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CHEM 1100 (7)
Chapter 3


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University of Guelph
CHEM 1100
Kim Bolton

CH. 3 ATOMS AND ELEMENTS Atoms • Atoms are the building blocks of the physical world • Compose all matter • Atom is the smallest identifiable unit of an element • There are 91 different elements in nature and 91 different kinds of atoms • Some elements in nature are inert  they do not tend to combine with other elements to form compounds  Ex. Helium and neon • Other elements in nature are reactive  they will form compounds with almost anything are not found in nature as pure elements  Ex. Fluorine and chlorine Protons Determine the Element • Atoms are composed of protons, neutrons, and electrons • The number of protons in the nucleus of an atom defines what element it is • The atomic number (Z) is the number of protons in the nucleus of an atom • Since the atoms that compose an element have a unique number of protons in the nuclei, each element has a unique number • Electrical charge is a property of both protons and electrons that causes them to exert forces on one another • Particles with like charges repel each other, and particles with opposite charges attract each other • The charge of a proton is assigned a numerical value of 1+ • The atomic number of an atom (Z) determines the nucleus’s charge • Protons constitute a large part of the mass of an atom • The mass of a proton is 1.0 atomic mass units (amu) • An amu is a unit of mass defined as 1/12 the mass of a carbon (12 nucleus and is equivalent to 1.67 X -24 10 g) • We denote each element by its chemical symbol, a one- or- two letter abbreviation for the element • All naturally occurring elements have probably been discovered • New elements-that don’t exist naturally-have been created by accelerating protons or neutrons into the nuclei of naturally occurring elements • The protons and neutrons are incorporated into the nucleus and elements of higher atomic number form • All known elements are listed in the periodic table in order of increasing atomic number CH. 3 ATOMS AND ELEMENTS Electrons • A neutral atom has as many electrons outside of its nucleus as protons inside of its nucleus • Electrons have a very small mass compared to protons (0.00055 amu) and are negatively charged • Electrons therefore experience a strong attraction to the positively charged nucleus • The charge of an electron is assigned a numerical value of 1 – and cancels the positive charge of a proton so that atoms containing equal numbers of protons and electrons are charge neutral • An atom can lose or gain one or more of its electrons  the charges of the electrons no longer exactly cancel the charges of the protons, and the atom becomes a charged particle called an ion • Positively charged ions (loses an electron) are called cations and are written with a superscript indicating the positive charge next to the element’s symbol • Negatively charged ions (gains an electron) are called anions and are written with a superscript indicating the negative charge next to the elements symbol CH. 3 ATOMS AND ELEMENTS • In nature, cations and anions always occur together, so that matter is charge neutral • Ex. Sodium (Na) which contains 11 protons and 11 electrons in neutral form, often loses one electron to + form the ion Na • Ex. Fluorine (F) which contains 9 protons and 9 electrons in neutral form, often acquires an extra – electron to form the ion F Neutrons • Atoms contain neutral particles called neutrons within the nuclei • Neutrons have almost the same mass as protons but carry no electrical charge • The number of neutrons in the atoms of an element can vary • Atoms with the same atomic number but different numbers of neutrons are called isotopes • Scientists can synthesize some isotopes that are not normally found in nature • The neutrons and protons in an atom account for nearly the entire mass of the atom • The sum of the number of neutrons and protons in then nucleus of a given atom is called its mass number and has the symbol A Specifying an Atom Subatomic Particles Mass (g) Mass Charge (amu) -24 Proton 1.6726 X 10 1.0073 1+ -24 Neutron 1.6749 X 10 1.0087 0 Electron 0.000911 X 10 0.000549 1- -24 A C Z X • Z= atomic number • A= mass number • C= charge • X= element • The charge is left out for charge neutral atoms • Because every element has a unique atomic number, the chemical symbol and the atomic number are redundant • A brief notation often used when referring to neutral atoms isX – A CH. 3 ATOMS AND ELEMENTS • X= chemical symbol • A= mass number • Protons = Z • Neutrons = A – Z • Electrons = Z – C Atomic Mass • Atomic mass the weighted average of the masses of each naturally occurring isotope for that element • Formula for atomic mass: Atomic mass = (fraction isotope 1) X (mass isotope 1) + (fraction isotope 2) X (mass isotope 2) + … • Percent abundances must be converted to fractional abundances by dividing by 100 The Periodic Law • Dmitri Mendeleev found when the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically • Niels Bohr links the macroscopic observation – that certain elements have similar properties that recur – to the microscopic reason – that the atoms comprising the elements have similarities that also recur A Theory That Explains the Periodic Law: The Bohr Model • Bohr Model a theory for the way electrons behave in atoms that explains the periodic law and links the macroscopic properties of elements to the microscopic properties of their atoms • Number of electrons outside the nucleus defines chemical behaviour • Electrons orbited at fixed distances from the nucleus  those fixed distances corresponds to fixed energies from the electrons (electrons closer to the nucleus have lower energy) • Orbit’s quantum number = n • The higher the quantum number, the greater the distance between the electron and the nucleus and the higher the electron’s energy • Orbits could only hold a maximum number of electrons • Bohr orbit with n=1 holds a maximum of 2 electrons • Bohr orbit with n=2 holds a maximum of 8 electrons • Bohr orbit with n=3 holds a maximum of 8 electrons CH. 3 ATOMS AND ELEMENTS • In an atom, an electron occupies the lowest energy orbit that is available • Bohr diagrams or electron configurations sh
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