What are chemoautotrophic bacteria ? How they obtain energy?

Chemoautotrophic Bacteria : (a) They are bacteria which are able to manufacture their organic food from inorganic raw materials with the help of energy derived from exergonic chemical reactions involving oxidation of an inorganic substance present in the external medium.
(b) The chemical energy obtained from oxidation reaction is trapped in ATP molecules.
© For example, nitrifying bacteria, sulphur oxidising bacteria and iron bacteria.