <a href="/facts/Lanthanide/rt3XIRCl">lanthanides</a> and <a href="/facts/Actinide/kHC4lxJV">actinides</a>, the number of valence electrons ranges from 3 to 16 (<i>n</i>s, (<i>n</i>−2)f and (<i>n</i>−1)d orbitals).</li></ul> <p>All other non-valence electrons for an atom of that element are considered core electrons. </p>