Atomic Structure and Chemical Bonds Why do atoms combine? How elements bond

Why do Atoms Combine Atomic Structure Center of atom is nucleus where protons and neutrons found (almost all of atoms mass is found here) Electron cloud positive nucleus (protons) surrounded by a cloud of negative (electrons) path of electrons are not predictable, rather can be found in predictable shells for a neutral atom, there is equal number of protons and electrons…. But later we will see that sometimes atoms aren’t neutral

Electron Arrangement Number and arrangement of electrons in the electron cloud are responsible for physical and chemical properties Electron energy: position of electrons are found closer to nucleus first, then in shells farther away from nucleus Only certain number of electrons can be found in each shell (or energy level) First shell contains maximum of 2 electron (closest to nucleus) 2nd shell holds maximum of 8, 3rd, maximum of 18, fourth maximum of 32) Have sub shells in 3rd shell and up, with maximum in sub shells of 8

Energy steps Electrons in the 1st energy level contains lowest amount of energy Electrons farther away have greater energy levels and are easier to remove from atom To calculate the maximum number of electrons found in an energy level use the formula 2n2 ( n equals the number of the energy level) It takes less energy to remove electrons that are farther away from nucleus, than those closer to nucleus

Periodic Table and Energy Levels You can determine the number of electrons in an atom by looking at its atomic number Number of electrons equal the number of protons in a neutral atom

Electron Configurations As you move left to right in same row on periodic table, you notice the atomic number increases by one 1st period contains H and He.. H has one electron in outer shell and He has 2 electrons which fills its outer shell 2nd period Lithium (atomic # 3) has 2 electrons in first shell and 1 electron in second shell Be (At # 4) has 2 electron in first shell, 2 in second shell B has 3 electrons in outer shell, C has 4, thru Ne which has 8 electrons and is full… it is stable

Electron configuration On 3rd period on periodic table, it can hold a maximum of 18, but only 8 can be in outer shell Na(sodium At # 11) .. 2 in first shell, 8 in 2nd energy level and 1 in outer shell Period 3 ends with Ar which has 8 electrons in outer shell and is stable(doesn’t combine with others)

Magnesium configuration

Element Families Groups or Families are represented by columns on periodic table depending on the number of electrons in outer shell All in group I (Alkali metals) have 1 electron in outer shell All in group II (Alkaline earth metals) have 2 Noble gases (group 18) are stable containing 8 electrons and are considered inert gases (completely unreactive) Noble gases when pass an electric current through a vacuum bulb, emit light of various colors (neon light)

Halogens Group 17: called Halogens Contain 7 electrons in outer shell, very reactive, Atoms are always trying to have outer energy shells filled Easier for Fluorine to pick up 1 electron to fill its outer shell than to lose 7 electrons, so it looks for an extra electron to fill its outer shell F is most reactive atom in halogen group

Alkali Metals Alkali metals have one electron in outer shell It is easier to lose the one electron than to pick up 7 electrons Less energy is needed to remove electron farther away from nucleus, so Fr is most reactive atom in group 1

Electron Dot Diagram Electron dot diagram is a shorthand showing the element and the number of electrons in outer shell You need to know the symbol and the number of electrons in outer shell Dots represent the number of electrons but they are written in specific order Write 1 dot on top, next dot to right, bottom and to left…. Fifth dot pairs at top, then fills as electrons are added reaching 8 total

Using Dot Diagram Electron Dot Diagrams show the elements and their outer configuration By knowing what electrons would be needed to fill outer shell you can determine its ability to bond to other atoms Chemical bonds is the force that holds two atoms together This allows atoms to be more stable and resemble noble gases

How elements bond Ionic Bonds… Loss and gain of electrons to fill outer shell Bonding of atoms into compounds depend on outer shell being filled… Some atoms loose electrons, some gain electrons to become stable with 8 electrons in outer shell Na is soft metal, very reactive, has one electron in outer shell Cl has 7 electrons, very reactive, posonous gas Cl will receive an electron from Na to become stable Thus electrons are gained or lost to reach stable configurations

Ionic bonding of Sodium and Chlorine

Ions Ions form when atoms loose or gain electrons and overall charge of atom is not neutral Atoms which loose electrons (negative charge) become positive ions Atoms which gain electrons become negative charged ions

Loosing and Gaining Electrons

Bond Formation Mg++ O -- MgO Si++++ Cl- SiCl4 Ionic bond: attraction which holds ions close together and a compound is formed NaCl Na+ Cl- NaCl Mg++ O -- MgO Si++++ Cl- SiCl4

Metallic Bonding--Pooling Metals can bond to nonmetals such as NaCl MgO, LiF, MnO, KBr Metals can bond to other metals in special bonding Electrons aren’t held to other atoms, rather they move freely among all the ions in the metals which allow you to hammer, or stretch certain metals and allows conductivity of electric currents to flow

Covalent Bonding: Sharing of electrons Rather than gaining or losing electrons, some atoms share electrons to form molecules (compounds are formed from ionic bonding) Shared electrons are attracted to the nuclei of both atoms No ions (charged particles) are formed Gases in atmosphere form covalent bonds Cl2 Single bond

Double and Triple Bonds Covalent Bond: CO2 Total Number of electrons : 16 Carbon does not follow the octet rule unless double bonds are formed. Double and Triple Bonds Carbon Dioxide CO2 N2

Polar and Nonpolar bonding Atoms of some molecules can share electrons unevenly and cause one side of bond to have greater negative and positive pulls thus called polar covalent bonds like water H2O O end of water molecule has slight negative charge and the H end has a slight positive charge (give molecule two opposite ends like a magnet) Positive ends of H attracts the O ends Gives water the special characteristics it has

Water Covalent Bond with polar bonds

Nonpolar Moecules These are covalent bonds that do not have uneven charges and differ slightly in its ability to attract electrons The triple bond of gases N2 is an example

Chemical Shorthand Symbols for atoms are comprised of one, two or three letters Can represent single letter C, O, N Others represent elements name in another language K (latin for kalium meaning potassium) First letter is capitalized second letter is small letter , Mg, Co

Symbols for Compounds Utilizes symbols of atoms in compounds and the number of each atoms within the compound or molecule NaCl refers to one atom of sodium and one atom of chlorine H2O: 2 atoms of hydrogen and one atom of oxygen (the 2 represents a subscript indicating number of H atoms)

Chemical Formula Chemical formula is a combination of chemical symbols and numbers that show which elements are present in a compound and how many atoms of each element are present Ag2S, NH3, H2SO4, HCl, HNO3, Na2CO3