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8th Chemistry of Matter Part I Chapters 14 & 15 Notes Page History of Atoms Demo: Aluminum Foil All material is made of particles, named atoms “cannot be divided” Scientists came to realize that all matter is made up of elements. An element is matter made of atoms of only one kind. Example: iron is made up of iron atoms. Democritus – Greek philosopher 440BC. Particles or smallest pieces cannot be divided any further. Atomos – “uncuttable”. John Dalton – English schoolteacher, early 19th century His idea about matter; 1. matter is made up of atoms 2. atoms cannot be divided into smaller pieces 3. all atoms of an element are exactly alike 4. different elements are made of different kinds of atoms He thought an atom was pictured as a hard sphere, that was the same throughout, something like a marble pg. 405 Figure 3 William Crookes, English scientist, 1870 did experiments to test Dalton’s theory. He developed a cathode-ray tube or CRT. He discovered that atoms have an Anode, which has a positive charge. He also discovered that atoms have a Cathode, which has a negative charge, J.J. Thomson, English physicist, 1897, he discovered that opposite charges attract each other. He named the negative Cathode charge, electron. He thought an atom was pictured as a ball of modeling clay with ball bearings mixed though. Pg. 408, Figure 8. He stated the positive charge of the clay is equal to the negative charge of the electrons. Therefore, the atom is neutral. Ernest Rutherford, 1906, discovered the positive charge of an atom which he called a proton was crammed into a small space in the middle of the atom, which he called the nucleus. He said the rest of the atom is empty space occupied by the atom’s smallest part, the almost massless electron. Twenty years later scientists discovered by using more modern experiments that atoms have neutrons in the nucleus that have a neutral charge. Pg. 411, Figure 13 Neils Bohr, physicist, twentieth century, he said that electrons will move around the nucleus, much like the moon orbiting around the Earth Later a new model of the atom was made, showing an electron cloud that electrons are constantly moving around in while the proton and neutron stay together in the nucleus. It was also determined that the electrons are more likely to be close to the nucleus rather than farther away because they are attracted to the positive charges of the proton. Pg. 413, Figure 15 Facts about atoms: 1. A sheet of paper is about 10,000 atoms thick. 2. There are 2,000 billion billion atoms of oxygen in one drop of water – and twice as many atoms of hydrogen. 3. Newspaper pictures are made from tiny dots of ink. Each dot contains about a billion billion atoms! Atoms of different elements contain different number of protons. The protons and neutrons stay together due to a strong nuclear force. The smallest of the atoms, the hydrogen atom has one proton in its nucleus. The heaviest of the atoms, Uranium, has 92 protons. When different elements are combined together a new element can be formed by the number of protons, neutrons and electrons changing positions. This can also change the charge of the element from positive to negative to neutral. This process creates energy and is called radioactive decay. When a proton leaves the nucleus and element changes into another element it is called transmutation. Loss and Gain of Particles; Alpha Particles – two protons and two neutrons undergo transmutation. This creates nuclear radiation. Beta Particles – a neutron becomes unstable and splits into a proton and an electron. The electron is released with a large amount of energy. The proton stays in the nucleus. Sharing of Particles; Metallic bonding – metal atoms share their pooled electrons. Covalent bond – a chemical bond between nonmetal atoms who share electrons. This forms a neutral particle with the same number of negative and positive charges. This new particle is called a molecule. Different Types of Elements Isotopes – atoms of the same element that have different number of neutrons. Pg. 415, Figure 16 Compound – is a pure substance containing two or more elements that are chemically bonded, ionic bond. Ion – an atom that is no longer neutral because it has lost or gained an electron. Molecule – compound that shares electrons. Periodic Table of the Elements Dmitri Mendeleev (men duh LAY uhf), Russian chemist, 1869, published the first version of the periodic table. Pg. 434, Figure 1. He arranged the elements in order of increasing atomic mass and saw a pattern. Atomic mass unit (amu) – measures the particles in an atom. The mass of a proton or neutron is about 1 mass unit. Example: Hydrogen 1.008 Elements with similar properties fell into groups on the table. Henry Moseley, English physicist, early twentieth century arranged periodic table according to atomic number instead of mass. Atomic number – number of protons in elements. Modern day Periodic Table Period – row of elements whose properties change gradually and predictably. Are labeled 1 – 7 Group – contains elements that have similar physical or chemical properties. Columns 1 – 18 Two sections – Representative elements - section 1, groups 1-2 & 13 -18 Transition and Inner transitions elements – section 2, groups 3 – 12 Assignment: #2 Families (Groups) of the Periodic Table Pgs. 436 – 437 Atoms By The Numbers – Bohr Chart Many atomic nuclei are stable when they have about the same number of protons and neutrons. Example; Carbon – 12 is the most stable isotope of carbon because it has 6 protons and 6 neutrons. Some nuclei are unstable because they have too many or too few neutrons. This causes repulsion to build up, the nucleus must release a particle to become stable. This will release energy called radioactive decay. This can also change the identity of the element. Assignment #3 Go over together, front page Examples on board; Students do back page independently Parts of an Atom Assignment #4 Go over together, front page Examples on board; backpage Bohr Models Electron Dot Diagram and the Ocet Rule Do front page together Examples on board – show whether to add or subtract according to octect rule