Which of these elements has the highest first ionisation energy?

1. Ionisation energy increases across a period because atoms get smaller. 2. Argon sits at the end of period 3, so its atoms are very small. 3. Argon is also a noble gas with a complete outer shell. 4. Removing an electron from a stable closed shell needs extra energy. 5. So argon has the highest first ionisation energy among these four. 6. Na has the lowest because it readily loses its single valence electron. _Source: Selina Concise Chemistry ICSE Class 10, Ch 1 'Periodic Table' (aplustopper.com extract)_