| A | B |
|---|
| atom | the smallest particle into which an element can be divided and still be the same substance |
| electrons | the negatively charged particles found in all atoms |
| nucleus | the tiny, extremely dense, positively charged region in the center of an atom |
| electron clouds | the regions inside an atom where electrons are likely to be found |
| protons | the positively charged particles of the nucleus |
| atomic mass unit | the SI unit used to express the measure of particles in atoms |
| neutrons | the particles of the nuceus that have no charge |
| atomic number | the number of protons in the nucleus of an atom |
| isotopes | atoms that have the same number of protons, but have different numbers of neutrons |
| mass number | the sum of the protons and the neutrons in an atom |
| atomic mass | the weighted average of the masses of all the naturally occurring isotopes of an element |
| ions | atoms with a positive or negative charge due to unequal numbers of protons and electrons |
| gravity | force that depends upon the mass of objects and their distance from one another |
| electromagnetic force | the force that holds electrons in their orbits around the nucleus |
| weak force | force that plays a key role in a neutron's changing into protons and electrons in unstable atoms |
| strong force | the force that counteracts the electromagnetic force so that protons stay together in the nucleus of an atom |
