3.1 Masses of Atoms and Molecules Flashcards
When we perform chemical calculations, we need to know
how heavy one atom is compared to another atom
The mass of a single atom is so small that it is impossible to weigh it directly. How is this problem overcome?
To overcome this problem, we have to weigh a lot of atoms. We then compare this mass with the mass of a standard atom. We define atomic mass in terms of a standard atom. We call this standard the unified atomic mass unit. Scientists have defined unified atomic mass in terms of the isotope of carbon-12
What is the formal definition of unified atomic mass?
One twelfth of an unbound neutral atom of the carbon-12 isotope in its ground state.
What is the ground state?
The lowest energy state
The symbol for unified atomic mas is
u; You will also see the symbol Da (Dalton) being used.
What is relative atomic mass
It is the ratio of the average mass of the atoms of an element in a given sample to the unified atomic mass unit
The values of the relative atomic masses in the periodic table are determined by
using the weighted average mass of the atoms of a particular element. This is because most elements are a mixture of isotopes
Why does the definition of relative atomic mass specify a particular sample
Because the proportion of different isotopes can vary slightly according to the location where the element is extracted however since the difference is so small that we can use the values given in the Periodic table.
Why does the relative atomic mass have no units
Because it is a ratio and the unis cancel
How is relative atomic mass symbolized?
Ar
Why is the Ar of chlorine 35.5
In a sample of chlorine, chlorine-35 makes up about three quarters of the chlorine atom and chlorine-37 makes up about a quarter of atoms.
What is relative isotopic mass
It is the mass of particular atom of an isotope compared to the value of unified mass unit. This term is used for the mass of a particular isotope of an element which has the Avogadro number of atoms
What is relative molecular mass
The weighted average mass of a molecule in a given sample of that molecule compared to the value of unified atomic mass.
How can we find relative molecular mass
By adding up the relative atomic masses of all the atoms present in one molecule
When calculating relative molecular mass, we use the simplest formula for the compound. Give one example of this
For silicon dioxide which has a giant covalent structure of silicon and oxygen atoms joined together, the simplest formula is Si02. The same is true for ionic compounds such as NaCl
