Atoms, Elements, Compounds, and Isotopes

Atomic Structure

The atom is the smallest particle of an element that can take part in a chemical reaction. Despite its small size, the atom has a complex internal structure consisting of a central nucleus surrounded by electrons.

The Nucleus and Subatomic Particles

At the heart of the atom lies the nucleus, which contains two types of particles: protons and neutrons. Because protons carry a positive charge, the nucleus is always positively charged. Almost all of the atom's mass is concentrated in the nucleus because the mass of an electron is considered negligible.

The third subatomic particle, the electron, is negatively charged and moves around the nucleus in specific regions called shells or energy levels.

The table below summarizes the properties of these particles:

Proton Number and Nucleon Number

To identify an element and its specific atoms, we use two important numbers:

1. Proton Number (Atomic Number): The number of protons in the nucleus of an atom. This number defines the element.
2. Nucleon Number (Mass Number): The total number of protons and neutrons in the nucleus.

In a neutral atom, the number of protons must equal the number of electrons to ensure the total charge is zero. If you know the nucleon number and the proton number, you can calculate the number of neutrons using this formula:

$\text{Number of neutrons} = \text{nucleon number} - \text{proton number}$

Electronic Configuration and Ions

Electronic configuration refers to the arrangement of electrons in the shells around the nucleus. Electrons follow specific rules when filling these shells:

• They fill the shells nearest the nucleus first (the lowest energy levels).
• The shells have specific capacities: the 1st shell holds up to 2 electrons, the 2nd shell holds up to 8, and the 3rd shell holds up to 8 (for elements up to calcium).
• The outermost shell can never hold more than 8 electrons.

Exam Tip: When determining the electronic configuration, always fill the shells from the inside out. For example, Calcium (atomic number 20) has the configuration (2, 8, 8, 2).

Ions

An ion is a charged atom or group of atoms formed when an atom either loses or gains electrons.

• Cations: Formed when an atom loses electrons, resulting in a positive charge.
• Anions: Formed when an atom gains electrons, resulting in a negative charge.

It is crucial to remember that the number of protons never changes when an ion forms; only the number of electrons changes. Generally, ions form so that they reach the stable electronic configuration of the nearest noble gas.

Isotopes and Relative Atomic Mass

Not all atoms of the same element are identical. Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons.

For example, Chlorine has two main isotopes: Chlorine-35 (17 protons, 18 neutrons) and Chlorine-37 (17 protons, 20 neutrons).

Because isotopes have the same number of electrons, they have the same electronic configuration and therefore exhibit the same chemical properties. However, because their masses differ, they have different physical properties, such as density and the rate of diffusion.

Relative Atomic Mass ($A_r$)

Since elements in nature exist as a mixture of different isotopes, we use the relative atomic mass ($A_r$) to represent the average mass of those isotopes. This is calculated by comparing the average mass to 1/12th of the mass of a carbon-12 atom.

To calculate the $A_r$ of an element, use the following formula:

$A_r = \frac{\sum (\text{mass of isotope} \times \% \text{ abundance})}{100}$

---

Summary Table of Key Terms