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Suppose we order a mole of marshmallows for a chemistry party. How much space here at Central would be required to store the marshmallows?

Chapter #2

Atoms and Molecules

- Atoms and Molecules
- Symbols and Formulas
- Inside the Atom
- Isotopes
- Relative masses of atoms and molecules
- Isotopes and atomic weights
- Avogadro’s number: the mole
- The mole and chemical formula’s

Elements

- Elements are pure substances made up of identical atoms.
- There are 115 known kinds of atoms. Each has its own symbol.
- The first character in an atoms symbol is an uppercase letter that corresponds the first letter of the elements name.
- Most elements have a second character in their symbol which is is a lowercase letter usually the second letter of the elements name.

Element Names

Elements discovered long ago have names and symbols with Latin or other origins, such as Au for gold (from aurum, meaning “bright dawn”), or Pb for lead (plumbum). Tungsten has a symbol of W, which is the first letter of its German name, wolfram.

Symbol and names for the known atoms are given in Table 2.1.

Periodic Chart

You can also find these symbols and names in the periodic table inside the front cover of your text although this chart does not have names provided.

Subatomic Particles

What are the atoms made of?

- Atoms are composed of subatomic particles most of which
- exist outside the stable structure of atoms for very short periods of
- time.
- Three subatomic particles are of interest to chemists:
- The proton (p)
- The neutron (n)
- The electron (e-)

Subatomic Particles

1 amu = 1.67 x 10-24 g

Neutrons and protons are about 2,000 times larger than electrons.

Atoms have a dense heavy, positively charged nucleus containing the neutrons and protons.

Outside the nucleus are the small, negatively charged electrons.

Combining Atoms

What happens when we combine atoms?

When two or more different types of atoms combine compounds are formed.

In a compound formula we write the symbol for each atom type present in the compound, if more than one atom of a given type then a subscript is used to indicate the number of each atom.

e.g. NH3 H2O SO2 CH4

Atomic Number

Atoms have no overall charge so contain the same number of protons (+) as electrons (-).

The number of protons in the nucleus of an atom is called the atomic number It is given the symbol Z.

All atoms of a given type or element have the same atomic number (Z).

The periodic table arranges atoms in order of atomic number.

Mass Number

Neutrons have no electrical charge. For a given atom type the number of neutrons may vary.

The sum of the number of neutrons and protons in an atom is called the mass number. It is given the symbol A.

Atoms with the same number of protons but different numbers of neutrons in their nucleus are called isotopes.

E

E

E

P

P

P

N

N

N

N

N

P

N

Hydrogen-3

Hydrogen-2

Hydrogen-1

1

2

3

H

H

H

1

1

1

Atomic Number (number of protons); found on the periodic chart

E

E

E

P

P

P

N

N

N

N

N

P

N

Hydrogen-3

Hydrogen-2

Hydrogen-1

1

2

3

H

H

H

1

1

1

Atomic Number (number of protons)

Mass number (sum of protons and neutrons); Not found on periodic chart

E

E

E

P

P

P

N

N

N

N

N

P

N

Hydrogen-3

Hydrogen-2

Hydrogen-1

1

2

3

H

H

H

1

1

1

Atomic Number (number of protons)

Mass number (sum of protons and neutrons); Not found on periodic chart

Oxidation number (Protons – electrons)

E

Z

Atomic SymbolsWe can distinguish isotopes with the following notation:

Where:

- E is the atom’s symbol
- A is the mass number
- Z is the atomic number

e.g. 612C, 613C, 614C, 11H, 12H and 13H

As all atoms with a given symbol have the same atomic number sometimes we skip writing Z.

e.g. 612C (carbon-12), 613C (carbon-13) and 614C (carbon-14)

Relative Atomic Mass

If we assign a mass of 12 atomic mass units (u or sometimes amu) to a carbon-12 atom then we can compute the relative atomic weight for any other atom.

Atomic weights or atomic masses are given in your periodic table.

Relative Atomic Mass

Some atoms naturally occur as a mixture of isotopes.

e.g. 11H, 12H and 13H

The atomic weights given in the periodic table take this into account.

They are the average atomic weight taking into account the amounts of each isotope present.

Relative Atomic Mass of Hydrogen

When computing the average atomic mass of any element the radioactive (unstable) isotopes are excluded since there relative abundances are slowly decreasing.

Hydrogen has three isotopes. The first two protium and deuterium are stable isotopes and the third tritium is unstable, thus excluded in the calculation.

The relative abundance of isotopes of a particular element is constant here on our planet Earth. Mass spectroscopy gives information of relative abundances and relative masses of isotopes.

Relative Atomic Mass of Hydrogen

Mass spectroscopy gives the following information for the two stable isotopes of hydrogen.

isotope namerelative abundanceMass (amu)

Hydrogen-1

(protium)

99.985 %

1.007825

Hydrogen-2

(deuterium)

2.01355

0.015 %

To calculate a weighted average, convert the percent to a decimal by moving the decimal two places to the left. Then multiply this number by the mass and add the two masses.

Relative Atomic Mass of Hydrogen

0.99985 X 1.007825 = 1.007673

0.0015 X 2.01355 = 0.003021

1.010694 amu

1.0107 amu

Relative Atomic Mass

we defined the relative atomic weight of each kind of atom as follows:

“assign a mass of 12 atomic mass units to a carbon-12 atom then compute the relative atomic weight for all other atom types.”

Atomic mass units are very small.

1 amu = 1.67 x 10-24 g

Not very practical !!!

Three molecules of O2 has the same mass as two molecules of O3 as they each contain a total of six oxygen atoms.

Relative Atomic MassConsider the following ozone has the formula O3. Two molecules of ozone contains 6 oxygen atoms. Oxygen has the formula O2. What is the mass of three molecules of O2?

In chemistry we are often more interested in the number of particles but we can only measure the masseasily.

The Mole

- So we have two problems:
- It is difficult to count particles as small as molecules, atoms, ions, protons, neutrons, or electrons.
- Atomic mass units are not very practical.

We need a system that allows us to switch between mass (in units which we can measure) and number of atoms or molecules.

The Mole

We can define a constant called Avogadro’s constant (NA):

“NA is the number of atoms needed to give a carbon-12 a mass of 12 grams”

An Avogadro’s number of hydrogen atoms would weigh 1.008g, or of any other element on the Periodic Chart their atomic weight.

The Mole

One Avogadro’s number of something is called a mole (mol).

NA = 6.022 x 1023

A mole is often compared to a “dozen”. Just as a dozen of something means there are 12 of them, a mole of something means there are 6.022 x 1023of them. Or, how about a ream?

How many pieces of paper are in a ream? Dozens and reams are convenient counting units for objects, but not for particles.

Mole is Latin for heap

Suppose we order a mole of marshmallows for a chemistry party. How much space here at Central would be required to store the marshmallows?

Mole is Latin for heap

Suppose we order a mole of marshmallows for a chemistry party. How much space here at Central would be required to store the marshmallows?

Would cover the entire 50 states 60 miles deep

Mole is Latin for heap

Suppose we order a mole of marshmallows for a chemistry party. How much space here at Central would be required to store the marshmallows?

Would cover the entire 50 states 60 miles deep

How about a mole of computer paper instead of a ream of computer paper, how far would that stretch?

Mole is Latin for heap

Would cover the entire 50 states 60 miles deep

How about a mole of computer paper instead of a ream of computer paper, how far would that stretch? Way past the planet Pluto!

Formula Weight Calculation

To calculate the molar mass of a compound we sum together the atomic weights of the atoms that make up the formula of the compound. This is called the formula weight (MW, M).

Formula weights are the sum of atomic weights of atoms

Making up the formula.

The following outlines how to find the formula weight of water

symbol

number

weight

=

2.02

1.01

2

H

X

16.0

1

=

X

O

16.0

g/mole

18.0

Percent Composition

Find the formula weight and the percent composition of

glucose (C6H12O6)

weight

number

symbol

72.0

6

=

x

12.0

C

12

=

12.12

x

H

1.01

96.0

O

x

=

16.0

6

180.1 g/mole

72.0

40.0 %C

X =

%C =

180.1

12.12

6.73 %H

X =

%H =

180.1

96.0

53.3 %O

X =

%O =

180.1

Formula Weight

We can define the molar mass or molecular weight of a substance can as:

“the mass of one mole of that substance.”

We give molar mass the symbol M and it has units g/mol-

For an atom the molar mass is equal to the atomic weight we find on the period table.

Its All About Moles

For a molecule the molecular weight is equal to the sum of the atomic weights of the atoms that make it up.

The mathematical relationship between mass and moles is:

m = nM

This can be summarized by the following diagram:

Mole Concepts

The key equation to remember for mole calculations is:

m = nM

Where:

M = molecular weight (gmol-1)

n = number of moles (mol)

m = mass of sample (g)

Mole Concepts

For a mole of a molecule the number of moles of each atom is determined by how many of that atom are in each molecule.

e.g. One mole of H2O contains:

One mole of oxygen atoms

Two moles of hydrogen atoms

In 5 moles of H2SO4 how many moles of oxygen atoms is there?

20 moles of O atoms.

Mole Conversions

In 50.0g of H2SO4 how many moles of oxygen atoms are there?

mole H2SO4

50.0g of H2SO4

=

98.0g of H2SO4

Mole Conversions

In 50.0g of H2SO4 how many moles of oxygen atoms are there?

mole H2SO4

50.0g of H2SO4

4mole O

=

2.04 mole O

mole H2SO4

98.0g of H2SO4

Mole Conversions

In 5 moles of H2SO4 how many atoms of oxygen are present?

Mole Conversions

In 5 moles of H2SO4 how many atoms of oxygen are present?

5 moles H2SO4

4 mole O

mole H2SO4

Mole Conversions

In 5 moles of H2SO4 how many atoms of oxygen are present?

5 moles H2SO4

6.02 x 1023 atoms O

4 mole O

=

mole O

mole H2SO4

1.20 x 1025 atoms

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