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Chapter 5 Atomic Structure & the Periodic Table 5.1 Atoms are the smallest form of elements. • 2400 years ago, Greek philosophers believed everything was made up of air, fire, water, and earth. Everything else was a mixture of these 4 substances. • Today, we know there are about 100 basic substances, known as the elements. • Sometimes, elements can be found by themselves. Most often they are found combined with other elements to make new substances. Atoms in the Earth & Living Things • Hydrogen atoms make up about 90% of the mass of the universe. Humans Earth's Crust 12% 23% 47% 28% Oxygen Oxygen Iron Nitrogen Silicon Other 8% 5% Other Aluminum 10% 3% 3% 61% Hydrogen Carbon Element Names • Magnesium = region in Greece known as Magnesia • Lithium = Greek word lithos, meaning stone • Neptunium = after the planet Neptune • Hydrogen (H), Sulfur (S), Carbon (C) • Gold (Au), Lead (Pb), Iron (Fe), Copper (Cu) = symbols come from latin names. Dalton’s Atomic Theory • British scientist, John Dalton, proposed that each element is made of tiny particles called atoms. • All of the atoms in a particular element are identical but are different from atoms of all other elements. • Atoms cannot be divided into anything simpler. (This was found to be untrue when protons, neutrons & electrons were discovered). Structure of Atoms • Atoms can either be positively or negatively charged. • Particles with the same type of charge repel each other. • Particles with different charges attract each other. • Atoms are composed of protons, neutrons & electrons. The Atomic Model Uncharged; inside nucleus Same Mass Contains protons & neutrons; positively charged Positive Charge; inside nucleus Negative charge; move around outside nucleus (electron cloud) . Millions of atoms could fit in a space the size of this dot. It would take you 500 years to count all the atoms in a grain of salt!! Relative Sizes • The electron cloud is about 10,000 times the size of the nucleus. • Electrons are MUCH smaller than protons or neutrons • Electrons remain evenly spread out in the electron cloud because they repel each other. However, they are attracted to the nucleus because it is positively charged. • Neutral atoms have no overall electrical charge because they have an equal number of protons and electrons. Atomic Number • The identity of an atom is determined by the number of PROTONS in its nucleus. This is called the ATOMIC NUMBER! • Hydrogen is atomic number 1. Therefore, it has one proton in its nucleus. • How many protons does Lithium have in its nucleus? – Nitrogen? – Gold? Atomic Mass • Atomic Mass = the total number of protons and neutrons in an atom’s nucleus. • Atoms of a certain element always have the same number of protons. However, they don’t always have the same number of neutrons. Therefore, not all atoms of a certain element have the same atomic mass. Isotopes • Isotopes are atoms of the same element that have a different number of neutrons. • Ex: Chlorine – All chlorine atoms have 17 protons. – Some chlorine atoms have 18 neutrons and some have 20 neutrons. These are chlorine isotopes. • Isotopes have different numbers of neutrons! How to find the number of neutrons in a particular isotope. Atomic Mass – Atomic Number = # of neutrons # of protons & neutrons # of protons Ions • An ion is formed when an atom loses or gains one or more electrons. • Since the number of electrons in an ion is different from the number of protons, an ion does have an overall electric charge. • See pgs. 142 & 143