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Atoms are the fundamental building blocks of matter. They are composed of three subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus of the atom, while electrons orbit the nucleus.

Subatomic particles

  • Protons are positively charged particles. The number of protons in an atom is called the atomic number. The atomic number of an element determines its chemical properties.
  • Neutrons are neutral particles. They have no charge. The number of neutrons in an atom can vary. Atoms with the same number of protons but different numbers of neutrons are called isotopes.
  • Electrons are negatively charged particles. They are much smaller than protons or neutrons. The number of electrons in an atom is equal to the number of protons in the nucleus.

Atomic number and mass number

The atomic number of an atom is the number of protons in the nucleus. It is always a whole number. The mass number of an atom is the sum of the number of protons and neutrons in the nucleus. It is also a whole number, but it can be a fraction if the atom has isotopes.


Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are isotopes of carbon. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.

Relative atomic mass

The relative atomic mass of an element is the average mass of the isotopes of that element. It is calculated by multiplying the mass of each isotope by its natural abundance and then dividing by 100.

Mass spectrometer

A mass spectrometer is a device that can be used to determine the mass of atoms or molecules. It works by ionizing the atoms or molecules and then accelerating them through a magnetic field. The mass of the atoms or molecules is determined by how far they are deflected by the magnetic field.

Atomic orbitals

Atomic orbitals are regions of space around the nucleus where electrons are likely to be found. They are defined by four quantum numbers:

  • n is the principal quantum number. It determines the size and energy level of the orbital.
  • l is the angular momentum quantum number. It determines the shape of the orbital.
  • m_l is the magnetic quantum number. It determines the orientation of the orbital in space.
  • m_s is the spin quantum number. It determines the spin of the electron.

Electron configurations

Electron configurations are the arrangements of electrons in atomic orbitals. They are determined by the Aufbau principle, which states that electrons fill orbitals in order of increasing energy.

Line spectra

Line spectra are the emission spectra of atoms. They are produced when electrons in an atom move from a higher energy level to a lower energy level. The different lines in a line spectrum correspond to different energy levels.


The basics of atomic structure are essential for understanding chemistry. By understanding the structure of atoms, we can understand how they interact with each other and how they form molecules.

Additional topics

In addition to the topics covered above, the IB Chemistry curriculum also includes the following topics related to atomic structure:

  • Ionization energy is the energy required to remove an electron from an atom.
  • Electronegativity is a measure of the ability of an atom to attract electrons.
  • Periodic trends are the patterns that exist in the properties of elements as a function of their atomic number.

These topics can be found in the IB Chemistry syllabus.

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