Click on the Rectangle Selection button or select another selection mode from the drop-down list: Lasso Selection or Structure Selection .

to select an atom, bond or group, click or select it (Rectangle Selection or Lasso Selection mode).

to select a structure you can double-click on an atom or a bond of it in the Rectangle Selection or Lasso Selection mode, too.

Lasso Selection : Click on the mouse button and move the mouse pointer around the fragment; release the mouse when you are ready.

Rectangle selection : Hold the mouse button down while selecting; release the mouse when you are ready.

To increase selection, press and hold down the Shift key while selecting.

To decrease selection, press the Shift key while clicking a selected atom, bond or group.

To unselect, click on an empty place of the canvas.