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
neutronsParticle 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
ex. , , ,
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 substance1 mol of substance molar mass of substance
12.01 g of C = 1.00 mol of C1.00 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.
= 64.00 g of O
Find the number of moles of iron in 16.00 g of iron.
= 0.286 mol of Fe