https://www.khanacademy.org/science/ap-biology/chemistry-of-life/elements-of-life/v/elements-and-atoms 2023-03-31T23:16:47.953017837Z Elements and atoms Elements are pure substances with specific properties. Atoms are the smallest units of elements that still retain the element's properties. Atoms contain electrons, neutrons, and protons. Each element is defined by the number of protons in its nucleus. Sal Khan video https://cdn.kastatic.org/googleusercontent/zfLHeNHjPfEhSYfKtZLu6Ft_uFg2p_L9gALjUyfahbjdBzZbrDLxrg-rk_mrsUBBVIKs44ALBBI1Gv8cm2O_Ppa05g Elements and atoms Elements are pure substances with specific properties. Atoms are the smallest units of elements that still retain the element's properties. Atoms contain electrons, neutrons, and protons. Each element is defined by the number of protons in its nucleus. https://cdn.kastatic.org/ka-youtube-converted/IFKnq9QM6_A.mp4/IFKnq9QM6_A.mp4 789 Electrons and the periodic table https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:atomic-structure/e/apply-atomic-structure 2026-06-25T14:10:39.916233732Z Apply: Atomic structure Apply your understanding of atomic structure in this set of free, standards-aligned practice questions. Megan Cohn exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atomic-models-and-periodicity/x2613d8165d88df5e:the-quantum-model/a/the-periodic-table-electron-shells-and-orbitals-article 2026-04-27T10:10:46.754795089Z Learn: The periodic table, electron shells, and orbitals article https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atomic-models-and-periodicity/x2613d8165d88df5e:the-periodic-table/a/determine-valence-electrons-using-the-periodic-table 2026-04-26T16:30:39.382257956Z Learn: Determine valence electrons using the periodic table Learn how to determine the number of valence electrons for an element using the periodic table. article https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atomic-models-and-periodicity/x2613d8165d88df5e:the-periodic-table/e/apply-number-of-valence-electrons 2026-06-25T10:00:38.725739878Z Apply: Number of valence electrons Apply your understanding of valence electrons for main group elements in this set of free, standards-aligned practice questions. Emily Cizmas exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atomic-models-and-periodicity/x2613d8165d88df5e:the-periodic-table/e/understand-structure-of-the-periodic-table 2026-06-25T12:40:44.68874125Z Understand: Structure of the periodic table Check your understanding of the structure of the periodic table in this set of free, standards-aligned practice questions. Donna fig exercise