Lecture 1.1

Refresh your high school chemistry

CS882, Fall 2006

Atoms and molecules

Atoms

Protons

Neutrons

Electrons(much smaller)orbiting around the nucleus

Atom models

Secondshell

Helium

2He

Firstshell

Thirdshell

Hydrogen

1H

2He

4.00Atomic mass

Atomic number

Element symbol

Electron-shelldiagram

Lithium

3LiBeryllium

4BeBoron

3BCarbon

6CNitrogen

7NOxygen

8OFluorine

9FNeon

10Ne

Sodium

11NaMagnesium

12MgAluminum

13AlSilicon

14SiPhosphorus

15PSulfur

16SChlorine

17ClArgon

18Ar

Electron-shell diagrams of the first 18 elements in the periodic table

Formation of covalent bondHydrogen atoms (2 H)

Hydrogenmolecule (H2)

1 In each hydrogenatom, the single electronis held in its orbital byits attraction to theproton in the nucleus.

When two hydrogenatoms approach eachother, the electron ofeach atom is alsoattracted to the protonin the other nucleus.

2

The two electronsbecome shared in a covalent bond,forming an H2

molecule.

3

+ +

+ +

+ +

Water molecule, H2O

Electron transfer and ionic bonding

Cl–

Chloride ion(an anion)

–

The lone valence electron of a sodiumatom is transferred to join the 7 valenceelectrons of a chlorine atom.

1 Each resulting ion has a completedvalence shell. An ionic bond can formbetween the oppositely charged ions.

2

Na NaCl Cl

+

NaSodium atom(an uncharged

atom)

ClChlorine atom(an uncharged

atom)

Na+

Sodium ion(a cation)

Sodium chloride (NaCl)

Sodium chloride crystal

Na+

Cl-

Hydrogen bond

Water(H2O)

Ammonia(NH3)

– +

O

H

H

+

–

N

H

H H

A hydrogenbond results from the attraction between thepartial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.+

+

+

Hydrogen bond

Copyright 2001 by Harcourt, Inc.

The Chemical Foundations of LifeThe Chemical Foundations of Life

So there is an So there is an attraction betweenattraction betweenthe positive andthe positive andnegative sides of negative sides of a water moleculea water molecule……this is this is hydrogenhydrogenbondingbonding..

Figure 2Figure 2--1212

Walking on water