https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/v/stability-of-atom 2026-04-29T07:20:45.495372596Z Stability of Atom Stability of Atom Khan Academy Pakistan video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Stability of Atom Stability of Atom https://cdn.kastatic.org/ka-youtube-converted/wp2YetnVVcY.mp4/wp2YetnVVcY.mp4 429 Atomic structure and the Periodic Table https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:atomic-structure/v/protons-neutrons-and-electrons-in-atoms 2025-11-01T06:31:04.963641538Z Protons, neutrons, and electrons in atoms Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, the dense region at the center of an atom. Electrons are found outside the nucleus. Protons are positively charged and have a mass of about 1 u. Neutrons are neutral (have no charge) and also have a mass of about 1 u. Electrons are negatively charged and have a much smaller mass of about 0.0005 u. Wieteke Holthuijzen video https://cdn.kastatic.org/ka_thumbnails_cache/b0e49086-7776-4d8b-8b13-7afbfd62b856_1280_720_base.png Protons, neutrons, and electrons in atoms Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, the dense region at the center of an atom. Electrons are found outside the nucleus. Protons are positively charged and have a mass of about 1 u. Neutrons are neutral (have no charge) and also have a mass of about 1 u. Electrons are negatively charged and have a much smaller mass of about 0.0005 u. https://cdn.kastatic.org/ka-youtube-converted/lz_gMkQr7YE.mp4/lz_gMkQr7YE.mp4 151 Atomic structure and the Periodic Table 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 Atomic structure and the Periodic Table https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/v/introduction-to-the-atom 2021-03-18T10:33:22Z Introduction to the atom Learn how atoms are made up of protons, neutrons, and electrons. Elements are defined by the atomic number, the number of protons in the nucleus. The mass of an atom is determined by the total number of protons and neutrons.  Sal Khan video https://cdn.kastatic.org/googleusercontent/Njmwm-XqkUEUaIUunvEoRCwM2nvwARZsykcUj2_FxWGG2qNU7u245UgzKsRdWANkgHALQB0iqj0Q3YGllGSQpOBN Introduction to the atom Learn how atoms are made up of protons, neutrons, and electrons. Elements are defined by the atomic number, the number of protons in the nucleus. The mass of an atom is determined by the total number of protons and neutrons.  https://cdn.kastatic.org/ka-youtube-converted/1xSQlwWGT8M.mp4/1xSQlwWGT8M.mp4 1265 Atomic structure and the Periodic Table https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:atomic-structure/v/elements-and-atomic-number 2025-12-03T14:30:51.007256483Z Elements and atomic number Atoms come in a variety of types, called elements, each with a unique set of physical and chemical properties. An element is defined by the number of protons in its atoms, known as its atomic number. The periodic table organizes the elements by atomic number and provides other key information about them such as their names and chemical symbols. Wieteke Holthuijzen video https://cdn.kastatic.org/ka_thumbnails_cache/f9b78e3a-4de0-46ed-a0cb-3e36d84fa21b_1280_720_base.png Elements and atomic number Atoms come in a variety of types, called elements, each with a unique set of physical and chemical properties. An element is defined by the number of protons in its atoms, known as its atomic number. The periodic table organizes the elements by atomic number and provides other key information about them such as their names and chemical symbols. https://cdn.kastatic.org/ka-youtube-converted/OQi8FwPSu5o.mp4/OQi8FwPSu5o.mp4 274 Atomic structure and the Periodic Table https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/v/worked-example-using-the-mass-number-equation 2025-12-03T14:30:51.007256483Z Worked example: using the mass number equation The sum of the number of neutrons and protons in an atom is called its mass number. If we know the mass number of an atom, we can determine the number of neutrons in its nucleus. Wieteke Holthuijzen video https://cdn.kastatic.org/ka_thumbnails_cache/6e21f6d7-2d25-4628-866c-a2a94729826f_1280_720_base.png Worked example: using the mass number equation The sum of the number of neutrons and protons in an atom is called its mass number. If we know the mass number of an atom, we can determine the number of neutrons in its nucleus. https://cdn.kastatic.org/ka-youtube-converted/T9UH12HdFdo.mp4/T9UH12HdFdo.mp4 121 Atomic structure and the Periodic Table https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/v/atomic-number-mass-number-and-isotopes-ur 2025-08-08T00:24:37.843818748Z Atomic number, mass number Created by Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Atomic number, mass number Created by Khan Academy https://cdn.kastatic.org/ka-youtube-converted/-F0KbgYuyhI.mp4/-F0KbgYuyhI.mp4 432 Atomic structure and the Periodic Table https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/v/electron-distribution-in-shells-ur 2025-08-08T00:24:37.843818748Z Electron distribution in shells Created by Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Electron distribution in shells Created by Khan Academy https://cdn.kastatic.org/ka-youtube-converted/9elgLcy86X0.mp4/9elgLcy86X0.mp4 605 Atomic structure and the Periodic Table https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/v/given-atomic-number-find-position-and-vice-versa-solved-example-ur 2025-08-08T00:24:37.843818748Z Given atomic number find position and vice-versa (Solved example) Created by Khan Academy video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Given atomic number find position and vice-versa (Solved example) Created by Khan Academy https://cdn.kastatic.org/ka-youtube-converted/DzLEbHyZC9I.mp4/DzLEbHyZC9I.mp4 668 Atomic structure and the Periodic Table https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:atomic-structure/e/understand-atomic-structure 2026-04-26T16:30:39.382257956Z Understand: Atomic structure Check your understanding of atomic structure in this set of free practice questions. Megan Cohn exercise https://www.khanacademy.org/science/class-9-chemistry/x46dd29ce84a663ea:structure-of-the-atom/x46dd29ce84a663ea:models-of-an-atom/e/atomic-structure 2026-04-03T18:30:01.793877038Z Atomic structure Test your knowledge of atomic structure! Ashley exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:atomic-structure/e/apply-atomic-structure 2026-04-26T16:30:39.382257956Z Apply: Atomic structure Apply your understanding of atomic structure in this set of free practice questions. Megan Cohn exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/lo-2-3-atomic-number-and-mass-number 2026-04-28T11:00:37.315336883Z Atomic number and mass number Let's solve some problems to better understand atomic and mass numbers. Aakash Bagga exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:ions/e/understand-ions 2026-04-28T11:00:37.315336883Z Understand: Ions Check your understanding of ions in this set of free practice questions. Megan Cohn exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/find-the-valence-electrons 2021-04-29T07:57:02Z Find the valence electrons Let's practice on finding the valence electrons of an element by counting the number of electrons present in the outermost shell. Chaitra exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/structure-of-an-atom-in 2025-02-02T06:10:19.671082841Z Structure of an atom Let's practice! vibhor exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/electronic-configurations-practice 2026-01-03T08:04:42.460383575Z Electronic configurations (part 2) Let's solve a few more questions based on how electronic configuration positions different elements in the periodic table. Revathi exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atomic-models-and-periodicity/x2613d8165d88df5e:the-periodic-table/e/apply-number-of-valence-electrons 2026-04-26T16:30:39.382257956Z Apply: Number of valence electrons Apply your knowledge of valence electrons for main group elements in this set of free practice questions. Emily Cizmas exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:the-periodic-table/x8bc8fb44369617ed:arrangement-of-elements/e/groups-and-periods--position-of-elements 2026-02-27T12:30:36.531197313Z Identify the elements belonging to the same group or period Let's practice identifying elements belonging to the same period or the group by writing their electronic configuration. Also, knowing the reasons for belonging to the same period or the group. Chaitra exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/find-the-group-and-period-number-of-an-element 2021-04-29T07:51:19Z Find the group and period of an element Let's practice to find position of an element in the periodic table by finding out the group and period number. Chaitra exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:the-periodic-table/x8bc8fb44369617ed:arrangement-of-elements/e/groups-and-periods--find-the-atomic-number 2021-04-29T07:53:56Z Groups and periods: find the atomic number Let's practice on finding the atomic number of an element by knowing its group and period number. Chaitra exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/atomic-number-and-electronic-configuration 2026-04-07T22:59:30.107127435Z Atomic number and electronic configuration (testing 1) Let's practice to write the electronic configuration of an element given its atomic number. Chaitra exercise https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:atomic-structure-and-the-periodic-table/e/electronic-configuration-and-atomic-number 2026-04-07T22:59:30.107127435Z Electronic configuration of elements: KLMN shells. Let's practice to write the electronic configuration of a neutral atom. Chaitra exercise https://www.khanacademy.org/science/ap-biology/chemistry-of-life/elements-of-life/a/matter-elements-atoms-article 2021-04-29T08:02:50Z Matter, elements, and atoms Learn about the structure of the atom, and how atoms make up matter. An atom is the smallest unit of matter that retains all of the chemical properties of an element.  article 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