There are three elements of symmetry: 1-axis of symmetry It's a line which cuts the molecule into two equal halves. 2-plane of symmetry It's a plane which cuts the molecule into two equal halves...
Yes, a parallelogram.
Yes. An isosceles triangle, for example, is symmetric about the bisector of its odd angle but has no rotational symmetry.
effectively yes. Diagonal symmetry is when the line of symmetry goes diagonally, rather than horizontally or vertically
parallelogram* * * * *A parallelogram does have rotational symmetry (order 2).