There are no periodic trends This section sentence is a bit wrong headed Periodic trends are those which repeat from period to period e g noble gas behavior recurring What this really means to talk about is horizontal trends across the period What the article means to say I guess is that horizontal trends are due to a quantum shell filling and since 1s fills within 2 electrons there s not much scope to look at trends It s also perhaps worth noting that when Mendelev put the periodic table together he didn t know the noble gases existed so all he had to place from what would become period 1 was hydrogen