Chemical Level of Organization Review

Electron Shells and Energy Levels

Electrons do not move randomly; they occupy an orderly series of energy levels within the electron cloud, which can be represented as concentric electron shells.

• The first shell (closest to the nucleus) is the lowest energy level.
• The outermost shell is known as the valence shell.
• The number of electrons in this outermost shell is critical because it determines the chemical properties of the element. Atoms with unfilled outer shells are "reactive" and tend to interact with other atoms to fill them, whereas atoms with full outer shells (like the noble gases) are "inert" and stable.

Exam Tip: Remember that atoms are normally electrically neutral because they contain an equal number of protons and electrons. If an atom gains or loses electrons, it becomes an ion (a cation if positive, or an anion if negative).

Defining Elements

An element is a substance composed entirely of atoms that share the same atomic number. The atomic number is defined by the total number of protons in the atom; this number is what gives an element its unique identity.

Atomic Properties and Isotopes

When studying specific atoms, we distinguish between several related terms:

• Mass Number: The total count of protons and neutrons in a specific atom.
• Isotopes: These are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have different neutron counts, they have different mass numbers, yet they retain identical chemical properties.
• Atomic Mass: The actual mass of a specific isotope, measured in atomic mass units (amu) or daltons.
• Atomic Weight: Because elements exist as a mixture of different isotopes, the atomic weight is the average mass of the element, accounting for all its isotopes in proportion.

From Atoms to Molecules

When atoms interact with one another, they produce larger, more complex structures called molecules. The specific characteristics of any given piece of matter are determined by the types of atoms present and the specific ways in which they interact.

Elements in the Human Body

In the human body, the elements present are categorized based on their abundance:

• Principal Elements: The thirteen most abundant elements by body weight.
• Trace Elements: The fourteen other elements that are present in the body in very small amounts.