If the ending substance has changed its properties. For example if iron rusts it is a chemical change because it is no longer the same as before it is now just rust. A physical change only changes the appearance but burning something or changing it's chemical properties is a chemical change.
You would expect 500 million atoms to be present. When a source experiences a half-life, it is a point in which atoms have degraded to half what they used to be in mass. 1/2(1 billion) = 500 million
