Which statement best explains how periods on the periodic table are organized? A. increasing atomic number from left to right B. increasing number of neutrons from left to right C. decreasing number of electrons from left to right D. decreasing atomic mass number from left to right

The best statement that explains how periods on the periodic table are organized is A. increasing atomic number from left to right.

Here's why:

1. The periodic table is a systematic way of organizing the chemical elements, which are arranged in order of increasing atomic number. The atomic number is the number of protons in an atom's nucleus, which also defines the chemical properties of an element and its place in the periodic table.

2. Each period in the periodic table corresponds to a higher energy level of an atom's outermost electrons. As you move from left to right across a period, each element has one more proton and electron than the previous element.

3. The number of neutrons (option B) does not determine the position of an element in the periodic table. Neutrons contribute to the atomic mass of an element, but they do not affect the chemical properties of an element.

4. The number of electrons (option C) increases from left to right across a period, not decreases. Each element in a period has one more electron than the previous element.

5. The atomic mass number (option D) generally increases from left to right across a period, not decreases. The atomic mass number is the total number of protons and neutrons in an atom's nucleus. However, there are exceptions due to the isotopes of certain elements.

So, the best answer is A. increasing atomic number from left to right.