Key concepts in chemistry Study Guide

Atoms are composed of protons, neutrons, and electrons. The atomic number, unique to each element, represents the number of protons. As atoms are neutral, they typically have an equal number of electrons and protons. The mass number is the sum of protons and neutrons in the nucleus. Ions are atoms that have gained or lost electrons, resulting in a net electrical charge. A positive ion (cation) forms when an atom loses electrons, meaning it has more protons than electrons. Conversely, a negative ion (anion) forms when an atom gains electrons, leading to more electrons than protons. The number of protons and neutrons remains unchanged during ion formation, as these are located in the nucleus and define the element's identity and mass.

Ionic compounds are formed when atoms transfer electrons to achieve a stable electron configuration, typically a full outer shell. This transfer results in the formation of charged particles called ions. Elements in Group 1 and Group 2 are metals with 1 and 2 valence electrons, respectively. To achieve stability, they readily lose these electrons, forming positively charged ions (cations). For example, a Group 1 atom loses one electron to form a 1+ ion, and a Group 2 atom loses two electrons to form a 2+ ion. Elements in Group 6 and Group 7 are non-metals. Group 6 elements have 6 valence electrons and gain two electrons to form 2- ions, while Group 7 elements have 7 valence electrons and gain one electron to form 1- ions (anions). The electrostatic attraction between the oppositely charged cations and anions then holds the ionic compound together.