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

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


- 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


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


- 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


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


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


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


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. , , ,


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


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


- 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?


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


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


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


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

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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