https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:isotopes/v/atomic-number-mass-number-and-isotopes 2021-03-23T09:46:01Z Atomic number, mass number, and isotopes Each atom has a charged sub-structure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. The number of protons and the mass number of an atom define the type of atom. Atoms of the same element with different mass numbers are called isotopes.  Jay video https://cdn.kastatic.org/googleusercontent/m8DpxjtEI4cOeDd6Gev17fgG0-R-_Dir4VamMrT-s7rq9Wq9o0yQ7b9BfwIJHrXVykAe-uwgygdKTkvsUuu6lLXfhQ Atomic number, mass number, and isotopes Each atom has a charged sub-structure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons. The number of protons and the mass number of an atom define the type of atom. Atoms of the same element with different mass numbers are called isotopes.  https://cdn.kastatic.org/ka-youtube-converted/I-Or4bUAIfo.mp4/I-Or4bUAIfo.mp4 584 Isotopes https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/v/average-atomic-mass 2022-07-29T13:48:23.370439724Z Average atomic mass The average atomic mass (sometimes called atomic weight) of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. Average masses are generally expressed in unified atomic mass units (u), where 1 u is equal to exactly one-twelfth the mass of a neutral atom of carbon-12. Sal Khan video https://cdn.kastatic.org/googleusercontent/1rWF_Sm-ov4z3_Tm3RojlNxBlONAGoAJE-b9W7zP9blvbt1VpVl3g7vXDYrT6ccW1nQG9pZLgV6kp-R-tibaw5XudchkKh6hQZ6y Average atomic mass The average atomic mass (sometimes called atomic weight) of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. Average masses are generally expressed in unified atomic mass units (u), where 1 u is equal to exactly one-twelfth the mass of a neutral atom of carbon-12. https://cdn.kastatic.org/ka-youtube-converted/YsqoF3hqdkg.mp4/YsqoF3hqdkg.mp4 518 Isotopes https://www.khanacademy.org/science/class-9-chemistry/x46dd29ce84a663ea:atoms-and-molecules/x46dd29ce84a663ea:what-is-an-atom/v/atomic-mass-2 2021-05-25T21:59:59Z Atomic mass Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. video https://cdn.kastatic.org/googleusercontent/5YusxUCLO6XEhz_eswj4WT-vKQncR_cKC41-TLReVPtw8G0u9t0sILZGODGLR_7DPWL70bk1HPlZDeiupbzr00Xp Atomic mass Visit us (http://www.khanacademy.org/science/healthcare-and-medicine) for health and medicine content or (http://www.khanacademy.org/test-prep/mcat) for MCAT related content. These videos do not provide medical advice and are for informational purposes only. The videos are not intended to be a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read or seen in any Khan Academy video. https://cdn.kastatic.org/ka-youtube-converted/b0sYucYw6F0.mp4/b0sYucYw6F0.mp4 513 Isotopes https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:mass-spectrometry-of-elements/v/isotopes 2022-09-07T08:21:01.673602243Z Isotopes Isotopes are atoms of the same element with different numbers of neutrons. Because they contain different numbers of neutrons, isotopes have different atomic masses. The average atomic mass of an element is calculated by taking the weighted average mass of the element's naturally occurring isotopes. Sal Khan video https://cdn.kastatic.org/ka_thumbnails_cache/0a383f46-3f5b-4b2c-ad63-6338ab2a71be_1280_720_base.png Isotopes Isotopes are atoms of the same element with different numbers of neutrons. Because they contain different numbers of neutrons, isotopes have different atomic masses. The average atomic mass of an element is calculated by taking the weighted average mass of the element's naturally occurring isotopes. https://cdn.kastatic.org/ka-youtube-converted/ShC3VoxxW3g.mp4/ShC3VoxxW3g.mp4 328 Isotopes https://www.khanacademy.org/science/cambridge-o-level-chemistry-cie/x8bc8fb44369617ed:atoms-elements-and-compounds/x8bc8fb44369617ed:isotopes/v/worked-example-to-identifying-isotopes-and-ions 2026-04-29T07:20:45.495372596Z Worked example to Identifying isotopes and ions Worked example Identifying isotopes and ions Khan Academy Pakistan video https://cdn.kastatic.org/googleusercontent/ZCdwTudJg6e6n-P2gsaUborP4izvMsGo71pvEVlX9dNYWcLXcP7VHkWpn2grt4TUP1KoJLQP9NswyHBuBLSFTBw Worked example to Identifying isotopes and ions Worked example Identifying isotopes and ions https://cdn.kastatic.org/ka-youtube-converted/htOGiwy8mSw.mp4/htOGiwy8mSw.mp4 225 Isotopes https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/e/understand-isotopes 2026-04-29T07:20:45.495372596Z Understand: Isotopes Check your understanding of isotopes in this set of free practice questions. Lauren Rosebrugh exercise https://www.khanacademy.org/science/class-9-chemistry/x46dd29ce84a663ea:structure-of-the-atom/x46dd29ce84a663ea:atomic-number-mass-number-isotopes-and-isobars/e/isotopes-and-isobars 2026-04-08T08:09:55.69362548Z Isotopes and Isobars Let's solve a few problems to understand the concept of isotopes and isobars. Aanchal Arora exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/e/apply-calculating-average-atomic-mass 2026-04-28T11:00:37.315336883Z Apply: Calculating average atomic mass Apply your understanding of average atomic mass in this set of free practice questions. Allen Ruch exercise https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/e/apply-isotopes 2026-04-29T07:20:45.495372596Z Apply: Isotopes Apply your understanding of isotopes in this set of free practice questions. Lauren Rosebrugh exercise https://www.khanacademy.org/science/ap-chemistry-beta/x2eef969c74e0d802:atomic-structure-and-properties/x2eef969c74e0d802:mass-spectrometry-of-elements/a/isotopes-and-mass-spectrometry 2022-01-11T22:17:00.362877679Z Isotopes and mass spectrometry article https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article 2021-04-29T07:51:16Z Atomic number, atomic mass, and isotopes Fundamental properties of atoms including atomic number and atomic mass. The atomic number is the number of protons in an atom, and isotopes have the same atomic number but differ in the number of neutrons. article