Science: Atoms & the Periodic Table: Standard Practice

Medium Level Guide Grade-level practice aligned to typical classroom expectations and unit assessments. Structure of the Atom Atoms have a dense nucleus containing protons and neutrons, surrounded by electrons in energy levels. Protons carry positive charge, electrons negative, and neutrons neutral. In a neutral atom, protons equal electrons. Atomic Number and Mass Atomic number equals the number of protons and defines the element. Mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons. The Periodic Table Elements are organized by increasing atomic number in rows (periods) and columns (groups). Groups share similar chemical properties. Metals, nonmetals, and metalloids occupy different regions of the table. Elements and Compounds An element cannot be broken down chemically into simpler substances. Compounds form when elements bond chemically in fixed ratios. Molecules are two or more atoms bonded together, like H₂O.