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Atomic structure and isotopes
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Our model of the atom changed as new evidence appeared. Isotopes explain why some atoms are radioactive.
The model of the atom
A tiny, dense, positive nucleus (protons and neutrons) surrounded by electrons in shells.
The nucleus is about 1/100 000 of the atom's diameter. Atoms are about 1 × 10-10 m across.
The nucleus is about 1/100 000 of the atom's diameter. Atoms are about 1 × 10-10 m across.
How the model developed
Dalton: solid spheres. Thomson: plum pudding (electrons in positive dough).
Rutherford's alpha scattering: most alpha particles passed straight through gold foil, but a few bounced back, so the atom is mostly empty space with a small, dense, positive nucleus.
Bohr: electrons in fixed shells.
Rutherford's alpha scattering: most alpha particles passed straight through gold foil, but a few bounced back, so the atom is mostly empty space with a small, dense, positive nucleus.
Bohr: electrons in fixed shells.
Isotopes and ions
Isotopes of an element have the same number of protons but different numbers of neutrons.
Electrons can move to a higher shell by absorbing EM radiation and drop back by emitting it.
An atom that loses an outer electron becomes a positive ion.
Electrons can move to a higher shell by absorbing EM radiation and drop back by emitting it.
An atom that loses an outer electron becomes a positive ion.
Carbon-14 has 6 protons. How many neutrons does it have?
- Mass number 14 = protons + neutrons
- 14 − 6
Answer: 8
Atoms have a tiny, dense, positive nucleus with electrons in shells. Rutherford's scattering showed the nucleus exists. Isotopes differ in neutrons. Electrons move between shells by absorbing or emitting radiation.
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