Why do atoms combine?How elements bond

Atomic Structure and Chemical Bonds

Why do Atoms CombineAtomic 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 shellsfor a neutral atom, there is equal number of protons and electrons…. But later we will see that sometimes atoms aren’t neutral

Electron ArrangementNumber 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 stepsElectrons in the 1st energy level contains lowest

amount of energyElectrons farther away have greater energy levels and

are easier to remove from atomTo 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 shell2nd 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 configurationOn 3rd period on periodic table, it can

hold a maximum of 18, but only 8 can be in outer shellNa(sodium At # 11) .. 2 in first shell, 8

in 2nd energy level and 1 in outer shellPeriod 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 shellAll in group I (Alkali metals) have 1 electron in outer

shellAll 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)

HalogensGroup 17: called HalogensContain 7 electrons in outer shell, very

reactive, Atoms are always trying to have outer

energy shells filledEasier 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 MetalsAlkali metals have one electron in outer shellIt is easier to lose the one electron than to pick up 7

electronsLess 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 DiagramElectron Dot Diagrams show the elements and their

outer configurationBy 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 bondIonic Bonds… Loss and gain of electrons to

fill outer shellBonding 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 shellNa 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 FormationIonic bond: attraction which holds ions

close together and a compound is formed NaCl

Na+ Cl- NaCl

Mg++ O -- MgO

Si++++ Cl- SiCl4

Metallic Bonding--PoolingMetals can bond to nonmetals such

as NaCl MgO, LiF, MnO, KBrMetals can bond to other metals in

special bondingElectrons 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 formedGases in atmosphere form covalent

bonds Cl2

Single bond

Double and Triple BondsCarbon Dioxide CO2

N2

Covalent Bond: CO2

Total Number of electrons : 16

Carbon does not follow the octet rule unless double bonds are formed.

Polar and Nonpolar bondingAtoms 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 endsGives water the special characteristics it

has

Water Covalent Bond with polar bonds

Nonpolar MoeculesThese 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 ShorthandSymbols for atoms are comprised of one, two or

three lettersCan represent single letter C, O, NOthers 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 CompoundsUtilizes 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 FormulaChemical 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