Is there an opening between the atria and the ventricles?

Not an opening as such but there is a valve, the atrio-ventricular valve, which allows blood to pass from the atrium to the ventricle. The valve is controlled according to pressure; when the atrial pressure is high and the ventricular pressure is low, the valve will open, allowing blood to flow through, this is also aided by contraction of the atrium to ensure blood is emptied into the ventricle.