Atomic Structure

This section covers the fundamental aspects of the atom, including its constituent particles, isotopes, the mass spectrometer, and electronic configuration. Understanding these concepts is crucial for comprehending chemical bonding and the properties of elements.

The Particles in the Atom

The atom is composed of three subatomic particles: protons, neutrons, and electrons.

Protons

• Located in the nucleus.
• Carry a positive electrical charge (+1).
• The number of protons defines the element (atomic number, Z).

Neutrons

• Located in the nucleus.
• Carry no electrical charge (neutral).
• Contribute to the mass of the atom.

Electrons

• Orbit the nucleus in specific energy levels or shells.
• Carry a negative electrical charge (-1).
• Responsible for chemical bonding.

Particle	Charge	Location	Mass (relative)
Proton	+1	Nucleus	1
Neutron	0	Nucleus	1
Electron	-1	Orbitals	0

Isotopes

Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons. This means they have the same atomic number but different mass numbers.

The mass number (A) is the sum of the number of protons and neutrons: $A = Z + N$

Different isotopes of an element have different masses, and their relative abundance is often expressed as a percentage.

The Mass Spectrometer

A mass spectrometer is an instrument used to measure the mass-to-charge ratio (m/z) of ions. It's a powerful tool for determining the isotopic composition of an element and identifying unknown compounds.

The process generally involves:

1. **Ionization:** The sample is ionized, creating positive ions.
2. **Acceleration:** The ions are accelerated through an electric field.
3. **Deflection:** The ions are deflected by a magnetic field. The amount of deflection depends on their m/z ratio.
4. **Detection:** Detectors measure the abundance of ions with different m/z ratios.

Electronic Configuration

The electronic configuration describes the arrangement of electrons in the energy levels and sublevels around the nucleus of an atom. It dictates the chemical properties of an element.

The electronic configuration can be represented using:

• **Noble gas notation:** Using the symbol of the preceding noble gas followed by the configuration of the electrons in the inner shells.
• **Orbital notation:** Specifying the energy level (principal quantum number, n), sublevel (azimuthal quantum number, l), and the number of electrons in each orbital.

The Aufbau principle states that electrons first fill the lowest energy levels. Hund's rule states that electrons will individually occupy each orbital within a sublevel before doubling up in any one orbital. The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers.

Example: Oxygen (Z=8)

Electronic configuration: $1s^2 2s^2 2p^4$

Noble gas notation: [He] 2s² 2p⁴

The outermost electrons, known as valence electrons, are primarily responsible for chemical bonding.