Atomic Structure and the Periodic Table Test on Feb. 21, 2014

The First Atom Democritus first proposed the idea that atoms make up all substances around 400 BC. He said matter cannot be cut into smaller pieces indefinitely. There must be a point at which you reach the smallest piece. He called the smallest pieces “atoms” which means “not to be cut”

Dalton’s Atomic Theory John Dalton developed some ideas about the atom based on observation in the early 1800’s. All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical. Atoms can neither be created or destroyed. Atoms can be combined to form compounds. Solid Spheres

Plum Pudding Model JJ Thompson discovered the electron in 1897 with the Cathode Ray Tube experiment. This meant the atom was made of smaller particles. He knew that the electron had a negative charge, but the overall atom was neutral. He developed the Plum Pudding Model of the atom – a positively charged atom with negatively charged electrons stuck inside.

Rutherford’s Gold Foil Experiment Ernest Rutherford discovered in 1911 that the atom is mostly empty space and that most of the mass is concentrated at the center of the atom.

Bohr Model Neils Bohr refined Rutherford’s model in 1913. He discovered that The electrons orbit the nucleus without losing energy The electrons only orbit in certain areas Electrons with high energy orbit farther from the nucleus and electrons with low energy orbit close to the nucleus.

The Neutron James Chadwick discovered the nucleus in 1932. This explained how you can have two version of an element – a radioactive one and a non-radioactive one.

Electron Cloud Model The current atomic model is called the Electron Cloud Model Instead of the electrons orbiting the nucleus on defined paths, the electrons are guaranteed to be in an undefined region around the nucleus. They are moving so fast they appear as a blur.

Quarks Protons and Neutrons are much bigger than electrons. Scientists hypothesized that protons and neutrons were made up of even smaller particles called quarks. Six different quarks have been discovered. Three quarks make up a proton and three quarks make up a neutron.

Finding Quarks Fermi Accelerator (Illinois) Electrons are accelerated to very high speeds. They then collide with protons. This causes the protons to break apart.

Atomic Number An element is classified by how many protons it has. Every element has a unique number of protons. The number of protons an atom has is called the atomic number.

Ions and Isotopes Normally, an atom has the same number of protons, electrons and neutrons. If an atom has a different number of electrons than protons, the atom is an ion. Ions have an electric charge because their positives and negatives don’t cancel out. If an atom has a different number of neutrons than protons, the atom is an isotope. The sum of neutrons and protons is called the mass number.

Charge = # of protons – # of electrons Electric Charge An atom has a charge if the number of protons and electrons are not equal. An atom with a charge is called an ion. To find the charge: Charge = # of protons – # of electrons

Ions and Isotopes If an atom is an ion, it will be written with the charge in the upper right hand corner. Cl-1 If an atom is an isotope, it will be labeled with the mass number Boron – 10

Atomic Mass The atomic mass and the mass number are not the same thing. The atomic mass is the average weighted mass of all the atoms of a certain element. The unit for atomic mass is amu (atomic mass unit). The atomic mass can be found on the periodic table.

Calculating Atomic Mass The atomic mass is the weighted average. Average = % occurrence x mass + % occurrence x mass + …