What are the horizontal (from left to right) parts of the periodic table called?

rows
periods
families

The correct answer is periods. See the explanation below, please.

Explanation:

The horizontal lines in the periodic table are called periods (ranging from 1 to 7) and vertical lines (ranging from 1 to 18).

The period that an element belongs to coincides with the last electronic layer of the element (that compound in period 3 will have 2 electronic layers).

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

b. accuracy of the data matters alot more than its number of digits, organization or amount of detail. quantitative observation is more accepted than the qualitative observation.