Counting Atoms

Atoms are much too small to be measured individually. But, we can analyze atoms quantitatively if we know certain fundamental properties of the atoms of each element.

Atomic Number The atomic number (Z) of an element is the number of protons of each atom of that element. Therefore, the atomic number identifies the element. On the periodic table, the atomic number is at the top of each box and the elements are in order of increasing atomic number.

Isotopes All hydrogen atoms have one proton However, hydrogen atoms can have different numbers of neutrons. There are three types of hydrogen atoms Protium Deuterium Tritium

Protium Protium is the most common type of hydrogen atom. 99.9885% of all hydrogen is protium It has one proton, just like all types of hydrogen. It also has one electron. It doesn’t have any neutrons.

Deuterium Deuterium accounts for 0.0115% of Earth’s hydrogen atoms. It has one proton, and one electron It also has one neutron

Tritium Tritium is the third type of hydrogen atom It is radioactive It exists in very small amounts in nature, but we can also prepare it artificially. It has one proton, one electron, and two neutrons.

Protium, Deuterium, and Tritium are isotopes of hydrogen. They all have one proton and one electron But, because they have different numbers of neutrons, they have different masses. Most elements consist of mixtures of isotopes.

Remember that an atom’s identity is given by its atomic number, or number of protons An isotope’s identity is given by both its number of protons and its number of neutrons. The mass number of an isotope is the total number of protons and neutrons in its nucleus.

The mass number is NOT the mass of the atom. It is simply the number of protons + neutrons in the nucleus

Atomic number (Z) Number of neutrons Mass number Protium 1 Deuterium 2 Tritium 3

Most isotopes don’t have distinct names They are usually identified by specifying the mass number. There are two methods for specifying isotopes Hyphen notation Nuclear symbol

Hyphen Notation Name of element – mass number Protium is Hydrogen-1 Deuterium is Hydrogen-2 Tritium is Hydrogen-3

Nuclear Symbol The symbol for the element is preceded by both superscript and a subscript. The superscript is the mass number The subscript is the atomic number Protium is Deuterium is Tritium is

Nuclide is a general term for a specific isotope of an element. There are three hydrogen nuclides. Table 3 in your text (p. 79) lists the three hydrogen nuclides and two helium nuclides.

Relative Atomic Masses Because atoms are so small, their masses are very small. The mass of a protium nuclide is 1.6735 x 10-27 kg For most chemical calculations, it is more convenient to use relative atomic masses.

To set up a scale for relative atomic masses, one nuclide was chosen as a standard and assigned a mass value. Then, the masses of all other atoms are expressed in relation to this standard. For example …

It has been assigned a mass of 12 amu. The standard used by scientists to compare units of atomic mass is the carbon-12 atom. It has been assigned a mass of 12 amu. Therefore, the standard for an amu is that 1 amu is exactly 1/12 the mass of a C-12 atom.

The atomic mass of ANY atom is determined by comparing it with the mass of a C-12 atom. A protium nuclide (a hydrogen-1 atom) is about 1/12 that of the carbon-12 atom, or about 1 amu An oxygen-16 atom has a mass that is about 16/12 times the mass of a C-12 atom or about 16 amu. Careful measurements have shown that the mass of an O-16 atom is 15.994915 amu

The masses of subatomic particles can also be expressed in amu An electron has a mass of 0.0005486 amu A proton has a mass of 1.007276 amu A neutron has a mass of 1.008665 amu

Mass number is always a whole number While the mass number of an atom and its atomic mass are close to each other, they are NOT the same thing. Mass number is always a whole number # protons + # neutrons Atomic mass is the actual mass of the atom On our periodic tables, the masses are rounded to two decimal places You will use two decimal places for all atomic masses this year!!

Most elements exist in nature as a mixture of isotopes. Remember that hydrogen is a mixture of protium, deuterium, and tritium. Since each isotope has a different mass, why is there only one mass listed on the periodic table???

Average Atomic Masses Because elements are a mixture of their isotopes, their mass is an average of the masses of the isotopes. However, it is NOT a simple average. Since 99.9% of all hydrogen is protium, the mass of protium must have a much larger influence on the average mass than deuterium or tritium.

Calculating Average Atomic Mass Multiply the mass of each isotope by its relative abundance Relative abundance is the percentage written as a decimal (so 12.5% would be 0.125) Add them up

Example Look at Table 4 on p. 82 Calculate the average atomic mass of copper:

Average atomic mass is listed on the periodic table. It is the number at the bottom of each box On the periodic table on your desk, the average atomic masses have been rounded to two decimal places. That is what we will use all year. Yes, I know I’m repeating myself – perhaps that means you should take note!!!!

We can relate masses in grams to numbers of atoms through 3 concepts: The relative atomic mass scale makes it possible to know how many atoms of an element are present in a sample of the element. We can relate masses in grams to numbers of atoms through 3 concepts: Mole Avogadro’s number Molar mass

Mole A mole is a collection of a specific number of atoms. One mole of hydrogen atoms contains the exact same number of atoms as one mole of nitrogen, one mole of oxygen, etc. The abbreviation for mole is mol Do NOT use m as an abbreviation for mole!

Avogadro’s Number This number is the number of particles in a mole of a pure substance. If the substance is an element, it is the number of atoms in one mole of that element If the substance is a compound, it is the number of molecules in one mole of that compound.

6.022 X 1023 602,200,000,000,000,000,000,000 602.2 sextillion!

Molar Mass The mass of one mole of a pure substance Written in units of g/mol Numerically equal to atomic mass Lithium has an average atomic mass of 6.94 amu The molar mass of lithium is 6.94 g/mol

Molar mass can be used to convert between grams and moles. For example, what is the mass of 3.50 moles of copper?

How many moles of calcium are in 3.46 g of calcium?

We can also do conversions with Avogadro’s number: How many atoms are in 0.875 mol Al?

How many moles are present in 5.5 X 1020 atoms of sodium?

We can use BOTH molar mass and Avogadro’s number together. What is the mass of 2.35 X 1024 atoms of phosphorus?

How many atoms are in 7.5 g of Neon?