In addition to properties of elements, list at least two other things or events that are periodic.

The term "periodic" has  to do with something that repeats regularly at certain intervals. two other things or events that are periodic include; Occurrence of day and night and vibration of a spring.

When we use the word "periodic" what immediately comes to mind is something that occurs in a regular repeating pattern just like the elements in the periodic table.

Day and night follows a periodic cycle which is the rotation of the earth and the period of this rotation is 1 earth day.

The vibration of a spring is another example of a regular repeating motion.

Day and night and the vibration of a spring are two other examples of things or events that are periodic.

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Major periodic trends include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, and metallic character. Periodic trends, arising from the arrangement of the periodic table, provide chemists with an invaluable tool to quickly predict an element's properties.

it means there are 3 atoms of oxygen

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