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 nucleus 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 Bernabeu 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 set mass to it Because Carbon is so prevalent in life we use it as a standard and assign the carbon-12 isotope a mass of 12. Everything else is compared to Carbon-12 –Hydrogen is 1.008

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

Group Work 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

The Periodic Table - History –Two scientists, Dmitri I Mendeleev (Russia) and Lothar Meyer (Germany) properties of elements did not change smoothly with increasing atomic mas. Instead the properties of the elements repeated periodically. –Periodic Law: the properties of the elements repeat periodically as the elements are arranged in order of increasing atomic number (# of protons) –This periodic law is used to form the Periodic Table

The Periodic Table - Categories Three Categories: Metals, Non-metals, Metalloids

–The elements in the same vertical column are called a group and have similar chemical properties Nobel Gases - Neon Lights, Lasers Halogens Details of the Periodic Table - Group Chlorine (Cl), Bromine (Br) and Iodine (I)

Details of the Periodic Table - More Groups Alkali earth metals Main-Group Elements Transition Metal Elements Lanthanide and Actinide Elements Alkali metals Potassium (K), in Water (H 2 O)

The elements in the same horizontal row are called a period Details of the Periodic Table - Period Hydrogen and Helium are in period 1 Lithium through Neon are in period 2 4