### Chapter 4.2 Notes

```Chapter 4.2 – The Structure of Atoms
- protons have a positive charge
- neutrons have no charge
- protons and neutrons have almost the same size and mass
- electrons have a much smaller mass than protons and
neutrons
Particle
Charge
Mass (kg)
Location in the atom
Proton
+1
1.67 x 10-27
In the nucleus
Neutron
0
1.67 x 10-27
In the nucleus
Electron
-1
9.11 x 10-31
Outside the nucleus
Chapter 4.2 – The Structure of Atoms
-
number of protons in the nucleus determines what element an atom is
atomic number (Z) – the number of protons in the nucleus of an atom
each element is defined by its atomic number
atoms have equal numbers of protons and electrons
atomic number is also equal to the number of electrons in an atom
atoms are neutral because the positive charge of the protons and the
negative charge of the electrons cancel out
- the attraction between positive protons and negative electrons holds
the atom together
Chapter 4.2 – The Structure of Atoms
How many protons does an oxygen atom have?
Z=8
How many electrons does an oxygen atom have?
e- = 8
How many protons does an iron atom have?
Z = 26
Chapter 4.2 – The Structure of Atoms
- if an atom gains or loses electrons it becomes charged
- ion – a charged atom
- in an ion the number of protons and electrons is not equal
- the number of protons stays the same, it is only the number of
electrons that changes
- because the mass of protons, neutrons, and electrons are so small we
describe their mass in atomic mass units
- in atomic mass units a proton has a mass of 1, a neutron has a mass of
1, and an electron has a mass of 0
Chapter 4.2 – The Structure of Atoms
mass number (A) –
the number of protons plus neutrons in an atom
- indicates the mass of an atom in atomic mass units
- the number of neutrons in an atom is equal to the mass number minus
the atomic number
A – Z = # of neutrons
Chapter 4.2 – The Structure of Atoms
How many neutrons are in an atom with a mass number of 14 and an
atomic number of 6?
14 – 6 = 8 neutrons
How many neutrons are in an atom with a mass number of 58 and an
atomic number of 26?
58 – 26 = 32 neutrons
If an atom has 19 protons and 20 neutrons what is its mass number?
19 + 20 = 39
mass number = 39
Chapter 4.2 – The Structure of Atoms
isotope –
an atom that has the same number of protons as other atoms of
the same element, but has a different number of neutrons
- isotopes of the same element have the same chemical properties but a
different mass
- many isotopes are unstable and decay into other isotopes
radioisotopes – unstable isotopes
Chapter 4.2 – The Structure of Atoms
two ways to write isotope names
1. write the element name followed by the mass number
ex. carbon – 14, hydrogen – 3, uranium – 235, iron – 54
2. use a nuclear symbol

54
ex. 146, 31, 235
,
92
26
Chapter 4.2 – The Structure of Atoms
many many atoms make up matter
to deal with the large numbers we use a different unit
mole (mol) –
a unit that describes the amount of a substance,
one mole is the number of carbon atoms in 12.00 grams of
carbon – 12
Chapter 4.2 – The Structure of Atoms
Avogadro’s number – number of particles in one mole of any substance
Avogadro’s number = 6.022 x 1023
In 1.00 mole of carbon – 12 there are 6.022 x 1023 atoms of carbon – 12
1.00 mol of C = 12.00 g of C = 6.022 x 1023 atoms of C
molar mass –
mass in grams of one mole of a substance
1 mol of carbon – 12 has a mass of 12.00 g
molar mass of carbon – 12 is 12.00 g/mol
Chapter 4.2 – The Structure of Atoms
- one mole of an element usually has many isotopes in it
- the molar mass of an element in g/mol is equal to its average atomic
mass in atomic mass units
- molar masses of elements can be found on the periodic table
- always round molar masses to 2 decimal places
What is the molar mass of H? N? Fe? Cl? Na? Al?
How many particles are in 1 mole of H? N? Fe? Cl? Na? Al? eggs?
marbles? people?
Chapter 4.2 – The Structure of Atoms
compounds also have molar masses
to find the molar mass of a compound, multiply the number of atoms of
each element by the molar mass of that element and then add each of the
masses together
Find the molar mass of H2O.
H = 1.01 g/mol
O = 16.00 g/mol
H2O = 2 x 1.01 + 1 x 16.00 = 18.02 g/mol
Chapter 4.2 – The Structure of Atoms
Find the molar mass of CO2.
C = 12.01 g/mol
O = 16.00 g/mol
CO2 = 1 x 12.01 + 2 x 16.00 = 44.01 g/mol
Find the molar mass of NaCl.
Na = 22.99 g/mol
Cl = 35.45 g/mol
NaCl = 1 x 22.99 + 1 x 35.45 = 58.44 g/mol
Chapter 4.2 – The Structure of Atoms
We can convert between mass in grams and number of moles
Use unit conversion and conversion factors
molar mass can be written as: 12.01 g of C = 1.00 mol of C
Use the molar mass as a conversion factor
molar mass of substance or
1 mol of substance
1 mol of substance
molar mass of substance
12.01 g of C
=
1.00 mol of C
1.00 mol of C
12.01 g of C
Chapter 4.2 – The Structure of Atoms
Find the number of grams in 4.0 moles of oxygen.
4.0
1
×
16.00
1
= 64.00 g of O
Find the number of moles of iron in 16.00 g of iron.
16.00
1
×
1
55.85
= 0.286 mol of Fe
```