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Atomic structure
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Everything is made of atoms. Three tiny particles inside them explain how elements behave and react.
Inside an atom
A tiny nucleus holds protons and neutrons. Electrons move around it in shells.
Atomic number and mass number
Atomic number = number of protons. In an atom it also equals the number of electrons.
Mass number = protons + neutrons.
So neutrons = mass number − atomic number.
Sodium has atomic number 11 and mass number 23, so it has 12 neutrons.
Mass number = protons + neutrons.
So neutrons = mass number − atomic number.
Sodium has atomic number 11 and mass number 23, so it has 12 neutrons.
Isotopes and electron shells
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Electrons fill shells in order: up to 2 in the first shell, then up to 8 in the next two. Sodium (11 electrons) is 2.8.1.
Electrons fill shells in order: up to 2 in the first shell, then up to 8 in the next two. Sodium (11 electrons) is 2.8.1.
Chlorine is 75% chlorine-35 and 25% chlorine-37. Find its relative atomic mass.
- Multiply each mass by its percentage: 35 × 75 = 2625 and 37 × 25 = 925.
- Add: 2625 + 925 = 3550.
- Divide by 100: 3550 ÷ 100 = 35.5.
Answer: 35.5
Protons = atomic number. Neutrons = mass number − atomic number. Isotopes differ only in neutrons. Electrons fill shells 2, 8, 8.
The interactive lesson includes the diagrams for this topic.
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