Atomic Structure - Two Observable Facts made in the 1800’s Law of Conservation of Mass –Matter is neither created nor destroyed in a chemical reaction Law of Constant Composition –The elements present in a compound are present in a fixed and exact proportion by mass

Dalton’s Atomic Theory From 1803 to 1808 an English schoolmaster named John Dalton came up with a very good Atomic Theory. –All matter is composed of atoms (Dalton thought they were indestructible) Atoms are the building blocks of matter –Atoms of one element are identical

–Atoms of different elements can be distinguished by their different masses –Compounds are combinations of atoms of different elements and possess properties different from those of their component elements –In chemical reactions, atoms are neither created nor destroyed but only exchanged between starting compounds to form new compounds Dalton’s Atomic Theory

Questions: How is a Hydrogen atom different from a Helium atom? Is there something that makes them different? We need to look at their structure to understand the difference

The Structure of Atoms Atoms aren’t the smallest particles of matter They are made up of three types of particles –Proton - a positively charged particle –Neutron - a neutral particle –Electron - a negatively charged particle (much lighter than a Proton or Neutron)

The nuclease is at the center of the atom and contains the protons and neutrons - it is very small compared to the atom –A pea at the center of the WSU football stadium is about the same ratio The electrons occupy the rest of the space of the atom A picture of the Atom

The charge of a proton is exactly opposite of the charge of an electron –The proton is +1 and the electron is -1 The number of protons in an atom is unique to each element and is called the atomic number An atom having a net electrical charge is called an ion. –A positively charged atom is a cation –A negatively charged atom is an anion The Charge of the Atom

Atomic Mass Each atom has a fixed mass Because the carbon 12 isotope is stable and common we use it as a standard and assign it a mass of exactly Everything else is compared to this carbon- 12 atom –Hydrogen is on this relative scale

Isotopes –Atoms of the same element have the same number of protons (atomic number) –The number of neutrons in an atom can vary a family of elements with atoms having the same chemical properties but slightly different masses. These atoms with differing masses are called isotopes –An isotope is identified by its mass number: the sum of the number of protons and the number of neutrons it contains

checksome Write the element (with atomic mass and atomic number) and charge for each –4 protons, 5 neutrons, 2 electrons –26 protons, 29 neutrons, 26 electrons –9 protons, 10 neutrons, 10 electrons –80 protons, 121 neutron, 78 electrons