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Why valence electrons matter
Chemistry happens at the edge of the atom. Sodium has one valence electron and gives it up to become Na⁺; chlorine has seven and takes one to become Cl⁻. Carbon’s four let it form four covalent bonds, the backbone of organic chemistry. Count them for any element using its electron configuration: add up the electrons in the highest-numbered shell.
Questions students ask
What are valence electrons?
The electrons in an atom’s outermost shell. They are the ones involved in bonding, so they decide how an element reacts, what charge its ions carry and how many bonds it forms.
How do you find valence electrons from the periodic table?
For main-group elements, use the group number: groups 1 and 2 have 1 and 2; groups 13 to 18 have 3 to 8 (take away 10). Helium is the exception with 2. For transition metals the count is less clear-cut: textbooks usually count the outer s electrons plus the d electrons, which is what the calculator shows.
How many valence electrons does oxygen have?
Six. Oxygen is in group 16 with configuration 1s² 2s² 2p⁴; the second shell holds 2 + 4 = 6.
How many valence electrons does carbon have?
Four (2s² 2p²), which is why carbon forms four bonds.
What is the octet rule?
Main-group atoms tend to gain, lose or share electrons until they have eight in their outer shell, the arrangement of a noble gas. Hydrogen and helium aim for two. There are exceptions, such as boron in BF₃ (six) and sulfur in SF₆ (twelve).