What general trend is found in Period 3 as the elements are considered in order of increasing atomic number?
heavier, smaller, higher ionization energy, lower reactivity, higher electron affinity
Increasing atomic number means increasing proton number. For an atom to exist, they tend to require about an equal amount of neutrons to balance the protons. Hence both increasing both protons and neutrons will increase the mass of an atom.
Atoms with higher proton number (aka atomic number) means that the nuclear charge will be more attractive hence the electron orbitals (or shells) are closer to the nucleus because opposites attract.
Because opposites attract, the electrons are strongly bonded together hence they are harder to ionize.
Because they are harder to ionize, they are harder to react with, and also they have higher electron affinity (release more energy when given an extra electron)