---

Structure 1.2.1—The nuclear atom

The Nucleus: Protons, Neutrons, and Nucleons

Atoms consist of three primary subatomic particles: protons, neutrons, and electrons. The core of an atom is the nucleus, which is both positively charged and dense. The nucleus is composed of protons and neutrons, collectively referred to as nucleons.

Protons carry a positive charge (+1), while neutrons are neutral. Both contribute significantly to the mass of an atom. Their number within the nucleus determines the identity and mass of the element.

Electrons: Orbiting the Nucleus

Surrounding the nucleus is a cloud of negatively charged electrons (–1). These are much lighter than protons and neutrons, with their mass often considered negligible. Electrons occupy specific regions called electron shells or energy levels.

Electrons play a crucial role in determining chemical properties and reactivity. They participate in chemical reactions and form bonds with other atoms. The arrangement and distribution of electrons dictate the behavior of elements in chemical processes.

The Nuclear Symbol: ^AX_Z

The nuclear symbol provides key information about an atom’s composition:

• A: Mass number (sum of protons and neutrons)
• X: Chemical symbol of the element
• Z: Atomic number (number of protons)

Note: The MASS number is always the more MASSive number of the two.

Deducing Protons, Neutrons, and Electrons

1. Protons = atomic number (Z). E.g. ²³Na₁₁ has 11 protons.
2. Neutrons = mass number – atomic number (A – Z). E.g. 23 – 11 = 12 neutrons.
3. Electrons: In a neutral atom, electrons = protons. For ions, adjust for charge. E.g. Na⁺ has 10 electrons (11 – 1).

Atomic Symbol	Atomic Number	Mass Number	Protons	Neutrons	Electrons
Mg
Cl
Ti
Sr
Ag
Xe
Ba
La
Pt

Questions

1. What is the atomic number (Z) and what does it represent?
2. How can you calculate the number of neutrons using the nuclear symbol?
3. Given ¹⁴N₇, how many protons, neutrons, and electrons are in a neutral nitrogen atom?
4. If oxygen (¹⁶O₈) gains two electrons to form O^2–, how many electrons will it have?
5. An atom has 12 protons, 12 neutrons, and 10 electrons. What is its nuclear symbol, and is it neutral or an ion?

Answers

---

Structure 1.2.2—Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. While isotopes share the same chemical properties (identical electron configurations), they differ in physical properties such as mass and density.

Calculating Relative Atomic Masses

The relative atomic mass is the weighted average of an element’s naturally occurring isotopes. It can be non-integer because it accounts for the distribution of various isotopes:

Relative atomic mass = Σ (isotope mass × isotope abundance)

For example, an element with isotope A (mass 10 u, abundance 70%) and isotope B (mass 11 u, abundance 30%):

(10 × 0.70) + (11 × 0.30) = 7 + 3.3 = 10.3 u

Physical Properties of Isotopes

Isotopes differ in mass, density, boiling and melting points, and rates of radioactive decay. For example, hydrogen’s isotopes—protium (¹H), deuterium (²H), and tritium (³H)—share chemical properties but differ in mass and stability. Tritium is radioactive and undergoes beta decay.

Questions

1. What is an isotope, and how do isotopes of the same element differ?
2. Why can relative atomic mass be a non-integer value?
3. An element has isotope A (15 u, 40%) and isotope B (16 u, 60%). Calculate the relative atomic mass.
4. How do physical properties of isotopes differ?
5. How can isotopes be used as tracers in chemical reactions?

Answers

---

Structure 1.2.3—Mass Spectrometry (HL only)

Mass spectrometry involves ionizing a sample and separating the resulting ions based on their mass-to-charge ratio. The mass spectrum shows the identity and relative abundance of isotopes.

Steps in a Mass Spectrometer

1. Ionization: The sample is bombarded with high-energy electrons, causing atoms to lose electrons and become positive ions.
2. Acceleration: Positive ions are accelerated by an electric field.
3. Deflection: A magnetic field deflects ions based on their mass-to-charge ratio (m/z). Heavier ions are deflected less.
4. Detection: A detector records the signal, producing a mass spectrum showing abundance vs m/z.
5. Analysis: Peak positions identify isotopes; peak heights indicate relative abundance.

By analyzing the mass spectrum, we can determine relative atomic masses based on isotopic composition. Many elements have multiple isotopes, and the isotopic composition can provide information about the origin of a sample.

Questions

1. What is mass spectrometry, and how does it separate ions?
2. How is a mass spectrum used to identify isotopes and their relative abundance?
3. How can mass spectrometry determine relative atomic masses from isotopic composition?
4. What is the relationship between peak position on a mass spectrum and the mass-to-charge ratio?

Answers