Introduction to Chemistry – Background for Nanoscience and Nanotechnology

Prof. Petr Vanysek

Introduction to Chemistry:

Compounds

Chemical Bonding

• Covalent bonds• Ionic bonds• Metal bonding

Covalent bonds

• Covalent bonding is when electrons are shared between to atoms or more.

• The number of covalent bonds an atom is likely to form is determined by its place in the periodic table and the number of valence electrons it has.

• An atom will share electrons with another atom so that it results in them both having a full valence shell. Usually this will be 8 electrons.

Ionic bonds

• When a metal and a non-metal form bonds they are typically ionic bonds where electrons are transferred from the metal to the non-metal.

• Some metals will lose enough electrons to achieve a complete valence shell.

• Non-metals will usually gain enough electrons to achieve a complete valence shell.

• Many metals are able to form ions with more than one charge.

Metal bonding

• In metals the atoms are held together by metal bonding. Electrons can easily transfer from one atom to the next. This suggests a model of positive ions in a sea of electrons. Metals can conduct electricity because electrons flow easily in any direction.

Covalent and Ionic BondingAtom # of Covalent Bonds Typical charge for an

Ion

H 1 +1

C 4

O 2 -2

N 3

F, Cl, Br, I 1 -1

S 2 -2

Si 4

Li, Na, K +1

Be, Mg, Ca, Sr, Ba +2

Polar Molecules

• Polar Molecules– If the electron density is not distributed evenly

around a molecule then they are polar.

Intermolecular BondingBonding between molecules

van der Waals forces• Hydrogen bonding

– This relatively strong type of inter-molecular bonding which typically occurs between a hydrogen atom of one molecule and an electron pair or electronegative atom of another molecule.

– Multiple hydrogen bonds hold the DNA double helix together.

• Dipole interaction• London forces

– These are induced forces caused by a temporary rearrangement of the electron clouds when molecules bump together.

Hydrogen Bonding

+

--+

H

OH

Hydrogen Bonding

OH

H

OH

H

Dipole Interaction

• http://www.chemguide.co.uk/atoms/bonding/vdw.html

• The partial positive and negative ends of the molecules hold the molecules together.

London Forces

Two hexane molecules approach.

The hexane molecules bump into each other.

The electron clouds rearrange to form a temporary dipole.

+ + +- - -

+ + +

- - -

London forces are induced dipoles caused by temporary rearrangement of the electron cloud.

Polymers

• Polymers are large chainlike molecules that are built from smaller molecules called monomers.

• For example polyethylene is formed from ethylene:

• Proteins are natural polymers.

• http://www.pslc.ws/macrog.htm

C C

H H

H H

)(n

nCH2=CH2

Chemical Reactions

• Involve the making or breaking of chemical bonds.

• Chemical reactions result in making a new substance with different properties from the original substance.

Structure of Materials

Grains Crystals Crystal

Unit CellElectron orbitalsAtom

Biological Organization

Tissue Cells Organelles

Proteins Nucleic AcidsMembranes

Review of Chemistry

• States of Matter• Atoms, Molecules and Ions• Subatomic particles• Periodic Table• Covalent and ionic bonding• Chemical reactions• Intra-molecular forces• Polymers