Why atomicity is not correct .Consider below Txn
read(x); x:=x-50; write (x); read(y); y:=y+20; write(y)
1.Initially X = $1000, Y= $2000 .SUM(X+Y) = $3000
2.Execute above Txn COMPLETELY(Atomically).Deduct 50 from X and Add 20 to Y .
3.Final X=$950 Y=$2020 . SUM(X+Y)=$2970
So even though we executed above TXN Atomically our SUM(X+Y) is inconsistent.
That's why Atomicity does't ensure that sum is unchanged after txn. But "CONSISTENCY" property explicitly says that if $50 is deducted from X then $50 must be added to Y.
REFERENCE: page no. 566, chapter 15,Database System concepts by Sudarshan-Korth ,Third line from bottom of page.