Question 21

Elements in the same row are arranged from left to right by an increasing number of protons in an atom's nucleus.

Elements in the same row of the periodic table are organized according to an increasing number of protons in their nuclei. This arrangement reflects the atomic number of each element, which is defined by the number of protons.

A) an increasing number of neutrons in an atom's nucleus

This option is incorrect because the arrangement of elements in a row does not consider the number of neutrons. Neutrons contribute to the atomic mass but do not affect the order of elements in terms of their atomic number.

B) a decreasing number of neutrons in an atom's nucleus

This option is also incorrect for the same reason as Option A. The periodic table's rows are not organized based on neutron count, thus making this statement irrelevant to how elements are sequenced.

C) an increasing number of protons in an atom's nucleus

This option is correct because the periodic table is structured with elements arranged by their atomic number, which corresponds directly to the number of protons in the nucleus. As you move from left to right across a row, each subsequent element has one more proton than the previous one.

D) a decreasing number of protons in an atom's nucleus

This option is incorrect as it contradicts the fundamental organization of the periodic table. Elements in a row do not decrease in proton number; instead, they increase sequentially.

Conclusion

The correct answer is Option C, as it accurately describes the organization of elements by their atomic number, represented by the increasing number of protons in their nuclei. All other options fail to reflect the actual principles governing the arrangement of elements in the periodic table, as they either misinterpret neutron involvement or incorrectly describe the trend in proton numbers.