Chapter 5.

Atoms and Molecules

Atoms are the smallest particles of an element that can exist and still have the properties of that element.

Atomic TheoryProposed by John Dalton, 1808

All matter is made up of small particles called atoms.

All atoms of a given type are similar, and different from atoms of other types.

The relative number and arrangement of different types of atoms in a substance determine its identity.

Chemical change is a union, separation, or rearrange-ment of atoms to give new substances.

Atoms are indivisible, so only whole atoms can participate in or result from any chemical change.

Atomic TheoryProposed by John Dalton, 1808

How big are atoms? Not very!

Diameter ~ 10-10 m

~ 1 million in diameter of printed period

Mass ~ 10-22 g

~ 1025 would weigh 1 pound

This was figured out in early 1900's

Atomic TheoryProposed by John Dalton, 1808

Molecules

A molecule is a group of two or more atoms that functions as a unit because the atoms are tightly bound together.

Molecules

Some elements exist as molecules.

Also H2, N2, O2, F2, Br2, I2

Molecules

In molecules, elements are bound in a specific order and geometry. They act as a unit.

MoleculesNot all compounds exist as molecules!

Chemical Formulas

A chemical formula is a notation that shows the chemical symbols of the elements present in a compound, with numerical subscripts to the right of each symbol to show how many atoms of each element are present in the compound.

Water H2O Methane CH4 Aspirin C9H8O4

Calcium hypochlorite Ca(OCl)2

Tetraethyl lead Pb(C2H5)4

Chemical Formulas

Writing chemical formulas:

Subscripts are omitted if only one atom of that element is present.

Elements are usually listed in order from left to right as they appear on the periodic table.

Sometimes a group of atoms acts as a unit within a compound. Such groups are en-closed in parentheses, followed by a sub-script to show how many groups are present.

Chemical Formulas

Types of chemical formulas:

Empirical formulas show the smallest whole-number ratio of the elements found in a compound. They are always used for non-molecular compounds.

Molecular formulas are usually used for molecular compounds. They are whole-number multiples of empirical formulas.

Chemical Formulas

How many of each type of atom are found in the following compounds?

Na2O C6H12O6

NH3 N2H4

Al(NO3)3 Fe2O3

COCl2 CoCl2

Which of these are empirical formulas? Which are molecular formulas?

Subatomic Particles

For almost 100 years, atoms were thought to be indivisible. In the late 1800's pieces of them started showing up!

By 1935, it was known that atoms are made up of three types of subatomic particles:

Electrons

Protons

Neutrons

Subatomic Particles

Diagram of a gas discharge tube used to study the behavior of “cathode rays.”

Subatomic ParticlesParticle Mass, Charge Notes

amu

Electron 0.0005 -1 Exists on periphery of atom Does chemistry

Proton 1.007 +1 Exists in nucleus Defines element Participates in nuclear reactions

Neutron 1.008 0 "Glue" in nucleus

Gives isotopes

Subatomic Particles

Rutherford’s gold foil experiment

Subatomic Particles

Where things are:

Subatomic Particles

Relative sizes:

If the nucleus were the

size of a pea, placed in

the center of a stadium,

the electrons would be

in the outer seats!

Subatomic Particles

Fun facts:

The atom is mostly

empty space!

Most of the mass is

packed into the nucleus

which has a density of

~1014 g/cm3!

Complete Atomic Symbols

The complete atomic symbol shows the num-ber of protons and neutrons in the nucleus, and the charge (if any) on the atom.

M C

E Z

Complete Atomic SymbolsE = Symbol for Element

Z = Atomic number = # of protons in nucleus Atomic number defines element Above symbol for element in Periodic Table

M = Mass number = sum of protons and neutrons in nucleus

C = Charge; Zero for neutral atom, when # of electrons = # of protons

Complete Atomic Symbols

Reading symbols of neutral atoms:

14 56 238

C Fe U 6 26 92

Alternate notation:

U-235 P-31 C-14

Isotopes

Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei.

Isotopes of an element have the same nor-mal chemistry (that's a job for electrons!) but different nuclear behavior. This can show up in magnetic properties and nuclear reactions.

Isotopes

Isotopes of some common elements:

1 2 3

H H H 1 1 1

12 13 14

C C C 6 6 6

Isotopes

Most elements have several isotopes. These are exceptions:

9 19 23

Be F Na 4 9 11

27 31

Al P 13 15

Atomic Masses

The atomic mass of an element is a weighted average mass of the isotopes of that element.

It appears below the element's symbol on the periodic table.

It represents the mass one would obtain by measuring the mass of a group of atoms.

Atomic Masses

Atomic mass units (amu) are used to give masses of atoms.

Atomic mass units are defined relative to 12C. One atom of 12C has a mass of exactly 12 amu.

1.00 amu = 1.99 x 10-23 g

Atomic Masses

Atomic mass of Magnesium

Isotope Atom % Mass, Contribution amu to total

24Mg 78.70 23.985 18.88 amu

25Mg 10.13 24.985 2.531 amu

26Mg 11.17 25.983 2.902 amu

Total = 24.31 amu

Charges

M C

E Z

C = Charge; Zero for neutral atom, when # of electrons = # of protons

# of electrons = # of protons - charge

Charges

Examples:

23 1+ 35 1- 63 2+

Na Cl Cu11 17 29

If we're working with a collection of ions, all isotopes are present, and we don't need M (or Z). Simplify:

Na1+ Cl1- Cu2+