Atomic Structure

Subatomic Particles

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. Each particle has distinct properties and plays a specific role in the structure and behavior of atoms.

• Protons: Positively charged particles found in the nucleus. The number of protons determines the atomic number and the identity of the element.

• Neutrons: Neutral particles also located in the nucleus. Neutrons contribute to the mass of the atom but do not affect its chemical properties.

• Electrons: Negatively charged particles that orbit the nucleus in electron shells. Electrons are responsible for chemical bonding and reactions.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number and Mass Number

The atomic number and mass number are fundamental properties used to describe atoms.

• Atomic Number (Z): The number of protons in the nucleus of an atom. It uniquely identifies an element.

• Mass Number (A): The sum of the number of protons and neutrons in the nucleus.

Formula:

where is the mass number, is the atomic number (protons), and is the number of neutrons.

Example: For , the atomic number is 78 (protons), and the mass number is 190. The number of neutrons is .

Structure of the Nucleus

The nucleus contains protons and neutrons, which account for nearly all the mass of the atom. However, the nucleus is very small compared to the overall size of the atom, and it does not account for most of the atom's volume.

• Key Point: The nucleus is made up of protons and neutrons, not just neutrons.

• Key Point: The volume of an atom is mostly due to the electron cloud.

Isotopes

Definition and Notation

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.

• Isotope Symbol: , where is the mass number, is the atomic number, and is the element symbol.

Example: is an isotope of potassium with 19 protons and 21 neutrons.

Properties of Isotopes

• All isotopes of a given element have the same number of protons and electrons (if neutral).

• Isotopes differ in the number of neutrons and thus in their mass numbers.

Example: Carbon-12 and Carbon-14 are both isotopes of carbon, with 6 protons but 6 and 8 neutrons, respectively.

Isotope Table Interpretation

Given a table of protons and neutrons, the isotope symbol can be constructed as follows:

Ions

Definition and Properties

Ions are atoms or molecules that have gained or lost electrons, resulting in a net charge.

• Cations: Positively charged ions formed by loss of electrons.

• Anions: Negatively charged ions formed by gain of electrons.

Key Point: The number of protons in an ion does not change; only the number of electrons changes.

Ion Table Interpretation

Subatomic Particle Differences in Ions

• Ions differ from their neutral atoms in the number of electrons.

• The number of protons and neutrons remains unchanged in ions.

Isoelectronic Species

Isoelectronic species are atoms or ions that have the same number of electrons.

• Example: Na+, Sr2+, and Ba2+ are isoelectronic.

Atomic Mass and Isotopic Abundance

Calculating Atomic Weight

The atomic weight of an element is the weighted average of the masses of its naturally occurring isotopes.

Formula:

Example: For sulfur:

• : 94.99%, 31.972 amu

• : 0.75%, 32.971 amu

• : 4.25%, 33.968 amu

• : 0.01%, 35.967 amu

Atomic weight calculation:

Calculating Isotopic Abundance

Given the atomic weight and isotope masses, the relative abundance of each isotope can be calculated using algebraic methods.

Example: Boron-10 and Boron-11 with known masses and atomic weight.

where is the fractional abundance of Boron-10.

Mass Spectrometry and Element Identification

Mass spectrometry is used to determine the masses and relative abundances of isotopes in a sample, allowing for calculation of the atomic mass and identification of the element.

• Each peak in a mass spectrum corresponds to a different isotope.

• The average atomic mass is calculated from the masses and abundances.

Semimetals (Metalloids)

Definition and Properties

Semimetals (also called metalloids) are elements that have properties intermediate between metals and non-metals.

• Semimetals are typically found along the "staircase" line on the periodic table, separating metals from non-metals.

• They exhibit both metallic and non-metallic characteristics, such as semiconducting behavior.

Example: Boron (B), Silicon (Si), and Arsenic (As) are common semimetals.

Classification

• Semimetals act as borders between metals (to their left) and non-metals (to their right).

• They are the second-largest classification for elements after metals.

Additional info: Semimetals are important in electronics due to their semiconducting properties.