Calculating particle numbers

Atomic Number and Mass Number

Understanding an element's identity is as simple as counting its positive charges. The atomic number (often called the proton number in OCR exams) tells you exactly how many protons are in the nucleus, and it is unique to each element.

The mass number (or nucleon number) tells you the total number of heavy particles in the nucleus. It is the sum of both protons and neutrons.

You can calculate the number of each subatomic particle using these simple formulas:

$$\text{Number of Protons} = \text{Atomic Number}$$ $$\text{Number of Electrons} = \text{Atomic Number} \text{ (for neutral atoms)}$$ $$\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$$

Extracting Data from the OCR Periodic Table

Have you ever noticed that standard science textbooks and your exam data sheet write elemental symbols differently? In standard nuclear notation, isotopes are written as $^{A}_{Z}\text{X}$, where the mass number ($A$) is at the top and the atomic number ($Z$) is at the bottom.

However, on the OCR B Periodic Table (Data Sheet J258), the layout is flipped. The atomic number is located at the top of the element cell, and the Relative atomic mass ($A_r$) is located at the bottom.

If an element has a decimal value for its relative atomic mass (like chlorine at $35.5$), you cannot calculate half a neutron. To find the particle counts for a typical atom, round the relative atomic mass to the nearest whole number to estimate the mass number of its most common form.

Calculating Subatomic Particles for Isotopes

Not all atoms of the same element are identical twins. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Because isotopes have the exact same number of electrons in their outer shells, they share identical chemical properties. However, their physical properties (such as density or boiling points) can differ because of their different masses.

Calculate the numbers of protons, neutrons, and electrons in an atom of Carbon-14.

Step 1: Identify the atomic number for protons and electrons.

• Using the Periodic Table, the atomic number for carbon is $6$.
• Therefore, the atom has $6$ protons. Because it is neutral, it also has $6$ electrons.

Step 2: Identify the mass number.

• The notation "Carbon-14" tells us the mass number is $14$.

Step 3: Calculate the number of neutrons.

• $\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$
• $\text{Number of Neutrons} = 14 - 6 = 8$ neutrons.
• Final Answer: $$ protons, neutrons, electrons.

Calculating Particles in Ions

What happens when an atom loses its perfect electrical balance? It becomes an ion, which is a charged particle formed when an atom gains or loses electrons.

A cation is a positive ion formed when an atom loses electrons, whereas an anion is a negative ion formed when an atom gains electrons. Crucially, the nucleus remains completely stable during this process; the number of protons and neutrons never changes when an atom becomes an ion.

Calculate the number of protons, neutrons, and electrons in an Aluminium ion ($^{27}_{13}\text{Al}^{3+}$).

Step 1: Identify protons and neutrons using standard rules.

• Atomic Number = $13$, so there are $13$ protons.
• Mass Number = $27$, so Neutrons = $27 - 13 = 14$ neutrons.

Step 2: Adjust the electron count for the charge.

• A neutral aluminium atom would have $13$ electrons.
• The $3+$ charge means it has lost $3$ electrons.
• $\text{Electrons} = 13 - 3 = 10$ electrons.
• Final Answer: protons, neutrons, electrons.