Introduction to Atomic Structure and Mass

Welcome to the captivating world of atoms! In the topic on Atomic Structure, students are introduced to the microscopic world of atoms, where its constituents such as protons, neutrons and electrons are explained in terms of its characteristics.

The unique arrangement of these subatomic components determines an atom’s characteristics, from its size and mass to its chemical reactivity. 

As we delve into the realm of atoms, we’ll explore their intriguing structure, delve into their electronic configurations, and uncover the remarkable ways in which they interact to form the diverse world of molecules and materials. 

Protons, Neutrons and Electrons

An atom is always called a “fundamental building blocks of matter”, but what are atoms made of? The answer is that protons, neutrons, and electrons are the three main subatomic particles that make up atoms.

Keeping in mind that the structure of an atom is made of these three subatomic particles, it then becomes paramount to understand the charges of each of these subatomic particles, as it affects the atomic number and hence the element that the numbers are meant to represent. 

Protons:

Charge: Protons carry a positive electrical charge.

Relative Charge: The relative charge of a proton is +1.

Relative Mass: The relative mass of a proton is approximately 1 atomic mass unit (u).

Neutrons:

Charge: Neutrons are electrically neutral, meaning they do not carry a net electrical charge.

Relative Charge: The relative charge of a neutron is 0.

Relative Mass: The relative mass of a neutron is also approximately 1 atomic mass unit (u), very similar to that of a proton.

Electrons:

Charge: Electrons carry a negative electrical charge.

Relative Charge: The relative charge of an electron is -1.

Relative Mass: Electrons are much lighter than protons and neutrons. Their relative mass is considered negligible, often approximated as nearly 0 atomic mass units (u).

Atom structure of Oxygen 

As one of the most common element on earth, the element of oxygen is as follows: 

Protons: Oxygen has 8 protons in its nucleus. The number of protons, also known as the atomic number (Z), uniquely identifies the element as oxygen. The atomic number for oxygen is 8.

Neutrons: The most common isotope of oxygen, known as oxygen-16 (O-16), has 8 neutrons in its nucleus. This isotope is the most stable and abundant form of oxygen found in nature. However, oxygen can also exist in other isotopic forms with varying numbers of neutrons, such as oxygen-17 (O-17) and oxygen-18 (O-18).

Electrons: Oxygen has 8 electrons orbiting around the nucleus. Electrons are arranged in electron shells or energy levels, with 2 electrons in the first shell and 6 electrons in the second shell.

With its chemical properties, it can be combined with other elements to form various compounds, such as H2O (Water) and Oxygen gas (O2). 

How to calculate the numbers of protons, neutrons and electrons based on the Atomic Number?   

For the O level chemistry syllabus, one of the learning outcomes of the chapter on atomic structure is to be able to deduce the numbers of protons, neutrons and electrons in atoms and ions given proton and nucleon numbers.

For the A level Chemistry syllabus, it is taken a step further, where students should be able to determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic or proton number, mass, or nucleon number and charge. Additionally, tests on the angle of deflection and orbitals are also part of the chapters range of tests. (Refer to a detailed guide on angle of deflection and A level MCQ questions in our curated guide.)

For a start, consider the following question: 

Determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic number 27.

Atoms:

The atomic number (Z) represents the number of protons in an atom. For an atom with an atomic number of 27, it is a cobalt atom (Co).

Number of Protons (Z): 27

To determine the number of neutrons, you can use the approximate atomic mass of cobalt, which is around 58.933 u (atomic mass unit). Subtract the number of protons (atomic number) from the atomic mass to find the number of neutrons.

Number of Neutrons (N) = Atomic Mass (A) – Number of Protons (Z)

= 58.933 – 27 = 31.933 ≈ 32

For neutral atoms, the number of electrons is equal to the number of protons.

Number of Electrons = Number of Protons = 27

Ions:

If the atom gains or loses electrons, it becomes an ion. An ion is represented with a charge (positive or negative). Let’s consider cobalt (Co) forming a +2 ion.

For a +2 ion, it means the atom has lost 2 electrons compared to the neutral atom.

Charge = +2

Number of Electrons (in ion) = Number of Protons – Charge 

= 27 – 2 = 25

So, for an atom with an atomic number of 27 (cobalt):

Number of Protons = 27

Number of Neutrons ≈ 32

Number of Electrons (in neutral atom) = 27

Number of Electrons (in +2 ion) = 25

Atomic Structure Worksheet

Apart from the calculation of protons, neutrons and electrons, students may also be tested on the MCQ sections through true and false statements. Head on to the Atomic Structure (Chapter 2) worksheet provided here to build up on the fundamentals!