Chapter 2 Study Guide Dr. Joseph Silver

this chapter deals with - structure of atoms - how atoms form molecules - the periodic table - the control of chemical reactions

anything that has mass (made up of atoms) and occupies space (no matter how small) is called matter

the word atom was used 2000 years ago in Greece and we still are learning more about atoms today

we have not yet been able to actually clearly see an atom but scientists have done many experiments which show that one or more electrons (-) circle about a nucleus containing protons(+) and neutrons(0 no charge)

atomic number = number of protons in an atom atomic mass = mass of protons+neutrons+electrons electron mass is so small that it contributes very little to the mass of an atom

Atomic mass is also referred to as daltons the mass of a proton = 1 dalton the mass of a neutron = 1 dalton the mass of an electron = 1/2000 th of a Dalton so atomic mass is mainly the mass of protons and neutrons

remember mass represents the amount of matter in any atom, molecule or anything else weight represents the amount of force gravity exerts on a substance if you go to Mars your weight changes but your mass does not change

the nucleus of an atom has protons and neutrons packed into the center of the atom and the electrons are located in circular orbits (orbitals) or layers about the nucleus -electrons travel so quickly that we cannot see one -they do not fly off due to their attraction to the nucleus -atoms with large atomic numbers have many orbital layers

when protons are equal to electrons - neutral atom when protons do not equal electrons – atom is an ion if there are more protons than electrons – positive cation if there are more electrons than protons – negative anion

C12 = 6P + 6E + 6 neutrons C13 = 6P +6E + 7 neutrons C14 = 6P + 6E + 8 neutrons atoms of an element with different number of neutrons are called isotopes some isotopes release radioactive energy as the extra neutrons are released

isotopes have many uses in industry, medicine, chemistry, and biology one use is to date the age of fossils isotopes have a half life we know how long it takes for one-half of the energy in an isotope to disappear and we use this information to compare the amount of an isotope in new material and in a fossil and arrive at the age of the fossil

electrons determine the chemical behavior of atoms how when the outer electron shell of an atom is not full it will give away electrons, or get electrons from other atoms, or share electrons with other atoms in order to have the outer shell filled

the farther an electron is from the nucleus the greater is the energy of the electron likewise the closer to the nucleus the less energy in an electron

look at the drawings on pages 34 the first electron layer can have 2 electrons the second layer can have 8 electrons these 8 are made up 4 orbitals each with 2 electrons (see fig 2.8) for the purpose of this class any layer or orbital past the second orbital is filled by 8 electrons

the electrons in the outer shell of an atom are referred to as valence electrons and these are the electrons available for chemical reactions

-electrons are first added to the first shell (level) -when it is full electrons are added to the next shell -when the 2 nd shell is full electrons are added to the 3 rd and so on for the 90 naturally occurring elements as well as the laboratory manufactured elements

an atom with 1 electron in its first outer shell would love to give it away or gain one so that the outer shell a would be complete

an atom with 7 electrons in the outer shell would love to find an atom to give it 1 electron and an atom with 6 electrons in the outer shell would love to find an atom to give it 2 electrons or 2 atoms to each give it 1 electron

in atoms with multiple shells gaining electrons can fill the last orbital level or giving away electrons can empty the last shell and the lower level shell when it is filled becomes the outer layer see fig 2.12 page 38

on page B-1 there is a full periodic table showing the 90 natural elements as well as 28 man made elements and on page 34 there is a drawing showing the first 18 elements

in the periodic table the elements are arranged according to their atomic number (the number of protons or electrons) they are arranged in columns according to the number of electrons in the outermost orbital shell

the electrons in the outer shell are called valance electrons and they are located in the valence shell these are the electrons which are involved in chemical reactions

energy provides the ability to do work when an electron is released from its valence shell the next lower orbit becomes the outer orbital shell and the released electron releases electron or atomic energy which is used to power chemical reactions

look at the periodic table in the textbook -put a 1 over the H -put a 2 over Be -put a 3 over B -put a 4 over C -put a 5 over N -put a 6 over O -put a 7 over F -put an 8 over He the number represents the number of electrons present in the outer shell of the elements in the column (H,Li,Na,K,Rb,Cs,Fr all have 1 electron in the outer shell) (Be,Mg,Ca,Sr,Ba, Ra all have 2 electrons in the outer shell)

an element with 1 electron in the outer shell can readily interact with an element having 7 valence electrons an element with 1 valence electron can also readily interact with elements having 6 valence electrons by exchanging or sharing electrons they fill the outer shells and form compounds with stable atoms

ionic bonds form when atoms exchange electrons and become positive and negative ions which are attracted to each other and form a new molecule or compound look at fig 2.9 sodium gives up its 1 outer electron chlorine adds 1 electron to its 7 and fills the shell sodium becomes positive by giving away an electron chlorine adds an electron and becomes negative the + and – ions attract to become sodium chloride

carbon has 4 electrons in its outer shell two carbons share their outer 4 electrons making an outer shell in each atom with 8 electrons thus each has a full outer shell with 8 electrons forming a covalent bond the electrons travel so fast that it is as if the 8 electrons are in both atoms at the same time

when the atoms forming covalent bonds are similar in size the bonds may be neutral but if one atom is much larger than the other then the covalent bond may show a degree of polarity (one end of the molecule may be more positive or negative than the other end of the molecule)

the ability of atoms to form chemical bonds is influenced by - temperature (2x for every 10C) - concentration -catalysts (enzymes) - pressure (keeps atoms closer together)

I will on many occasions mention the shape of a molecule why because molecules cannot talk to each other molecules cannot use a phone to tell you something so molecules react according to their shape they fit together like pieces of a puzzle