The aufbau principle, from the German Aufbauprinzip (building-up principle), also called the aufbau rule, states that in the ground state of an atom or ion, electrons fill atomic orbitals of the lowest available energy levels before occupying higher levels.
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those are all physical changes and most physical changes can be undone like crushing a pice of metal you can flatten it back out you anwser is physical change
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protons = atomic numbermass number = protons + neutronsatomic number = mass number - neutrons = 78-38 = 40
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All around you there are chemical reactions taking place. Green plants are photosynthesising, car engines are relying on the reaction between petrol and air and your body is performing many complex reactions. In this chapter we will look at two common types of reactions that can occur in the world around you and in the chemistry laboratory. These two types of reactions are acid-base reactions and redox reactions.
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