The Atom

Everything is made of atoms. Atoms are the smallest part of matter. Atoms are made of 3 subatomic particles: electrons, protons, and neutrons.

Nucleus Center of the atom Contains protons and neutrons Contains almost all of the mass of the atom

Our modern model of the atom comes from the contributions of several scientist over millennia.

Democritus (BC 460-370) Greek scientist proposed that there had to be a smallest part of matter, which he called atomos (indivisible) Indivisible atoms are hard spheres.

John Dalton 1808 Published a theory of the atom that had these important points: All atoms of a particular element are the same. Atoms of different elements have different properties, mass, and chemical reactivity. Atoms are not changed by chemical reactions, just rearranged in order or number.

J.J. Thomson 1897 Discovered electrons Knowing atoms were neutral, he thought that electrons were like negative plums in positive pudding. Plum pudding model:

Ernest Rutherford 1911 Discovered the nucleus shot alpha particles at gold foil, most but not all passed through – some were bounced back (positive nucleus) the atom is mostly empty space with a solid, central nucleus

Niels Bohr 1913 while studying light, realized that electrons can only exist in certain energy levels or orbitals

Modern Day Scientists have now split the atom and even split protons, neutrons, and electrons into even smaller particles called quarks.

Subatomic Particles “smaller than the atom”. Each particle has different properties and locations.

Protons Positively charged particles Located in the nucleus Determine what element as atom is

Neutrons Neutral particles Located in the nucleus Gives mass but no charge to the atom Determines the isotope

Electrons Negatively charged particles Located outside of the nucleus Can be lost or gained to and from other atoms Very small mass; 1/2000th of a proton Determines the ion

An atom is a single piece of an element that retains the element’s properties.

All elements are found on the periodic table of elements. An element has only one kind of atom. All elements are found on the periodic table of elements.

A molecule is any combination of two or more atoms. Not all molecules are compound.

A compound is any combination of two or more different atoms.

Beginning to Read the Periodic Table

Changes in Atoms Isotopes Ions When an atom loses or gains neutrons Everything else stays the same When at atom loses or gains an electron The atom becomes electrically charged