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S.MORRIS 2006 Important Rule To Remember • There are 2 types of charges: positive and negative (think about a magnet.) • - Like charges (positive-positive or negative-negative) repel; • - Unlike charges (positive-negative) attract. What is an Atom? An atom is the smallest particle that has mass and makes up most types of matter.(They are too small to be seen by human eyes.) Every type of matter is made up of only one kind of atom. Example: every gold atom in gold looks exactly the same. Parts of the Atom • The Nucleus – The center area of an atom, it is where the most mass is found. It is considered to be positively charged, because of the protons (+) that are there (and stay there.) – There are also neutrons (neutrally charged) particles that add mass to the nucleus. PROTONS • Protons are small, positively-charged (+) particles inside the nucleus. Because they are the only charged particles in the nucleus, the nucleus has a positive charge. Because the protons remain in the nucleus, they make up the atomic number of the element. Electrons - Are tiny negatively charged particles (little mass) outside of the nucleus - They circle the nucleus because they are attracted to the positively charged nucleus. - Electrons are scattered (they can’ t get near each other) Electrons Because electrons are outside the nucleus, they can travel. If they are attracted to another nucleus, they join with another atom and cause chemical bonding. They can’t get near each other, but they are all attracted to the nucleus, so they circle the nucleus in pathways or shells. Electron Cloud • The area where electrons are located is called the electron cloud. This area is a negative energy field. Electron Shells Because electrons repel each other, you can only have so many per shell (path) before they get into each other’s negative fields. In the first shell (the smallest), there can only be 2 electrons. No matter how many electrons there are in an atom, the final shell can only hold up to 8 electrons. These are called Valence Electrons. Valence Electrons Because valence electrons are on the last shell, they are the ones that are furthest from the positive field of the protons. This means that the valence electrons could be attracted to the nucleus of more positive atoms. If the electrons join the valence electrons of other atoms, you have chemical bonding. ATOMIC STRUCTURE ctrons are arranged in Energy Levels or ells around the nucleus of an atom. The tire negative area is called the electron cloud. • first shell a maximum of 2 electrons • second shell a maximum of 8 electrons • third shell •fourth shell a maximum of 18 electrons a maximum of 32 electrons The History of the Discovery of the Atom HISTORY OF THE ATOM 460 BC Democritus develops the idea of atoms he pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called ATOMA (greek for indivisible) Law of Conservation of Matter • Lavoisier stated that matter is neither created or destroyed, it only changes form. HISTORY OF THE ATOM 1808 John Dalton suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them ATOMS HISTORY OF THE ATOM 1898 Joseph John Thompson found that atoms could sometimes eject a far smaller negative particle which he called an ELECTRON HISTORY OF THE ATOM 1904 Thompson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge like plums surrounded by pudding. PLUM PUDDING MODEL HISTORY OF THE ATOM 1910 Ernest Rutherford oversaw Geiger and Marsden carrying out his famous experiment. they fired Helium nuclei at a piece of gold foil which was only a few atoms thick. they found that although most of them passed through. About 1 in 10,000 hit the foil HISTORY OF THE ATOM helium nuclei gold foil helium nuclei They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back. HISTORY OF THE ATOM Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus. He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction However, this was not the end of the story. HISTORY OF THE ATOM 1913 Niels Bohr studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons. Bohr’s Atom electrons in orbits The Electron Cloud Model Section 2: Elements and the Periodic Table • An element is matter that is made up of only one kind of atom. • There are 115 known elements. • 90 of them are naturally occurring in the Earth, while others are synthetic (manmade). The Periodic Table • For many years scientists knew that there had to be a pattern among the atoms of elements, but no one could figure out a system that worked. • In 1869, Dmitri Mendeleev realized that, if the atoms of elements were arranged by the charged particle that doesn’t change numbers, the proton, that similarities could be seen. The Periodic Table • Mendeleev realized that, when atoms are arranged by the number of protons, then their electron shell configurations are similar (because atoms on the Periodic Table are in a “neutral” state.) • When the atoms are then arranged by their shells, Mendeleev realized that the chemical properties of each element in a column were similar. Column 1: Alkali Metals Column 18: Noble Gases How Elements Appear on the Periodic Table The most important number for each element is the atomic number, which is the number of protons that can be found in every nucleus for every atom of that element. The mass number is the number of protons plus neutrons in the nucleus. Chemical Symbols On the Periodic Table, elements are represented by a CAPITAL letter, or a capital letter with lower cap letters. The symbols represent the element’s name in Latin, so that is why some of the symbols don’t “match” the English term! (i.e., potassium = K) Reading the Periodic Table The columns (groups or families) show the electron configuration is similar, so they have similar physical and chemical characteristics. The rows, or periods, are divided into 3 main areas: the metals (the largest), metalloids, and non-metals. Atomic # increases Isotopes An isotope is when the nucleus of the atom has a different number of neutrons than protons. This is very common for most elements. Neutrons don’t affect the charge; they only affect how “large” (how much mass) the atom has. Isotopes Carbon 14 • A carbon 14 atom still has 6 protons, but it has 14 neutrons. • Because it is so large, it takes a long time to decay. • Carbon 14 decays at a steady rate (halflife), so it is easy to figure out when something died. Decay of Carbon 14 Atomic Mass Unit • If you were to calculate the actual mass of an element, you have to account for all the isotopes. • On the periodic table, all the protons and neutrons of the element are added up (including isotopes) and the average is found. • This number is rarely a whole number, due to averaging in isotopes. The exception are the synthetic elements. Atomic Mass Unit (AMU) The Noble Gases • The elements in the last family (column 18) are unique in that they already have 8 valance electrons. • This means these gases are found by themselves, as they cannot bond with other elements. • Because of this, we call them the Noble Gases. Electron Shells The N Level represents the last, or valance electrons. The magic number (the most this shell can hold) is 8. ATOMIC STRUCTURE There are two ways to represent the atomic structure of an element or compound; 1. 2. Electronic Configuration Dot & Cross Diagrams ELECTRONIC CONFIGURATION With electronic configuration elements are represented numerically by the number of electrons in their shells and number of shells. For example; Nitrogen 2 in 1st shell 5 in 2nd shell configuration = 2 , 5 2 + 5 = 7 N 7 14 ELECTRONIC CONFIGURATION Write the electronic configuration for the following elements; a) Ca 20 b) Na 40 2,8,8,2 d) Cl 17 35 2,8,7 11 23 c) 2,8,1 e) Si 14 28 2,8,4 O 8 16 2,6 f) B 5 11 2,3 DOT & CROSS DIAGRAMS With Dot & Cross diagrams elements and compounds are represented by Dots or Crosses to show electrons, and circles to show the shells. For example; X Nitrogen X X N XX X X N 7 14 Ions • Remember, atoms are considered “neutral” (same number of protons and electrons) when represented on the Periodic Table. • In reality, atoms often lose or gain electrons. This means the atom becomes either positively or negatively charged. Any charged atom (positive or negative) is an ion. Ions If an atom loses an electron, then it will have more protons than electrons. The atom will then be positively charged. If an atom gains an electron, then it will have more electrons than protons. The atom will then be negatively charged. How Sodium (Na) + Chlorine (Cl) becomes Table Salt The Periodic Table Song • http://www.youtube.com/watch?v=GFIvXV MbII0&safety_mode=true&persist_safety_ mode=1&safe=active