Atoms and Ions

Matter is anything that occupies space and has mass. Everything in the universe is made of matter, and matter is composed of atoms.

Atoms Atoms are the smallest unit of an element that retains its chemical properties. Every atom is composed of a nucleus containing protons and neutrons. Although the nucleus occupies very little space, it contains nearly all of the atom's mass. The nucleus is surrounded by an electron cloud in which electrons are found. The electron cloud accounts for most of an atom's volume and is responsible for chemical bonding and reactions.

Subatomic Particles Atoms are composed of three subatomic particles:

Protons: Positive charge (+1), located in the nucleus

Neutrons: No charge (0), located in the nucleus

Electrons: Negative charge (−1), found in the electron cloud

These particles determine an atom's identity, mass, and overall charge. A neutral atom contains equal numbers of protons and electrons, resulting in a net neutral charge.

Mass of Subatomic Particles Mass is measured in atomic mass units (amu).

Protons ≈ 1 amu

Neutrons ≈ 1 amu

Electrons ≈ 0 amu (about 1/2000 the mass of a proton)

Because electrons have very little mass, nearly all of an atom's mass is found in the nucleus from neutrons and protons.

Elements An element is a pure substance composed of atoms that all have the same number of protons. Each element has unique physical and chemical properties. Examples include hydrogen (H), oxygen (O), carbon (C), and sodium (Na).

Atomic Number (Z) The atomic number is the number of protons in an atom's nucleus. Every element has a unique atomic number, making it the defining characteristic of an element's identity. The atomic number equals the number of protons. In a neutral atom, the number of electrons equals the number of protons.

Mass Number (A) The mass number is the total number of protons and neutrons in the nucleus of a specific atom.

Mass number = number of protons + number of neutrons

Isotopes Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are isotopes of carbon.

Atomic Mass Atomic mass is the average weighted mass of an element's naturally occurring isotopes, measured in atomic mass units (amu). Because elements can exist as multiple isotopes with different masses and natural abundances, the atomic mass shown on the periodic table is usually not a whole number.

For example, chlorine has an atomic mass of approximately 35.45 amu because naturally occurring chlorine contains multiple isotopes.

Ions Ions are atoms or molecules that have gained or lost electrons, giving them a net charge:

Cations: Positively charged ions formed by losing electrons (e.g., Na⁺, Ca²⁺, K⁺)

Anions: Negatively charged ions formed by gaining electrons (e.g., Cl⁻, OH⁻, SO₄²⁻)

Ionic charge strongly influences a compound's solubility, physical properties, and interaction with biological membranes.