Atoms and Molecules

Symbols and Formula

Masses of Atoms and Molecules

The Atom

Isotopes

The Mole

Chemical Formula

Atomic Symbols

• Each element is assigned a unique symbol.

• Each is 1-2 letters and the first is capitalized.

• Symbol may not match the name - often had a different name to start with.

arsenic As potassium Kbarium Ba nickel Nicarbon C nitrogen Nchlorine Cl oxygen Ohydrogen H radon Rnhelium He titanium Tigold Au uranium U

The atomic symbol

AA - Atomic mass - Total protons & neutrons

ZZ - Atomic number - # of protons or electronsCC - Charge - + or - values

## - Number - # of atoms in a formula.

XA

Z

C

#

FormulaFormula are used to represent the elements in a compound.

• Lists the elements in a compound.

• Tells how many of each element there are.

• May also show how the elements are connected to each other.

HH22OO - water 2 hydrogen 1 oxygen

CHCH33CHCH22OHOH - ethyl alcohol 2 carbon, 6 hydrogen and 1 oxygen (shows how atoms are arranged)

Molecular representations

HH22OO - water

CHCH33CHCH22OHOH - ethyl alcohol

• Electrons moved around nucleus in orbitals

• Electrons are found only in orbitals.

• Orbitals have fixed energy values called quantum levels.

Simple model of the atom

Commonly called the Bohr model.

The Bohr ModelAtoms don’t really look like this.We know that the model is incorrect but it is good enough to help us understand many important concepts.

Niels Bohr

Atomic structure

Atom.

The smallest unit of an element that retains its chemical properties.

Atoms can be split into smaller parts.

Particle Symbol Charge AMU grams

electron e- -1 5.45x10-4 9.07x10-28

proton p +1 1.00 1.67x10-24

neutron n 0 1.00 1.67x10-24

Structure of the atomAtoms have a specific arrangement.

NucleusSmall, dense, positive charge in the center of an atom that contains protons & neutrons.

ElectronsSurround the nucleus. Diffuse region of negative charge.

Nucleus is a very small part of an atom.If it was the size of a marble, the atom would fill a football stadium.

Structure of the atomAn electron is so small (~0.05% of a proton) that its mass is ignored in normal chemical calculations.

ExampleIf an electron was the size and mass of a penny, (2.5 g), would would be the mass of a proton?

gp= 2.5 g e- 5.45x10-4AMUe-

1.00 AMUp = 4587 g

gp=4.59 kg (if an electron weighed 2.5 g)

Isotopes

IsotopesIsotopes Atoms of the same element buthaving different masses.Each isotope has a different number of neutrons

Isotopes of hydrogen H H H

Isotopes of carbon C C C

11

21

31

12 6

13 6

14 6

Isotopes Most elements occur in nature as a mixture of isotopes.

ElementElement # of stable isotopes # of stable isotopesH 2C 2O 3Fe 4Sn 10

This is one reason why weights arenot whole numbers. They are basedon averages.

The atomic symbol & isotopes

Determine the number of protons,neutrons and electrons in each of thefollowing.

P3115 Ba

138 56 U

238 92

The moleNumber of atoms in 12.000 grams of 12C

1 mol = 6.022 x 1023 atoms = grams / formula weight

Atoms, ions and molecules are too small todirectly measure - measured in AMUAMU.

Using moles gives us a practical unit.

We can then relate atoms, ions and moleculeswhich are too small to an easy to measure unit- gramsgrams.

The mole

If we had one mole of water and one mole of hydrogen, we would have the name number of molecules of each.

1 mol H2O = 6.022 x 1023 molecules

1 mol H2 = 6.022 x 1023 molecules

We can’t weigh out moles -- we use grams.

We would need to weigh out a different number of grams to have the same number of molecules

Moles and weightsAtoms come in different sizes and masses.

A mole of atoms of one type would havea different weight than a mole of anothertype.

H H - - 1.008 AMU or grams/mol1.008 AMU or grams/molO O - - 16.00 AMU or grams/mol16.00 AMU or grams/molMo Mo - - 95.94 AMU or grams/mol95.94 AMU or grams/molPb Pb - - 207.2 AMU or grams/mol207.2 AMU or grams/mol

We rely on a straight forward system torelate mass and moles.

Masses of atoms and molecules

Atomic weightAtomic weight• The average, relative mass of an

atom in an element.

Atomic mass unit (Atomic mass unit (amuamu))• Arbitrary mass unit used for atoms.• Relative to one type of carbon.

Molecular or formula massMolecular or formula mass• The total mass for all atoms in a

compound.

Molar massesOnce you know the weight of an atom, ion, or molecule, just remember:

Mass of one unit - use AMU

Mass of one mole of units- use g/mol

The numbers DON’T change - just the units.

Masses of atoms and molecules

HH22OO - water

2 hydrogen 2 x 1.008 amu1 oxygen 1 x 16.00 amu

mass of molecule 18.02 amu18.02 g/mol

Rounded off basedon significant figuresRounded off based

on significant figures

Another example

CHCH33CHCH22OHOH - ethyl alcohol

2 carbon 2 x 12.01 amu6 hydrogen 6 x 1.008 amu1 oxygen 1 x 16.00 amu

mass of molecule 46.02 amu46.02 g/mol

Molecular mass vs. formula mass

Formula massThe sum of all atomic masses in a chemical formula. This term is valid for both molecular and ionic compounds.

Molecular massCalculated the same as formula mass but only valid for molecules.

Both have units of either AMU or g/mol.

The terms “formula weight” or molecular weight are also commonly used.

Formula mass

Sum the atomic masses of all elements in acompound based on the chemical formula.

You must use the atomic masses of the elementslisted in the periodic table.

CO2 1 atom of C and 2 atoms of O

1 atom C x 12.011 amu = 12.011 amu2 atoms O x 15.9994 amu = 31.9988 amu Formula Weight Formula Weight = 44.010= 44.010 amu amu

or g/molor g/mol

Example - (NH4)2SO4OK, this example is a little more complicated.

The formula is in a format to show you how thevarious atoms are hooked up.

( NH( NH4 4 ))2 2 SOSO44

We have two (NH4) units and one SO4 unit.

Now we can determine the number of atoms.

Example - (NH4)2SO4

Ammonium sulfate contains - 2 nitrogen,8 hydrogen, 1 sulfur and 4 oxygen.

2 N x 14.01 = 28.028 H x 1.008 = 8.0641 S x 32.06 = 32.064 O x 16.00 = 64.00

Formula WeightFormula Weight = 132.14= 132.14

Units are either AMU or grams / mol.