What is the smallest possible piece of an element called?

            The smallest possible piece of an element is called an Atom.

Explanation:

                    The word Atom is derived from atomos meaning indivisible particle. An atom is a very small sized particle and its size is approximately 100 picometers.

                    Atoms are composed of a Nucleus which contains protons and neutrons and the nucleus is surrounded by orbiting electrons. Also, the nucleus is positively charged due to +ve protons and the electrons revolving around nucleus are -vely charged making overall atom neutral in nature.

                   The number of protons present in nucleus defines the element to which an atom belongs. For example, if the number of protons are 10, it means that atom belongs to Neon element with atomic number 10.

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the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.