Atomic Structure

3. The historical evidence for atoms was based largely on indirect measurements and inferences far removed from direct experience. • Recently, instruments have been built that produce images of individual atoms, confirming what was inferred earlier as a result of overwhelming evidence from many scientific experiments. • Most scientists come to know the atomic theory is true by repeatedly using the concepts and principles presented in the theory to explain observed properties and predict changes in matter.

4. Atomic Structure • Atoms are mostly empty space • the center of an Atom is a tiny, dense nucleus • The nucleus consists of two types of particles; protons & neutrons • Protons have a positivecharge • Neutrons have a neutral charge • Electrons have a negative charge • Electrons orbit around the nucleus in a ‘cloud’ • Each electrons has about 1/2,000 the mass of a proton or neutron • Each electron has definite energy • The energy of each electron in an atom keeps it in motion around the positive nucleus to which it is attracted • The structure of multi-electron atoms is understood in terms of electrons filling energy levels that define orbitals.