Colour in the Chemical Families

Explaining the Periodic Table Nelson: Science Perspectives 9 – Pg

Learning Goals Define atomic number, mass number, and atomic mass number and describe what information we obtain from each Draw accurate Bohr-Rutherford diagrams Describe patterns that we see in the periodic table in relation to electrons and number of orbits/shells

Atomic Number Atomic Number  the number of protons in an atom’s nucleus What makes atoms unique is the number of protons they contain in their nucleus Example: Only gold (Au) has 79 protons in its nucleus … no other elements can have 79 protons Combining protons from one element into the nucleus of another would require a nuclear reaction

Mass Number Mass Number  the number of protons and neutrons in an atom’s nucleus If an element has different number of neutrons, it is called an isotope EXAMPLE: Lithium (Li) has an atomic number 3, but can have a mass number of 6 or 7. Li-6 = 3 protons and 3 neutrons Li-7 = 3 protons and 4 neutrons

Atomic Mass Atomic Mass  the mass of an atom in atomic mass units (u) Atomic Mass is not often a whole number, as it is an average of the isotopes of an element. EXAMPLE: Lithium’s atomic mass = 6.94 u By rounding the atomic mass, you can determine the most common isotope that an element is found EXAMPLE: Lithium is rounded to 7. Therefore, the most common isotope of Li is one with 4 neutrons

Problem: Problem:Find the number of neutrons in the most common isotope of aluminum Given:atomic mass of Al = atomic number = 13 Required: number of neutrons

Solution: Analysis:Round the atomic mass of the element to the nearest whole number to get the mass number of the most isotope. mass number of Al = 27 u (rounded up) mass number – atomic number = number of neutrons Solution:27 – 13 = 14 Statement:The most common isotope of aluminum contains 14 neutrons

Bohr – Rutherford Diagrams of an Atom Bohr-Rutherford diagram  a simple drawing that shows the numbers and locations of protons, neutrons, and electrons in an atom The number of protons = the atomic number The number of neutrons = the difference between the mass number and the atomic number The number of electrons = the number of protons in a neutral atom

Steps: 1)Determine the number of protons and number of neutrons from the atomic number and mass number 2)Draw a small circle for the nucleus 3)Write the numbers of protons (p+) and neutrons (n0) inside the nucleus (the number of electrons (e-) must equal the number of protons if the atom is neutral) 4)Draw a circle outside the nucleus (electron orbit), draw dots on the circle to represent electrons until the orbit is full 5)Draw another orbit outside of the first and add electrons as necessary (until full) REMEMBER: 1 st orbit = 2 electrons, 2 nd orbit = 8 electrons, 3 rd orbit = 8 electrons

EXAMPLE:

Patterns in the Periodic Table As you go down within each family, the # of electron orbits increases a new orbit for each new row Within each family, all atoms have same number of electrons in their outermost orbits/shell The farther down you go in the chemical family, the less attractive force between the outer electrons and the nucleus  more reactive

Patterns in the Periodic Table As you move across each period (row) from left to right, one more electron is added

To Do: Attempt questions on page 240