Isotopes
To understand isotopes, we must first define the components of an atom. Every atom consists of a central nucleus containing protons and neutrons, surrounded by electrons in shells.
The identity of an element is determined solely by the number of protons (atomic number). The total mass is determined by the sum of protons and neutrons (mass number).
We use standard notation to represent atoms:
^{A}_{Z}X
Where:
- X is the chemical symbol of the element.
- Z is the atomic number (number of protons).
- A is the mass number (total number of protons + neutrons).
From this notation, we can derive the number of subatomic particles:
- Number of protons = Z
- Number of neutrons = A - Z
- Number of electrons (in a neutral atom) = Z
| Location |
|---|
| Nucleus |
| Nucleus |
| Shells outside nucleus |
Because they are the same element, they share the same atomic number (Z). Because they have different numbers of neutrons, they have different mass numbers (A).
Building on the notation ^{A}_{Z}X, isotopes will always have the same Z but different values for A.
Consider the following atoms:
- Atom A: ^{35}_{17}Cl
- Atom B: ^{37}_{17}Cl
Both have atomic number 17 (17 protons), so they are both Chlorine. However, Atom A has 35 - 17 = 18 neutrons, while Atom B has 37 - 17 = 20 neutrons. Since the proton count is identical but neutron count differs, Atom A and Atom B are isotopes.
Example 2: Ions vs Isotopes
Consider Lithium:
- ^{6}_{3}Li: Neutral atom. Protons=3, Neutrons=3, Electrons=3.
- ^{7}_{3}Li^+: Ion. Protons=3, Neutrons=4, Electrons=2.
^{6}Li and ^{7}Li are isotopes of each other (same protons, different neutrons). The + charge indicates it has lost an electron, but this does not change its identity as an isotope of Lithium.
| Isotope Relationship |
|---|
| Isotopes of each other |
| Isotopes of each other |
| Not an isotope of Cl |
Correction: The atomic number (Z) determines the element. If Z is the same, it is the same element, regardless of the mass number. Isotopes are defined by having the same protons and different neutrons. If two atoms have different numbers of protons, they are different elements entirely.
- They are atoms of the same element.
- They have the same number of protons (or same atomic number).
- They have different numbers of neutrons (or different mass numbers).
Why this is accepted: The markscheme requires 'same element' to establish identity, 'same protons' to define that identity chemically, and 'different neutrons' to distinguish the specific variation. Omitting any of these three components will result in lost marks.
Example Answer: "Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons."
Reasoning: Chemical reactions involve the interaction of electrons. Because isotopes of the same element have the same number of protons, neutral atoms of these isotopes also have the same number of electrons. Consequently, they have the same electronic configuration (arrangement of electrons in shells). Since the outer electron structure is identical, their chemical behavior is identical.
Why this is accepted: Examiners look for the causal chain: Same Protons \rightarrow Same Electrons \rightarrow Same Electronic Configuration \rightarrow Same Chemical Properties. Mentioning only 'same electrons' is often insufficient; you must explicitly state same electronic configuration to fully address the definition of chemical reactivity.
Example Answer: "Isotopes have the same number of protons, so neutral atoms have the same number of electrons and therefore the same electronic configuration. Chemical properties are determined by electronic configuration, so they react identically."