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Chapter 2

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Biological Sciences

My Notes Chapter 23Lecture 3 Material Chapter 21 What are the chemical elements that make up living organisms y Atoms all matter is composed of them and they are tinyeach atom consists of a dense positively charged y Nucleus One or more negatively charged electrons move around the nucleusThe nucleus contains one or more protons and may contain one or more neutrons y Electrons A subatomic particle outside the nucleus carrying a negative charge and very little mass A subatomic particle with a single positive chargeThe number of protons in the y Protonsnucleus of an atom determines its element y Neutrons One of the three most fundamental particles of matter with mass approximately 1 amu and no electrical charge y Mass measures the quantity of matter present the greater the mass the greater the quantity of matter the mass of a proton serves as a standard unit of measure y Atomic mass unit amuy Charges that are not alikeattract each other whereas charges that are alikerepel each other y Atoms are electrically neutral because the number of electrons in an atom equals the number of protons y Element is a pure substance that contains only one kind of atomy 98 of the mass of every living organism is composed of just six elements carbon hydrogen nitrogen oxygen phosphorus and sulphur y Atomic number is unique to each element and does not changey Mass number of an atom is the total number of protons and neutrons in its nucleus y In text immediately preceding the symbol for an element the atomic number is written at the lower left and the mass number at the upper left y Isotopes the same element all have the same definitive number of protons but differ in their number of neutronsis the average of the mass numbers of a representative sample of atoms of y Atomic weightthe element with all isotopes in their normally occurring proportions y Radioisotopes are unstable and spontaneously give off energy in the form of alpha beta or gamma radiation radioactive decay this release of energy transforms the original atom from the atomic nucleusthe region of space where the electron is found at least 90 percent of the time y Orbitaly Electron shells energy levels around the nucleusFirst shell the innermost electron shell consists of just one orbital called an s orbitalSecond shell the second shell contains four orbitals an s orbital and three p orbitalsHolds up to eight electronsAdditional shells elements with more than ten electrons have three or more electron shellsThe farther a shell is from the nucleus the higher the energy level is for an electron occupying that shell y The outermost electron shell the valence shell determines how the atom combines with other atoms
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