Ch 3 Section 3 Flashcards
Atoms are much too small to be
Measured individually
Chemists can analyze atoms
Quantitatively by knowing fundamental properties of the atoms of each element
Mole
A special unit used by chemists to express amounts of particles, such as atoms and molecules
All atoms are composed of the
Same basic particles
Atoms of different elements have
Different numbers of protons
Atoms of the same element all have the same number of
Protons
The atomic number (Z) of an element is
The number of protons of each atom of that element
All atoms of the element hydrogen have
One proton
The atomic number identifies
An element
The simplest atoms are those of
Hydrogen
Like many naturally occurring elements hydrogen atoms can have
Different numbers of neutrons
The most common type of hydrogen is sometimes called
Protium
Protium accounts for
99.9885% of the hydrogen atoms found on earth
The nucleus of a protium atom consists of
One proton only and it had one electron moving about it
Another form of hydrogen is deuterium which accounts for
0.0115% of earths hydrogen atoms
Each deuterium atom has a
Nucleus with one proton and one neutron
The third form of hydrogen is known as tritium which is
Radioactive
Tritium exists in very small
Amounts in nature but can be prepared artificially
Each tritium atom has one
Proton, two neutrons, and one electron
Isotopes are atoms of the same element that have
Different masses
The isotopes of a particular element all have the same number of
Protons and electrons but different numbers of neutrons
Most of the elements consist of
Mixtures of isotopes
Identifying an isotope requested knowing both the
Name or atomic number of the element and the mass of the isotope
The mass number is the
Total number of protons and neutrons that make up the nucleus of an isotope
Isotopes are usually identified by
Specifying their mass number
There are two methods for
Specifying isotopes
In the first method the mass number is written with a
Hyphen after the name of the element (hyphen notation)
The second method shows the composition of a
Nucleus using the isotopes nuclear symbol
The superscript in nuclear symbol indicates the
Mass number (protons + neutrons)
The subscript in nuclear symbol indicates the
Atomic number (number of protons)
The number of neutrons is found by
Subtracting the atomic number from the mass number
Nuclide is a general term for a
Specific isotope of an element
Masses of atoms expressed in grams are
Very small
For most chemical calculations it is more convenient to use
Relative atomic masses
In order to set up a relative scale of atomic mass one atom has been
Arbitrarily chosen as the standard and assigned a mass value
The masses of all other atoms are expressed in
Relation to this defined standard
The standard used by scientists to compare units of atomic mass is the
Carbon-12 atom
One atomic mass unit or 1 amu
Is exactly 1/12 the mass of a carbon-12 atom
The atomic mass of any other atom is determined by comparing it with the mass of the
Carbon-12 atom
Isotopes of an element may occur naturally or they may be
Made in the laboratory (artificial isotopes)
Although isotopes have different masses they do not
Differ significantly in their chemical behavior
The masses of subatomic particles can also be expressed in the
Atomic mass scale
The mass of the electron is
0.000 5486 amu
The mass of the proton is
1.007 276 amu
The mass of a neutron is
1.008 665 amu.
The mass number and relative atomic mass of a given nuclide are
Quite close to each other
Mass number and relative atomic mass are not identical because the
Proton and neutron masses deviate slightly from 1 amu and the atomic masses include electrons
A small amount of mass is changed to energy in the creation of a
Nucleus from its protons and neutrons
The percentage of each isotope in the naturally occurring element on earth is
Nearly always the same no matter where the element is found
The percentage at which each of an elements isotopes occurs in nature is taken into account when
Calculating the elements average atomic mass
Average atomic mass is the
Weighted average of the atomic masses of the naturally occurring isotopes of an element
The average atomic mass of an element depends on both the
Mass and the relative abundance of each of the elements isotopes
The average atomic mass of copper can be calculated by
Multiplying the atomic mass of each isotope by its relative abundance (in decimal form) and adding the results
Most atomic masses are known to
Four or more significant figures
The relative atomic mass scale makes it possible to know
How many atoms of an element are present in a sample of the element with a measurable mass
Three very important concepts- mole, Avogadro’s number, and molar mass- provide
The basis for relating masses in grams to numbers of atoms
A mole is the amount of a
Substance that contains as many particles as there are atoms in exactly 12 g of carbon-12
The mole is a
Counting unit
The number of particles in a mole has been
Experimentally determined in a number of ways
The best modern value for the number of particles in a mole is
6.022 141 79 x 10^23 which means that exactly 12 g of carbon contains that number of carbon-12 atoms
The number of particles in a mole is known as
Avogadro’s number
Avogadro’s number is named for the 19th century Italian scientist whose ideas were
Crucial in explaining the relationship between mass and numbers of atoms
Avogadro’s number is the number of
Particles in exactly one mole of a pure substance
For most purposes Avogadro’s number is rounded to
6.022 x 10^23
An alternative definition of mole is the amount of a substance that cot subs
Avogadro’s number of particles
The molar mass of a substance is the
Mass of one mole of a pure substance
Molar mass is usually written in units of
g/ mol
The molar mass of an element is numerically equal to the
Atomic mass of the element in atomic mass u to
The molar mass of an element contains
One mole of atoms
Chemists use molar mass as a
Conversion factor in chemical calculations
Avogadro’s number can be used to find the
Number of atoms of an element from the amount in moles or to find the amount of an element in miles from the number of atoms
Such problems are useful in demonstrating the
Meaning of Avogadro’s number
In these calculations Avogadro’s number is expressed in
Units of atoms per mole
Neon is a
Minor component of the atmosphere
