An atom of an element or a molecule of a compound.
Light is quite complicated as it has wave-particle duality. Sometimes it acts like a particle other times like a wave. But technically no, it is not matter, it is made of energy
An Atom
As an "atom" is to an element - the smallest unit, so a "molecule" is to a compound.
Acceleration = force/mass
Einstein. He stated that light acts as both a particle and a wave.
photon
Light acts as a particle.
if it acts on a charged particle
No it does not as it acts like a liquid but is a solid
Light acts like both a particle and a wave.
In that case, basically no force acts on the particle, and the particle moves at a constant speed. This constant speed may, or may not, be zero.
an atom