What is an atom?

An atom is the smallest unit retaining an element’s chemical identity. It consists of a positively charged nucleus and electrons occupying quantum states around it. Everyday materials depend on the kinds of atoms present, their electronic arrangements and their interactions. An atom is not an indivisible solid sphere; it has smaller constituents.

What determines atomic structure and identity?

1. Nucleus and electrons The nucleus contains protons and, in most atoms, neutrons. It holds most of the atom’s mass. Electrons carry negative charge and should not be pictured as planets following sharply defined paths. Orbitals describe probability distributions for finding electrons in particular energy states. 2. Atomic number and elements The proton count is the atomic number and determines the element. An atom with six protons is carbon; changing that count produces a different element. A neutral atom has equal numbers of electrons and protons. Gaining or losing electrons can form an ion without changing elemental identity. 3. Isotopes Atoms of one element with different neutron counts are isotopes. Mass number is the sum of protons and neutrons. Some isotopes are stable and others radioactive. For most elements, the atomic weight displayed in the periodic table averages naturally occurring isotopes according to their abundances.

How do atoms form materials?

4. Electrons and bonding Outer electrons strongly influence chemical behavior. Electron sharing and attractions between charges produce different bonds. The resulting substance can have properties very different from those of its constituent elements in isolation. 5. Testing atomic models Scattering experiments, spectra and advanced microscopy reveal atomic structure. Colored textbook spheres are explanatory models rather than photographs to scale. A useful model need not show every detail. 6. Chemical and nuclear changes Chemical reactions mainly rearrange electrons while generally preserving nuclei. Nuclear processes can alter the nucleus itself. This distinction explains why combustion and radioactive decay are different kinds of events.