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 The atom https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-atom/e/structure-of-an-atom 2023-08-13T07:34:46.711956309Z Structure of an atom Let's solve some questions! vibhor exercise https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-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/chemistry/atomic-structure-and-properties/history-of-atomic-structure/v/rutherfords-gold-foil-experiment 2022-09-27T05:27:31.474935863Z Rutherford’s gold foil experiment Learn about Rutherford's discovery of the nucleus and the development of the nuclear model of the atom. video https://cdn.kastatic.org/googleusercontent/GUca1q02zeVjSuVtBIXH6p52GSGoDXIJ1yEl6cYQ4J1y8JlCk2lZbfkLapmLiIsRJkG0n-vitdeSdc31H9toW26p Rutherford’s gold foil experiment Learn about Rutherford's discovery of the nucleus and the development of the nuclear model of the atom. https://cdn.kastatic.org/ka-youtube-converted/bVlwH1kfDeg.mp4/bVlwH1kfDeg.mp4 684 The atom https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-atom/a/discovery-of-the-electron-and-nucleus 2021-07-18T18:44:33.041351326Z Discovery of the electron and nucleus Thomson's cathode ray experiment and Rutherford's gold foil experiment article https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:ions/v/introduction-to-ions 2023-12-18T19:15:22.874322042Z Introduction to ions An atom with an electric charge is called an ion. A positively charged ion (a cation) has more protons than electrons. A negatively charged ion (an anion) has more electrons than protons. video https://cdn.kastatic.org/googleusercontent/UAwfb6XNMO4aUhZwvvmHQIdu3oURWLZJAN_mgOX6sOKBYgq3IYFVWVOwlkBsC4Q_ry-upfT73FEpiQZ9YXvrRuHB Introduction to ions An atom with an electric charge is called an ion. A positively charged ion (a cation) has more protons than electrons. A negatively charged ion (an anion) has more electrons than protons. https://cdn.kastatic.org/ka-youtube-converted/zTUnjPALX_U.mp4/zTUnjPALX_U.mp4 397 The atom https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:ions/a/ions 2026-05-11T04:40:42.021307954Z Learn and try: Ions Learn how atoms gain and lose electrons to form ions. article 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/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:ions/e/apply-ions 2026-04-28T11:00:37.315336883Z Apply: Ions Apply your understanding of ions in this set of free practice questions. Megan Cohn exercise 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 The atom https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-atom/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 The atom https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-atom/e/atomic-and-mass-number 2023-08-13T07:34:46.711956309Z Atomic and mass number Let's solve some questions! vibhor exercise 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 The atom https://www.khanacademy.org/science/hs-chemistry/x2613d8165d88df5e:atoms-isotopes-and-ions/x2613d8165d88df5e:isotopes-and-ions/v/hs_isotopes 2025-12-03T14:30:51.007256483Z Isotopes Isotopes are atoms of the same element with different numbers of neutrons. Since isotopes of an element have different numbers of neutrons, they also have different masses. The sum of the number of neutrons and protons in an atom is called its mass number. Isotopes can be represented in writing using isotope notation. Wieteke Holthuijzen video https://cdn.kastatic.org/ka_thumbnails_cache/df82de5e-1e56-4266-8fc4-45e02720aca1_1280_720_base.png Isotopes Isotopes are atoms of the same element with different numbers of neutrons. Since isotopes of an element have different numbers of neutrons, they also have different masses. The sum of the number of neutrons and protons in an atom is called its mass number. Isotopes can be represented in writing using isotope notation. https://cdn.kastatic.org/ka-youtube-converted/HDniYwGEl0o.mp4/HDniYwGEl0o.mp4 250 The atom https://www.khanacademy.org/science/cambridge-o-level-physics-cie/x0e04e0cb682fb793:nuclear-physics/x0e04e0cb682fb793:the-atom/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 The atom 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 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/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