How does the familiar orbital filling rule follow from antisymmetry?

• Electrons must stay as far apart as possible
• Every orbital holds at most one electron
• No two electrons share all four quantum numbers

Why does the 4s orbital fill before 3d in a neutral atom?

• 4s penetrates closer to the nucleus
• The 4s orbital is smaller than 3d
• Electrons always fill in strict n order

Core electrons shield the nucleus well, while electrons in the same shell shield poorly.

Iron, with outer configuration 3d6 4s2, loses its first electron. Which one leaves?

• A 4s electron
• A 3d electron
• Either, since they fill together

Moving across a row, the effective charge climbs. What happens to atomic size?

• The atom gets smaller
• The atom gets larger
• The atom keeps the same size

Why does the first ionisation energy generally rise across a row?

• New shells pile on outside the core
• Penetration fades down every group
• The growing effective charge grips harder

Magnesium ends in 3s squared and aluminium in 3s squared 3p to the one. Which has the lower first ionisation energy?

• Aluminium, since its 3p electron penetrates less
• Magnesium, since its 3s electrons are farther out
• Neither, since neighbours always tie

A student says 3d must fill before 4s because 3 is smaller than 4. What is the error?

• The 3 in 3d is smaller than the 4 in 4s
• Penetration, not n alone, sets the order
• d orbitals always fill before s orbitals