2.1 Structure of atoms

Atoms are the building blocks of matter, and understanding their structure is crucial for grasping chemistry. This section breaks down the components of atoms: protons, neutrons, and electrons. We'll explore their properties, locations, and how they determine an atom's identity.

Diving deeper, we'll examine how these particles interact to form stable atoms and ions. We'll also learn about isotopes, which are atoms with the same number of protons but different numbers of neutrons. This knowledge forms the foundation for understanding chemical behavior.

Identify the three main subatomic particles and their properties

Subatomic particles and their charges

• The three main subatomic particles are protons, neutrons, and electrons.
• Protons have a positive charge (+1), neutrons have no charge (0), and electrons have a negative charge (-1).
• The charges of these particles play a crucial role in determining the overall charge of an atom or ion.

Mass and location of subatomic particles

• Protons and neutrons have approximately the same mass (1 atomic mass unit or amu), while electrons have a much smaller mass (about 1/1836 amu) compared to protons and neutrons.
• Protons and neutrons are located in the dense, positively charged nucleus at the center of an atom.
• Electrons occupy the space around the nucleus in regions called electron shells or energy levels.

Atomic number and mass number

• The number of protons in an atom determines its atomic number (Z) and element identity (e.g., an atom with 6 protons is always carbon).
• The total number of protons and neutrons in an atom's nucleus determines its mass number (A).
  • For example, an atom with 6 protons and 6 neutrons has a mass number of 12 (6 + 6 = 12).

Describe the structure of an atom, including the relative positions and charges of protons, neutrons, and electrons

Nucleus and electron shells

• An atom consists of a dense, positively charged nucleus surrounded by negatively charged electrons.
• The nucleus contains protons and neutrons, which are collectively called nucleons.
• Electrons occupy the space around the nucleus in regions called electron shells or energy levels (e.g., the first shell, second shell, etc.).

Electrostatic attraction and atomic stability

• The attraction between the positively charged nucleus and the negatively charged electrons holds the atom together.
  • This attraction is called electrostatic attraction or Coulomb force.
• The number of electrons in a neutral atom is equal to the number of protons, resulting in an overall neutral charge.

Mass and volume distribution in an atom

• The vast majority of an atom's mass is concentrated in the nucleus, while the electron shells occupy most of the atom's volume.
  • For example, the nucleus of a hydrogen atom accounts for about 99.95% of its mass but only occupies a tiny fraction of its volume.

Determine the number of protons, neutrons, and electrons in an atom or ion given its atomic number and mass number

Atomic number and proton count

• The atomic number (Z) of an element is equal to the number of protons in its nucleus.
  • For example, an atom with an atomic number of 8 has 8 protons in its nucleus and is identified as an oxygen atom.

Calculating the number of neutrons

• The mass number (A) of an atom is equal to the sum of the number of protons and neutrons in its nucleus.
• To determine the number of neutrons in an atom, subtract the atomic number from the mass number (A - Z).
  • For example, an atom with a mass number of 14 and an atomic number of 7 has 7 neutrons (14 - 7 = 7).

Electron count in neutral atoms and ions

• In a neutral atom, the number of electrons is equal to the number of protons, which is the atomic number.
• Ions are atoms that have gained or lost electrons, resulting in a net positive or negative charge.
  • For cations (positively charged ions), the number of electrons is less than the number of protons.
    • For example, a sodium cation (Na+) has 11 protons and 10 electrons.
  • For anions (negatively charged ions), the number of electrons is greater than the number of protons.
    • For example, a chloride anion (Cl-) has 17 protons and 18 electrons.

Explain the concept of isotopes and how they differ in terms of their subatomic composition

Definition and subatomic composition of isotopes

• Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
• Isotopes of an element have the same atomic number (Z) but different mass numbers (A).
  • For example, carbon-12, carbon-13, and carbon-14 are isotopes of carbon with 6, 7, and 8 neutrons, respectively.

Chemical properties and electron configuration

• The chemical properties of isotopes are nearly identical because they have the same electron configuration.
  • This is because chemical behavior is primarily determined by the number and arrangement of electrons in an atom.

Stability of isotopes

• Isotopes can be classified as stable or unstable (radioactive) depending on the ratio of protons to neutrons in their nuclei.
  • Stable isotopes have a balanced ratio of protons to neutrons and do not undergo radioactive decay.
  • Unstable isotopes have an imbalanced ratio of protons to neutrons and will undergo radioactive decay to achieve greater stability.

Average atomic mass and isotopic abundance

• The average atomic mass of an element is calculated based on the relative abundances and masses of its naturally occurring isotopes.
  • For example, the average atomic mass of carbon is 12.01 amu because carbon-12 (12 amu) makes up 98.93% of naturally occurring carbon, while carbon-13 (13.00335 amu) makes up 1.07%.