The most reactive element on the table is just one electron short of doing nothing at all.
Fluorine (configuration 2, 7) will tear an electron off almost anything it touches. Add one electron and you get the configuration of neon (2, 8), which is chemically inert. The two are neighbors, one electron apart, and that single electron is the whole story. By the end, you can read an atom's structure and count the few electrons that do chemistry.
Figure 1-1a.0: One electron separates reactive from inert: Fluorine (2, 7) versus Neon (2, 8).
Core Claims
- Atomic Structure: Atoms consist of a dense positive nucleus (protons + neutrons) surrounded by electron shells.
- Shell Energy Levels: Electrons fill shells from the inside out. Maximum capacities are 2, 8, 18, and 32 (2n2).
- Valence Count: Outermost shell electrons are the valence electrons; they dictate all chemical behavior.
Shell Filling Map
Retrieval Checklist
- Count protons, neutrons, and electrons given atomic mass & number.
- Write Bohr electron configuration sequences (e.g. 2, 8, 1).
- Identify the number of valence electrons for main-group elements.
Most of an atom never does chemistry. A handful of electrons do all of it.
An atom is almost entirely empty space around a tiny, dense nucleus of protons and neutrons. The proton count is the atomic number (Z). It is the atom's fingerprint: change it and you have a different element. The electrons live in shells at set distances out from the nucleus, filling from the inside out.
Inner electrons are locked away, and they do nothing chemically. Only the electrons in the outermost shell, the valence electrons, get close enough to another atom to be lost, gained, or shared. To predict how an element behaves, you almost always reason about that outer shell alone.
An atom's chemistry is almost entirely about its outermost electrons. Building the shells, writing the configuration, and counting valence all serve one job: finding and reading that outer shell.
Counting every electron as a valence electron. Fluorine has 9 electrons, but only the 7 in its outer shell take part in chemistry.
Neutrons add mass without charge. They do not change the electron arrangement, so they do not change how an element bonds or reacts. Atoms of one element with different neutron counts are isotopes. Isotopes matter for mass and nuclear behavior, not for chemical reactivity. That is why this page reasons from protons and electrons.
Build any atom, one proton at a time.
Electrons fill from the inside out. The first shell holds 2, and the next holds 8. Step up one proton at a time and watch each new electron find its place. The shell configuration is the count in each shell, written out.
How do the electrons fill the shells as the proton count rises?
The explanation appears after you predict.
- Inner electron
- Valence electron (outer shell)
- Nucleus, labeled with its protons
- Shell
- Load sodium and press Record. Look for: shells 1 and 2 are full (2 and 8), and one electron sits alone in shell 3.
- Load chlorine and press Record. Look for: 7 valence electrons, one short of a full shell of 8.
- Load argon and press Record. Look for: shell 3 holds 8, and the status says the outer shell is full.
- Load potassium and press Record. Look for: shell 4 opens while shell 3 stays at 8.
Use your evidence
After argon, shell 4 opens before shell 3 is full.
The first three shells fill the way you would expect: 2, then 8, then 8. That takes you from hydrogen to argon.
Shell 3 can hold 18, so you might expect the next electron to go there. It does not. At that point shell 4 sits at slightly lower energy than the rest of shell 3. The next electrons go there first. Potassium and calcium open shell 4: 2, 8, 8, 1 and 2, 8, 8, 2.
After calcium, ten more electrons finish shell 3. Shell 4 then picks up again at gallium. The builder above shows every one of these configurations.
Practice predicting configurations
Writing 2, 8, 9 for potassium. Shell 3 can hold 18, but shell 4 opens first. Potassium is 2, 8, 8, 1.
EnrichmentWhy shell 4 opens first, and what comes after calcium
Extra depth. The rest of the lesson works without it.
Calcium to zinc is past the main-group boundary of this course. It explains the wrinkle, but you will not be tested on it.
“Slightly lower energy” needs one more sentence, because it sounds like hand-waving and is not. Inside shell 3 there is a region called the 3d subshell. It holds electrons 9 through 18 of that shell. After shell 3 reaches 8, the 3d subshell sits above the first subshell of shell 4 (4s).
Electrons do not care about your shell numbering. They fill the lowest-energy vacancy. So electron 19 takes 4s. Once 4s holds two electrons, electron repulsion shifts the subshell energies. The 3d subshell dips lower. The next ten electrons, scandium through zinc, then finish shell 3. Shell 4 resumes at gallium.
Shell 3 pauses at 8 before 4s opens
Electron 19 takes 4s because 4s sits below 3d
4s fills completely with two electrons
3d opens: transition metals begin filling shell 3
3d finishes at 10: shell 3 reaches full capacity of 18
Shell 4 resumes: 4p opens and valence increases to 3
Two transition metals break even this pattern. Chromium (2, 8, 13, 1) and copper (2, 8, 18, 1) pull an electron from the fourth shell into the third. A half-full or completely full third shell is slightly more stable. You do not need to predict these exceptions. Simply notice the lone outer electron in the builder.
For the main-group elements, valence is still a clean count of the outer shell. The transition metals are the exception. Their outer-shell count does not track their chemistry in the simple way.
The number of valence electrons tells you what the atom wants to do.
Atoms are most stable with a full outer shell. That means 8 valence electrons for most main-group elements, and just 2 for hydrogen and helium. An atom "wants" the shortest route to that full shell. The valence count tells you the route.
1–3 valence electrons → the atom loses them. Shedding a few is the short path to the full shell underneath. These are the metals. The fewer electrons an atom has to lose, the more reactive it is. Group 1 atoms are the eager ones.
5–7 valence electrons → the atom gains the rest. Pulling in one or two completes the octet. These are the reactive nonmetals. Atoms that need just one electron (Group 17) grab hardest.
8 valence electrons → the atom does nothing. The shell is already full, so there is no reason to lose or gain. These are the noble gases, and that is exactly why they are inert.
Assuming an atom takes the first route it finds. Sulfur could lose 6 electrons, but gaining 2 is shorter, so sulfur gains.
Whether an atom loses or gains electrons, and how easily, drives every reactivity trend in 1-1b. Radius, ionization energy, and electronegativity are this same idea, measured and mapped across the table.
Given the element, produce the configuration from memory.
No diagram this time: write the shell configuration and the valence count yourself, then check. Separate the shells however you like (2, 8, 7 or 2 8 7 both work). Reading a diagram is recognition; producing the configuration is the skill that actually gets tested.
Sulfur
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
The number of protons in the nucleus is the , and it sets the element's identity. Electrons fill shells from the inside out: the first shell holds and the second holds . The electrons in the outermost shell are called the electrons, and they are the ones that react. An atom with 1–3 valence electrons tends to them. An atom with 5–7 tends to electrons. Both routes aim for a full outer shell of .
Write your answer first. Then grade yourself.
7 valence electrons (one short of a full shell of 8)
Neon signs and fireworks rely on electron configuration. When atoms absorb energy, their electrons jump to higher shells. When electrons drop back down to ground state, they release that energy difference as colored photons. This transition paints the sky with characteristic atomic colors.
Adaptive Competency Practice: HS-PS1-1
Test your mastery of valence electrons, atomic structure, and periodic trends one question at a time. The adaptive engine identifies individual skill gaps, provides tutoring hints on missed questions, and awards an official authenticated completion receipt.