Atomic size decreases across a period due to an increase in the effective nuclear charge, zeff. a. true b. false

It is a true statement that atomic size decreases across a period due to an increase in the effective nuclear charge, zeff.

Atomic size is a periodic trend that decreases across the period. Across the period, more electrons are added without a corresponding increase in the number of shells.

Also, the effective nuclear charge, zeff increases across the period as more positive charges are added to the nucleus. As such, the attraction between the nucleus and electrons increases leading to a decrease in atomic size.

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The answer is true. As you move across the periodic table, the elements have a higher atomic charge due to an increase in the number of protons and neutrons. Since the neutrons have no charge, the protons (that have a positive charge) attract the electrons clouds that are negative in charge (unlike poles attract). The more the protons in an atom, the more the attraction on the electron orbits so they get closer to the nucleus of the atoms. This is the reason the atomic diameter reduces across the periodic table.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.