Section 4.3 How Atoms Differ

What information can be used for one’s home address? Atoms have identifying information too.

Atomic Number Henry Moseley (1887-1915) Discovered that atoms of each element contain a unique positive charge in their nuclei The # of protons in an atom identifies it as an atom of a particular element.

Atomic Number The number of protons in an atom is referred to as the atomic number The number of electrons in a NEUTRAL atom is also equal to the atomic number

DOES NOT ALWAYS EQUAL the number of neutrons!!! Atomic number DOES NOT ALWAYS EQUAL the number of neutrons!!! So we cannot determine the number of neutrons from the atomic number

More on the Atomic Number All Atoms are neutral, so therefore, the number of electrons in an atom equals the atomic number. When an atom has a charge, it is called an ion… not an atom.

Let’s fill in a table Element Atomic # # of protons # of electrons F 4 29 35 18 23 Cu

Isotopes and Mass number Atoms with the same # of protons but different numbers of neutrons are called isotopes. Example: 3 types of potassium. All 3 types contain 19 protons and 19 electrons 1 contains 20 neutrons another contains 21 neutrons And another contains 22 neutrons

Mass of Isotopes When an isotope contains more neutrons, it has a greater mass than the isotopes that contain less neutrons. Even with different masses, the isotopes have the same chemical behavior.

Isotope notation Every isotope of an element is identified with a number called the mass number.

The mass number is the sum of the atomic number (or number of protons) and the number of neutrons in the nucleus.

Let’s break it down Mass number = atomic number +number of neutrons So, then we can say…. Number of neutrons = Mass number − atomic number

Time for another table  Element Atomic # Mass # # of p+ # of n0 # of e- Cd 36 107.868 14 8 11 12

Isotope notation We can write isotopes in 2 manners One method is simply to write the element name followed by a hyphen and the atomic mass (Ex: carbon-14) Or we could simply write in the format of 𝑍 𝐴 𝑋 , where A is the atomic mass and Z is the atomic number (Ex: 6 14 C )

Isotope notation Given one isotope notation, write the other corresponding isotope notation: Zinc-70 20 43 Ca Silicon-29 Silver-106

Isotope notation Hydrogen has 3 isotopes: Hydrogen-1, Hydrogen-2, and Hydrogen-3. Isotopes of hydrogen are special, because hydrogen-2 and hydrogen-3 have special names. We simply call hydrogen-1 “hydrogen,” while we call hydrogen-2 “deuterium” and we call hydrogen-3 “tritium”

Atomic mass unit: arbitrary number chosen by scientists many years ago that is equal to the mass of 1/12 of a C-12 atom (most commonly written amu)

Most elements are found in nature as mixtures of isotopes. Usually any sample of an element contains the same relative abundance of each isotope So, if there is a different mass for each isotope, then why is there only 1 mass number for an element?

How’s that calculated? Average atomic mass The mass number on the PT is the AVERAGE atomic mass, calculated by using the relative abundance of each isotope. How’s that calculated? (Mass of isotope)(% abundance) +(mass of isotope)(% abundance)+… 100

Example Element X has 2 isotopes. The isotope with a mass of 10.012 amu has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu has a relative abundance of 80.09%. Calculate the atomic mass of this element. (10.012)(19.91)+(11.009)(80.09) =10.810 amu 100

Some final official definitions The atomic mass of an element is the weighted average mass of the isotopes of that element.