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.
The Invert selection (Ctrl+Shift+I) will change the current selection such that everything previously unselected will now be selected and vice versa. If there is no selection , the entire image area will be selected.
To unselect, click on an empty place of the canvas.