Elements, Compounds, Bonds

Elements, Compounds, Bonds

Matter How is it organized?

Elements, Atoms & Compounds • Element: simplest type of matter with unique

chemical properties – 92 elements occur naturally

• Atom: Smallest particle of an element that has the chemical characteristics of that element

• Compound: Substance containing two or more elements in a fixed ratio

Atoms

• Composed of three subatomic particles: – protons (+) & neutrons (0)

make up the nucleus – electrons (-, e-) in orbitals

(clouds)

Periodic Table

Atoms

• Mass of atom = summed mass of its protons and neutrons – 1 proton = 1 atomic mass unit

or amu – 1 neutron ~1 amu

• Mass = Atomic weight

Summary of subatomic particles

Proton Neutron Electron Mass (amu) 1 ~1 ~0 Charge + 0 - Location Nucleus Nucleus “Orbitting”

nucleus

Approximate atom

electron (-)

proton (+) neutron (0) nucleus

Pea = nucleus

Atoms • Stable atoms always have same # protons. #

protons = atomic # – Carbon [C] = 6

• Atoms may have differing #’s of neutrons. # protons + # neutrons = Atomic weight (mass number) – 12C has 6 protons and 6 neutrons – 13C has 6 protons and 7 neutrons

Periodic Table

Importance (by weight)

Valence and reactivity

• Remember: electrons are arranged in valence shells or energy levels that “orbit” the nucleus.

• # of electrons in atoms influence their reactivity, relative to other atoms.

2 electrons

8 electrons 2 electrons

Atoms with unfilled electron shells are reactive…

• Atoms are “greedy” (want full e- orbitals) and “lazy” (don’t want to work hard to achieve them)

• Results in formation of chemical bonds • Three types of bonds

– Ionic bonds – Covalent bonds – Hydrogen bonds

Ionic bonds

• Ionic bonds form between anions (atoms that stripped an e- from another atom) and cations (atoms that lost an e-)

• Salts form by ionic bonding • Basically, a strong electrostatic attraction

Covalent bonds

• Covalent bonds form when atoms share e-. – Single or double

• H-H (H2) • O=C=O (CO2)

• # of bonds formed = # of e- needed to fill outer shell

Non-polar Covalent bonds

• Equal sharing of e- – Nuclei of similar size

have similar elecronegativity: pull on shared electrons in a covalent bond

Polar Covalent bonds • Form due to unequal

sharing of e- • Relative size and

electrical attraction of nuclei differ

• Partial + and partial – sides of molecule result

Hydrogen bonds

• Weak attraction between opposite charged poles of polar molecules

• IMPORTANT bond – determines structure of large,

complex molecules – Produces cohesion (water

droplets)

Water is a solvent • It’s polar; slides

between inorganic compounds bound by ionic bonds; dissociation

• Dissolves ions • Dissolves organic

molecules with charged parts (hydrophilic; “water loving”)

Lubricates

• H-bonding causes water molecules to adhere to each other – Creates thin, but

unbroken film – Perfect for coating and

lubricating moving parts

pH of common stuff

• Acids donate H+ ions • Bases accept H+ ions • Pure water and human

blood are neutral • Humans run a little

acidic

Hydrogen bonds give H20 it’s amazing properties

Structural bonds

• Responsible for secondary structure of proteins

• α-helix • β-pleated sheet

Allow water transport • H-bonding creates surface tension between

water molecules • As H20 evaporates, it pulls “attached” H20

molecules along with it

Ionic & covalent bonds form: • Molecules: atoms held together by covalent

bonds – H20, O2, H2, CO2

• Compounds: chemical substance made of atoms of 2 or more elements, regardless of type of bond joining them. – H20, NaCl, CO2.

Concept Check

• The reactive properties or chemical behavior of an atom mostly depend on the number of

1) the electrons in each electron shell of the atom. 2) the neutrons found in the nucleus. 3) the filled electron shells. 4) the electrons in the outer electron shell of the atom.

Answer

• The reactive properties or chemical behavior of an atom mostly depend on the number of

4) the electrons in the outer electron shell of the atom.

Concept Check

• Water molecules form hydrogen bonds because

1) the water molecule is polar. 2) the oxygen molecule is

positively charged. 3) the water molecule forms a

tetrahedron. 4) the hydrogen atoms are

negatively charged.

Answer

• Water molecules form hydrogen bonds because

4) the water molecule is polar.