Choose an element from hydrogen to argon and watch electrons fill the orbital boxes in the order 1s, 2s, 2p, 3s, 3p. Arrows show spin. In a p subshell, parallel spins are placed into separate boxes before any box is paired, which is Hund's rule.
• The first three periods of the periodic table, an orbital energy ladder, electron spin arrows and a filling cursor. • A neutral-atom selector from 1 to 18, an animate-filling toggle and a toggle for explanatory particles. • Readouts: selected atomic number, electrons currently placed, valence electrons at completion, period, group and unpaired electrons at the current step. • Experiments: oxygen (1s² 2s² 2p⁴, two unpaired electrons) and an argon filling tour that ends with filled 3p orbitals and eight valence electrons.
Subshell capacities are 2 for s and 6 for p. The lab fills in the order 1s → 2s → 2p → 3s → 3p, and Hund's rule places one electron in each degenerate p box before pairing. Period comes from the highest occupied shell, and valence electrons from that outer shell.
The lab is restricted to neutral hydrogen through argon ground-state configurations. Orbital spacings and arrow animation are symbolic. Transition-metal exceptions, excitation energies and spin dynamics are not modeled.
Electrons occupy degenerate orbitals in a subshell singly, with parallel spins, before any orbital gets a second electron. You can see it in the 2p boxes for nitrogen and oxygen in the simulator.
Two. Oxygen is 1s² 2s² 2p⁴, and in the 2p boxes three electrons go in singly before the fourth pairs with one of them.
They are the electrons in the outermost shell when filling is complete. The readout shows eight for argon and one for sodium.
No. It covers only hydrogen through argon, which avoids the exceptions that appear once 3d and 4s orbitals compete.