Structure of the Atom, Atomic Symbols and Isotopes – HSC Chemistry


This is part of the HSC Chemistry course under the topic of Atomic Structure and Atomic Mass

HSC Chemistry Syllabus

  • investigate the basic structure of stable and unstable isotopes by examining:

– their position in the periodic table

– the distribution of electrons, protons and neutrons in the atom

– representation of the symbol, atomic number and mass number (nucleon number)

    Structure of the Atom

    This section of the video will discuss what atoms are and what subatomic atoms consist of. It will also discuss the Rutherford model as a pivotal model for the development of our understanding of an atom's structure. 


    The Atom

    Atoms are the fundamental building blocks of matter. Each element on the periodic table consists of unique atoms that determine its properties. The understanding atomic structure is a cornerstone in the field of chemistry and physics.

    Rutherford's Atomic Model

    In 1911, physicist Ernest Rutherford introduced a groundbreaking model to illustrate the structure of an atom. Drawing upon experimental evidence, Rutherford proposed that an atom comprises a densely concentrated nucleus, possessing a positive charge and housing the majority of the atom's mass. This nucleus, he hypothesised, was encircled by a cloud of orbiting electrons.

    Rutherford's hypothesis stemmed from his innovative experiment involving alpha particle scattering. He bombarded a thin sheet of gold foil with alpha particles (helium nuclei) emitted from a radioactive source. The setup included a screen coated with zinc sulfide (ZnS), which flashed light whenever struck by an alpha particle.

    The observations were intriguing: most alpha particles passed through the foil with little to no deflection, suggesting that atoms were mostly empty space. However, some particles were deflected at large angles, and a few even bounced back, indicating the presence of a dense, central positively charged core – the nucleus



    Rutherford’s model likened the atom to a miniature solar system. In this analogy, the nucleus was akin to the sun – compact and massive, holding the majority of the atom's mass. Orbiting this nucleus were electrons, similar to planets revolving around the sun. 

    The positively charged particles in the nucleus are known as protons, which, despite carrying an equal but opposite charge to electrons, are considerably larger and heavier. 

    In 1932, the discovery of the neutron added another dimension to this model. Neutrons, neutral particles residing in the nucleus alongside protons, provided an explanation for the stability and cohesiveness of the nucleus. 


    Rutherford's atomic model


    Key components of an atom in Rutherford's model:

    1. Nucleus:

    Protons: Protons are positively charged particles which are found in the nucleus. The number of protons defines the atomic number, and thus, the identity of the element itself.

    Neutrons are neutrally charged particles that reside in the nucleus. neutrons add to the atomic mass without affecting the charge.  

    2. Electrons:

    Electrons are negatively charged particles that orbit the nucleus. The number of orbiting electrons depend on the element, and should equal to the number of protons in the nucleus for any atom; this leads atoms to have an overall neutral charge.

    3. Empty Space

    The experimental observation of most alpha particles passing through the gold foil suggested that atoms are mostly empty space.

    Atomic Symbols 

    Atomic symbols identify the element, and provide information on its atomic composition in terms of electrons, protons, and neutrons. For a neutral atom, the number of electrons equals the number of protons. 


    Figure 1. The image above demonstrates a general notation for atomic symbols on the left and the atomic symbol for carbon on the right.


    • Atomic Number (Z) represents the number of protons in an atom. It defines the element and can be found on the periodic table alongside each element's symbol
    • Mass Number (A): The total count of protons and neutrons in the atom's nucleus. This is also known as the nucleon number. The number of neutrons in an atomic can therefore be calculated by subtracting the atomic number (Z) from the mass number (A-Z)



    Each element on the periodic table is assigned an atomic number, which is crucial for determining its properties. The relative atomic mass considers the atomic mass of all stable isotopes (see below) of the element.

    For instance, Carbon (C) is number 6, indicating it has 6 protons, while Uranium (U) is number 82, reflecting its 82 protons. Since protons and neutrons have nearly equal masses, the mass number represents the sum of these two particles in the nucleus. The atomic mass of carbon is 12, which means the number of neutrons equals 12 – 6 = 6 neutrons. Since the number of electrons equals the number of protons in any atom, there are also 6 electrons in an atom of carbon.


    Isotopes are variants of an element that have different numbers of neutrons in their nuclei. Isotopes have the same number of protons (otherwise this changes the identity of element) but the vary number of neutrons leads to different atomic mass. The variance of atomic mass among isotopes of an element is reflected in its relative atomic mass which is provided in the periodic table. 

    Each element on the periodic table has several isotopic forms. Notably, the chemical properties of an element's isotopes are often nearly identical due to the unchanging number of protons and electrons.

    However, an exception is found in hydrogen's isotopes. Hydrogen's primary isotope has an atomic mass of 1, so any addition of neutrons significantly alters its atomic mass. While the chemical properties of isotopes of an element are generally similar, their physical properties can vary considerably. This variation is predominantly due to differences in mass, which can influence properties like density, rate of diffusion, and nuclear stability.


    Figure 2. The images above demonstrate the atomic symbol notation for the primary isotopes of hydrogen with one proton, and another isotope 'deuterium' which has one proton and one neutron. They have an atomic mass of 1 and 2 respectively. 


    Isotopes can be classified as being natural or artificial

    • Natural Isotopes: Natural isotopes are those which are found in nature. As previously mentioned, hydrogen's primary isotopic form exists with an atomic mass of one, but can also exist in nature with as an isotope containing one neutron (deuterium). 
    • Artificial Isotopes: These isotopes are manufactured in nuclear laboratories by bombarding the elements with subatomic particles, and usually have an extremely short lifespan because of their unstable nature and radioactivity. 


    Figure 3: The image above demonstrates the atomic symbols for two isotopes of carbon – carbon 12 and carbon 14. They both have the same atomic number (6) but have different mass numbers – 6 and 8. Carbon-12 isotope has 6 neutrons whereas carbon-14 isotope has 8 neutrons. Both isotopes have the same number of protons (6) and electrons (6). 


    Previous Section: Percentage Composition

    Next Section: Relative Mass and Calculating Relative Mass