Elements to Compounds NCSCOS 4.02 Big Picture Elements are pure substances with only ONE type of atom. Most matter is a combination of elements Atoms.

Elements to Compounds

NCSCOS 4.02

Big Picture

• Elements are pure substances with only ONE type of atom.

• Most matter is a combination of elements

Atoms elements molecules compounds

Vocab terms: compound, molecule, chemical formula, subscript, ionic bond, covalent bond

2.1D – ELEMENTS combine to form Compounds

• 1) 2 or more DIFFERENT elements

• 2) Held together by CHEMICAL BONDS

• 3) The type of ATOM & the BOND determine the properties

Compounds…..

Important characteristics of COMPOUNDS:1. have a definite composition2. can be broken down into simpler substances by

chemical means3. can be identified by their physical properties4. Compounds usually have very different properties

than the elements that compose themEX: Ca (soft, silvery metallic solid; needed by humans);

Cl (greenish-yellow gas; poisonous to humans) BUT..CaCl (nonpoisonous white solid used to melt ice on streets)

Molecules

Def: A group of atoms held together by a covalent bond.

Molecules: Compounds

Atoms: Elements

Compounds and molecules are often expressed as chemical formulas:

Chemical formulas= use chemical symbols and subscripts to show how many atoms of those elements are combined in the compound (RATIO!)

Chemical Formulas

Formulas are written with chemical symbols and subscripts which indicate the # of times that element appears in the molecule.

Elements without a subscript have an imaginary “1”

H2O: subscript indicates 2 hydrogen atoms and 1 oxygen atom

CO2: 1 carbon with 2 hydrogen

C3H8: 3 carbon with 8 hydrogen

C6H12O6: 6 carbon, 12 hydrogen, and 6 oxygen

Fill in the Chart (pls place in ntbk)

Compound Symbol and # of atoms

Total # of atoms

Na2SO4 Na=2,S=1,O=4 7

Al2O3

CaSo4

C2H6

Fe

NH3

SAME ELEMENTS…. DIFFERENT COMPOUNDS

• The SAME elements can be used to form VERY DIFFERENT compounds with DIFFERENT properties.

WATER vs HYDROGEN PEROXIDE

• H2O• 2:1• Clear• Colorless• Odorless• Needed for Survival

• H2O2

• 2:2• Clear• Colorless• Odorless• Thick, Syrupy• Kills Bacteria• Poisonous if ingested• Can be used as fuel

• NITROGEN & OXYGEN• Nitrogen Monoxide: By product of

combustion of substances in air, engine, fossil fuels, power plants, lightning

• Nitrous Dioxide: Brown toxic gas, major air pollutant, used in racing

• Nitrous Oxide: Used by dentist, surgery as mild anesthetic, greenhouse gas

NO ( 1N , 1O)

NO2 ( 1N , 2O)

N2O ( 2N, 1O)

N2O ~ Nitrous Oxide

Chemical Bonds

Chemical bonds are forces (the “glue”) that hold atoms together. 2 types:

1. Ionic Bonds: valence electrons transferred

2. Covalent bonds: valence electrons shared

IONIC BONDS

• Attraction between (+) & (-) ions = ionic bonds

• Ionic compounds are very stable & their crystals are very

strong

• Naming ionic compounds – Positive Ion always named 1st

• Negative Ion is named by dropping last part of name & adding the suffix “ide ”

• EX: Sodium Chloride (Na+Cl-)

PROPERTIES OF IONIC BONDS

IONIC BONDS….

*occur when metals react with nonmetals

* hard (think of a LEGO structure)

*brittle

*high melting & boiling points

*transfer of electrons

*conduct electricity when dissolved in water

EX: salt, concrete, sulfuric acid

COVALENT BONDS

• Electrons are SHARED…neither atom “loses or gains”…SO…NO IONS are formed!

• SHARED ELECTRONS are attracted to both positively charged nuclei

• EX: H2, N2, O2…..

• The # of covalent bonds an atom can form depends on the # of electrons available for sharing

• POLAR COVALENT: when electrons are shared unequally (polar-anything that has 2 extremes)

• EX: H2O

PROPERTIES OF COVALENT BONDS

• COVALENT BONDS……*electrons shared between nonmetallic elements

*not hard (think of a plastic ball pit)

*not brittle

*low melting & boiling points

*don’t conduct electricity well & usually don’t dissolve in water; are usually more flammable

EX: O2, CO2, methane, hydrochloric acid, candy corn

Atomic Theory

Proposed in 1808 by John Dalton.

1. All matter is composed of atoms

2. Atoms of a given element are identical

3. Atoms of different elements combine in simple, whole- number ratios to form chemical compounds