Question

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elements in the same group have similar chemical properties.
elements in the same period have identical ionization energies.
neutrons determine chemical reactivity.
atomic radius increases across a period.
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Explanation:

The Periodic Law states that when elements are arranged in order of increasing atomic number, their physical and chemical properties show a periodic pattern. Elements in the same group have the same number of valence - electrons, which leads to similar chemical properties. Elements in the same period have the same number of electron shells, but ionization energies vary across a period and are not identical. Neutrons mainly affect the mass and stability of an atom, not chemical reactivity directly. Atomic radius decreases across a period.

Answer:

Elements in the same group have similar chemical properties.