Atomic Theory Chapter 17 • Atom: the smallest particle which an element can be divided into and still be the same • The theory has been around for over 2000 years. But no one saw an atom until 1981 Theories • Democritus proposed the idea of the atom around 440 BC, called the smallest particle atomos meaning indivisible • Aristotle thought he was wrong so Democritus got • 1700’s scientists knew elements combine in specific proportions based on mass Dalton • 1700’s scientists knew elements combine in specific proportions based on mass • Not all of this was exactly correct • John Dalton published his atomic theory in 1803 • 3 parts • All substances are made of atoms. Atoms cannot be created, divided or destroyed • Atoms of the same element are exactly alike • Atoms join with other atoms to form new substances Plum Pudding • J.J. Thomson discovered the electron in 1897, revealing that atoms were made of smaller particles • Plum pudding model: the atom is mostly positively charged with electrons scattered throughout. Nucleus • Ernest Rutherford: 1909 discovered the nucleus and the proton • Most of the atoms mass is in the nucleus. • Protons and neutrons are found in the nucleus • Rutherford calculated that the nucleus must be 100,000 times smaller then the diameter of the atom • The atom is mostly empty space • Neutron discovered in 1932 by Chadwick • Electron- charge = -1 • Mass = 9.10938188 × 10-31 kg • Proton = charge =+1 • Mass = 1.67262158 × 10-27 kg • Neutron = charge = 0 • Mass = 1.67492716 × 10-27 kg • The electron can’t go half way – Can you stand between the rungs of a ladder? • Electrons exist in energy levels • Often called orbits but they aren’t really • Higher the orbit – the higher the energy • As they change levels they absorb or give off energy in exact amounts • These are called photons • The number of electrons is equal to the number of protons in a neutral atom Electron cloud • Electron exist in a cloud surrounding the nucleus • we can only predict about where something is- the exact path can’t be predicted if we try crazy things happen Elements • The atomic number is the number of protons in the atoms nucleus • The number of protons determine the element and the properties Atomic mass • Atomic mass- the sum of the protons and neutrons in an atom • Average atomic mass- weighted average of all of the naturally occuring isotopes AMU • Amu- atomic mass unit • amu’s are used to make adding the masses of elements easier • Proton = 1 amu • Neutron = 1 amu • Electron = 0 amu Isotopes • An isotopes are atoms with the same number of protons but different numbers of neutrons. • Most elements have several naturally occuring isotopes Quarks • Discovered in 1968 • 6 types • Top, bottom, up, down, strange, and charm • Top quark discoved in 1995 • Dmitri Medeleev-Father of the modern periodic table. – Mendeleev’s scheme was based on chemical properties of the elements. – It was noticed that the chemical properties of elements increased in a periodic manner. – The periodicity of the elements was demonstrated by Medeleev when he used the table to predict to occurrence and chemical properties of elements which had not yet been discovered. • Mendeleev left blank spaces in his table when the properties of the elements above and below did not seem to match. The existence of unknown elements was predicted by Mendeleev on the basis of the blank spaces. When the unknown elements were discovered, it was found that Mendeleev had closely predicted the properties of the • Introduction – The periodic table is made up of rows of elements and columns. – An element is identified by its chemical symbol. – The number above the symbol is the atomic number – The number below the symbol is the rounded atomic weight of the element. – A row is called a period – A column is called a group • Periodic Patterns – The chemical behavior of elements is determined by its electron configuration – Energy levels are quantized so roughly correspond to layers of electrons around the nucleus. – A shell is all the electrons with the same value of n. • n is a row in the periodic table. – Each period begins with a new outer electron shell – Each period ends with a completely filled outer shell that has the maximum number of electrons for that shell. – The number identifying the A families identifies the number of electrons in the outer shell, except helium – The outer shell electrons are responsible for chemical reactions. – Group A elements are called representative elements – Group B elements are called transition elements. Lewis Dot Structure- Element symbol with outer electrons shown as dots surrounding the symbol Metal: Elements that are usually solids at room temperature. Most elements are metals. Non-Metal: Elements in the upper right corner of the periodic Table. Their chemical and physical properties are different from metals. Metalloid: Elements that lie on a diagonal line between the Metals and non-metals. Their chemical and physical properties are intermediate between the two. – When an atom or molecule gain or loses an electron it becomes an ion. • A cation has lost an electron and therefore has a positive charge • An anion has gained an electron and therefore has a negative charge. • Octet Rule - Atoms prefer to gain or lose electrons to fill their outer shells, usually with 8 electrons – Elements with 1, 2, or 3 electrons in their outer shell tend to lose electrons to fill their outer shell and become cations. • These are the metals which always tend to lose electrons. – Elements with 5 to 7 electrons in their outer shell tend to gain electrons to fill their outer shell and become anions. • These are the nonmetals which always tend to gain electrons. – Semiconductors (metalloids) occur at the dividing line between metals and nonmetals. Indicated by a staircase line EXAMPLE What would the charge be on a sodium ion? Since sodium in in Group IA it is a metal and so would LOSE an electron You can tell how many would be lost by the group number Group 1A elements lose 1 electron So the charge would be +1 Remember an electron is negatively charged. When you lose them atom becomes positively charged… when you gain them it becomes negatively charged EXAMPLE How would you right the symbol for the sodium CATION? Na +1 How many outer electrons does sodium have before it loses one? It has 1…remember the group number!