2.5: Isotopes Flashcards
What is atomic mass
Atomic mass is simply the mass of an elements atom. Different elements have different atomic masses. Atomic mass is measured in Atomic mass units (AMU), a special unit which is a standard of 1, based on the mass of a hydrogen atom.
What was daltons original system of Atomic mass
Dalton choose Hydrogen because he thought it was the simplest element (which is true), and because he thought the masses of all other elements would be multiples of hydrogen (which is false). He rounded off the mass to whole numbers, which lead to errors. He also had several formulas wrong, like HO for water, this lead to him having the wrong atomic mass for elements like oxygen. He also did not know of polyatomic molecules like phosphorous.
What was the new system in 1828
in 1828 Berzelius published a new table with hydrogen weighed as 1, and other elements with non integral masses, like oxygen as 15.9 u.
What was the system change before the 1860s
By the 1860s, many chemists decided to change oxygen to 16, because it was used in many experiments and hydrogen had to be adjusted to 1.008
What is the modern system that was designed in 1961
In 1961 a new standard was adopted that was based on Carbon 12, and worked specifically with mass, rather than weight. An atom of Carbon 12 was given the precise mass of 12.0000u, all other atoms and particles can related to this.
A proton has a AMU of p+= 1.00728 u, and a neutron has a AMU of n = 1.008 66 u, both of which are rounded to 1
Why was carbon 12 chosen as a standard
The specific isotope of carbon 12 had to be chosen because the number of neutrons can vary within an atom and one molecule of oxygen 18 has a different mass than oxygen 16. Carbon 12 was chosen because it was solid at room temperature, because it was 99.9% of all carbon and earth, and because carbon is common on earth.
How do we determine atomic mass (general)
The atomic mass of an element is based on the average of all isotopes of that element, in their proportionality. so lithium has isotopes of 6 (7%) and 7 (93%) so the atomic mass of lithium is 6.93 u.
When an atom is so unstable that it cannot be measure the atomic mass is given based on the number of neutrons and protons it has.
What does varying the number of each subatomic particle give
Varying the number of protons gives a different element
Varying the number of electrons gives ions
and varying the number of neutrons gives different isotopes
What is mass number
Isotopes of an element display different neutron numbers and mass numbers (the sum of the proton and neutron counts).
Calcium 40, has 20 neutrons and 20 protons, so a mass of 40u
Calcium 42, has 22 neutrons and 20 protons, so a mass of 42u
Mass Number = Atomic Number (# protons) + # neutrons
How do you determine atomic mass
The atomic mass of an element is based on the weighted mass of its isotopes, if an element has two isotopes, found in equal measure, of 10u and 11u. the atomic mass would be 10.5
If the isotope 11 is more common than the atomic mass would be between 10.5-11.0 depending on the proportionality, and vice versa.
The atomic masses of a proton and of a neutron are almost exactly 1 μ. Electrons have negligible atomic mass. Hence, the atomic mass of an isotope is the sum of the number of protons and neutrons
Here’s the formula:
(Atomic mass = (percent found x isotope 1 mass) + (percent found x isotope 2 mass) + (percent found x isotope 3 mass))
