THE ATOM

Four important pieces of information… Atomic Number Number of protons in an element Defines the element Chemical Symbol 1 or 2 letter symbol 1 st letter is always capitalized If there is a second letter is it lowercase Element Name Atomic Mass - More on this later

Atomic Number The atomic number of an element = the number of protons The number of protons is what defines an element! IF the element is neutrally charged (not an ion) then… the number of protons = the number of electrons Mass Number Mass number = number of protons + number of neutrons This is NOT the number shown on the periodic table, the periodic table shows the atomic mass – more on this later.

Determining an Atom’s Composition If you know the atomic number and the mass number of an atom then you know how many protons, neutrons, & electrons it has. Atomic Number = Number of protons & electrons Mass Number = Protons + Neutrons SO… Neutrons = Mass Number – Atomic Number

Practice – Complete the chart below ElementProtonsElectronsNeutrons Helium Titanium Sodium Krypton

Practice – Complete the chart below ElementProtonsElectronsNeutrons Helium 222 Titanium 2226 Sodium Krypton 36 42

Isotopes For many elements there are multiple types of that element, called isotopes. Isotopes have the same number of protons, but different numbers of neutrons. Because they have different numbers of neutrons their mass numbers are different. Isotopes have the same chemical properties because they have the same number of protons and electrons Isotopes can (but don’t always) have different physical properties because of their difference in neutrons.

Isotopes are identified by the element name, followed by a dash with the mass number of that isotope. For example: Carbon-12 and Carbon-13 are two isotopes of Carbon. Because they are still Carbon, they both have 6 protons (where did I find this?) Carbon - 12 has a mass number of 12 Carbon - 13 has a mass number of 13 Since the Number of neutrons = Mass Number – Atomic Number Carbon – 12 has 6 Neutrons (12 – 6) Carbon – 13 has 7 Neutrons ( )

HW due Wednesday Side 1 (chart) of worksheet

Atomic Mass The mass that is found on your periodic table is the atomic mass. Atomic mass is the average of the masses of an element’s different isotopes adjusted for how frequently each one is found in nature. So what does this mean? Let’s look at an example…

Atomic Mass Example 1 Carbon-12 is found in 99% of all Carbon atoms while Carbon -13 is found in only 1% So the atomic mass of carbon is 99% of the Carbon-12 mass + 1% of the Carbon-13 mass

Atomic Mass Example 2 What is average atomic mass of Lithium if 7.42% exists as Lithium–6 and 92.58% exists as Lithium–7? Average Atomic Mass = (6 *.0742) + (7 *.9258) = round to the thousands = For these problems the mass will not be the exact same as the mass listed on the periodic table, other isotopes have been left out or percentages may be slightly changed… otherwise you could just look at your periodic table for the answers

Atomic Mass Example 3 Neon has two major isotopes, Neon-20 and Neon-22. Out of every 250 neon atoms, 225 will be Neon-20 and 25 will be Neon-22. What is the average atomic mass of Neon? = (20 * (225/250)) + (22 * (25/250)) OR (20*.9) + (22 *.1)

Atomic Mass Example 4 Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on earth? (Hint: look at the periodic table) Answer: Bromine on the periodic table has a atomic mass of 79.8, because the number is closer to 80 then it is to 79 we know that there is a greater percentage of Bromine-80 on Earth.

Atomic Mass Example 5 Magnesium has three naturally occurring isotopes % of Magnesium atoms exist as Magnesium % exist as Magnesium % exist as Magnesium-26 What is the average atomic mass of Magnesium? = (24*.787)+(25*.1003)+(26*.1117) = or