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 ParticleChargeMass (kg)Location in the atom Proton x In the nucleus Neutron01.67 x In the nucleus Electron9.11 x 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? = 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

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 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 = x In 1.00 mole of carbon – 12 there are x atoms of carbon – mol of C = g of C = x atoms of C molar mass – mass in grams of one mole of a substance 1 mol of carbon – 12 has a mass of g molar mass of carbon – 12 is 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 H 2 O. H = 1.01 g/molO = g/mol H 2 O = 2 x x = g/mol

Chapter 4.2 – The Structure of Atoms Find the molar mass of CO 2. C = g/molO = g/mol CO 2 = 1 x x = g/mol Find the molar mass of NaCl. Na = g/molCl = g/mol NaCl = 1 x x = 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: g of C = 1.00 mol of C Use the molar mass as a conversion factor molar mass of substanceor1 mol of substance 1 mol of substancemolar mass of substance g of C=1.00 mol of C 1.00 mol of C12.01 g of C

Chapter 4.2 – The Structure of Atoms