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| a form of electromagnetic radiation of shorter wavelength than light and capable of penetrating solids and of ionizing gases |
| the attractive effect of matter on other matter; the region surrounding an astronomical body in which the force of gravitation is strong |
| atomic structure | |
| —n | |
| the concept of an atom as a central positively charged nucleus consisting of protons and neutrons surrounded by a number of electrons. The number of electrons is equal to the number of protons: the whole entity is thus electrically neutral | |