The correct answer is option 'A' - the number of electrons determines the chemical properties of an element.

Explanation:
Chemical properties refer to the behavior of an element when it undergoes a chemical reaction or interacts with other substances. These properties are determined by the arrangement and behavior of the electrons in the atom of that element.

Electrons are negatively charged particles that orbit around the nucleus of an atom. They occupy specific energy levels or shells around the nucleus. The outermost shell of an atom is called the valence shell, and the electrons in this shell are called valence electrons.

The number of valence electrons in an atom is crucial in determining its chemical properties. It determines how easily an atom can form chemical bonds with other atoms to achieve a stable electron configuration. The valence electrons participate in chemical reactions by either gaining, losing, or sharing electrons with other atoms.

Key Points:
- The chemical properties of an element are determined by the number of electrons in its atom.
- Valence electrons, located in the outermost shell of an atom, play a significant role in chemical reactions.
- The number of valence electrons determines how readily an element can form chemical bonds.
- Elements with the same number of valence electrons tend to have similar chemical properties.
- The periodic table is organized based on the number of valence electrons, which allows us to predict the chemical behavior of elements.

In conclusion, the number of electrons, specifically the number of valence electrons, determines the chemical properties of an element. This knowledge is crucial in understanding the behavior of elements in chemical reactions and their interactions with other substances.