Isotopes, Atomic Mass, the Periodic Table, and the Mole: Study Notes

Isotopes: Counting Protons, Neutrons, and Electrons

Subatomic Particles and Atomic Structure

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. The number of each particle determines the atom's identity and properties.

• Proton: Positively charged particle found in the nucleus. The number of protons defines the atomic number and the element.

• Neutron: Neutral particle found in the nucleus. The number of neutrons can vary, resulting in different isotopes of the same element.

• Electron: Negatively charged particle found outside the nucleus. In a neutral atom, the number of electrons equals the number of protons.

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

Isotope	Protons	Neutrons	Electrons	Atomic Number	Mass Number
Na	11	12	11	11	23

Periodic Table

Organization and Groups

The periodic table arranges elements in order of increasing atomic number. Elements in the same vertical column (group) have similar chemical and physical properties.

• Periods: Horizontal rows.

• Groups: Vertical columns; elements in a group share similar properties.

Group Names and Specific Properties

Group	Name	Properties
1A	Alkali metals	Soft, highly reactive metals; react vigorously with water; not found free in nature
2A	Alkaline earth metals	Reactive metals; not as reactive as group 1A; only found in compounds
7A	Halogens	Nonmetals; very reactive; form salts with metals
8A	Noble gases	Inert gases; least reactive elements

Atomic Mass: The Average Mass of an Element's Atoms

Weighted Average and Isotopic Abundance

The atomic mass of an element is the weighted average mass of all naturally occurring isotopes of that element. It is calculated using the percent abundance and mass of each isotope.

• Percent abundance: The percentage of a specific isotope in a natural sample of the element.

Formula:

$\text{Average Atomic Mass} = \sum (\text{Fractional abundance} \times \text{exact mass})$

Example Calculation:

• Given isotopes with masses and abundances, multiply each mass by its fractional abundance and sum the results.

Isotope	Mass (amu)	Percent Abundance
Ne-20	19.9924	90.51%
Ne-21	20.9938	0.27%
Ne-22	21.9914	9.22%

Atoms and the Mole: How Many Particles?

Avogadro's Number and the Mole

The mole is a counting unit in chemistry, representing $6.022 \times 10^{23}$ entities (atoms, molecules, or ions). This number is known as Avogadro's Number.

• Avogadro's Number: $6.022 \times 10^{23}$ entities = 1 mole

• Used to convert between number of particles and moles.

Example: Calculate the number of copper atoms in 2.45 moles of copper.

$\text{Number of atoms} = \text{moles} \times 6.022 \times 10^{23}$

Molar Mass of Elements

Molar mass is the mass in grams of 1 mole of a substance. The units are grams per mole (g/mol).

• Used to convert between mass and moles of a substance.

Element	Atomic Mass (amu)	Molar Mass (g/mol)
C	12.01	12.01
H	1.008	1.008
O	16.00	16.00
Na	22.99	22.99
Cl	35.45	35.45

Example: Calculate the number of grams in 0.90 moles of carbon.

$\text{Mass (g)} = \text{moles} \times \text{molar mass (g/mol)}$

Example: Calculate the number of moles in 8.57 grams of carbon.

$\text{Moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$