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Alveoli are thin walled, there are many which increases the surface area.
Gas exchange takes place in the alveoli. Having a thin wall makes it easy and large amount of gasses can diffuse across due to the large surface area.

The proton has a positive charge while the neutron has no charge they both make up the nucleus of the atom while the elections have a negative charge and surround the nucleus of the atom. the protons and electrons are used to calculate the atom's mass whereas the neutrons don't affect the mass