1. Subatomic Particles
Atoms are the fundamental building blocks of matter. They consist of a dense, positively charged nucleus surrounded by a cloud of negatively charged electrons. The nucleus contains protons and neutrons, collectively known as nucleons.
| Particle | Relative Charge | Relative Mass | Location |
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1836 (≈0) | Orbitals/Shells |
Key Facts
- The nucleus contains virtually all of the atom’s mass due to the much larger mass of protons and neutrons compared to electrons.
- Electrons reside in specific regions of space around the nucleus called atomic orbitals, grouped into energy levels or shells. These shells are not fixed paths like planets orbiting a star, but rather represent regions of probability for finding an electron.
- Atoms are electrically neutral because the number of protons (positive charge) equals the number of electrons (negative charge).
Additional Properties of Subatomic Particles
| Particle | Discovery | Naming Origin | Mass (grams) | Charge (coulombs) |
| Proton | Rutherford (1919), but evidence earlier | From Greek “protos” meaning “first” | 1.67262 × 10⁻²⁴ | +1.602 × 10⁻¹⁹ |
| Neutron | Chadwick (1932) | From “neutral” | 1.67493 × 10⁻²⁴ | 0 |
| Electron | Thomson (1897) | From Greek “elektron” meaning “amber” | 9.10938 × 10⁻²⁸ | -1.602 × 10⁻¹⁹ |
2. Atomic and Mass Numbers
- Atomic Number (Z): The number of protons in the nucleus of an atom. This number defines the element. All atoms of a given element have the same number of protons.
- Mass Number (A): The total number of protons and neutrons in the nucleus of an atom.
Formula: A = Z + N, where N is the number of neutrons.
Example: Carbon-12 (¹²C) has an atomic number (Z) of 6 (6 protons) and a mass number (A) of 12. Therefore, it has 12 – 6 = 6 neutrons.
Exam Tip: The atomic number is unique to each element. The mass number can vary for atoms of the same element due to the presence of isotopes.
3. Isotopes
Definition: Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, and therefore different mass numbers.
Examples:
- Carbon isotopes: ¹²C (most abundant), ¹³C, and ¹⁴C (radioactive).
- Hydrogen isotopes: ¹H (protium), ²H (deuterium), and ³H (tritium).
Uses of Isotopes
- Radiocarbon dating: ¹⁴C is used to determine the age of organic materials up to about 50,000 years old.
- Nuclear power: ²³⁵U (uranium-235) is a fissile isotope used in nuclear reactors.
- Medical applications: Radioactive isotopes like ¹³¹I (iodine-131) are used in diagnosis and treatment of thyroid conditions. ⁶⁰Co (cobalt-60) is used in radiotherapy.
- Tracers: Stable isotopes can be used as tracers to follow the movement of substances in chemical reactions or biological systems.
Exam Tip: Isotopes have the same chemical properties because their electron configurations are identical (same number of protons and electrons). However, they have different physical properties, such as mass, density, and rates of diffusion, due to the difference in the number of neutrons.
4. Relative Atomic Mass (Ar)
The relative atomic mass (Ar) of an element is the weighted average mass of its isotopes compared to 1/12th the mass of a ¹²C atom. It’s a weighted average because it takes into account the relative abundance of each isotope.
Formula:
Ar = Σ (Isotopic mass × Percentage abundance) / 100
Example Calculation
Chlorine has two isotopes: ³⁵Cl (75% abundance) and ³⁷Cl (25% abundance).
Ar = (35 × 75) + (37 × 25) / 100 = 35.5
Exam Tip: Relative atomic masses are usually given to one decimal place.
5. Ions
Ions are formed when atoms gain or lose electrons, resulting in a net electrical charge.
- Cations: Positively charged ions formed when atoms lose electrons. Example: Na⁺ (sodium ion).
- Anions: Negatively charged ions formed when atoms gain electrons. Example: Cl⁻ (chloride ion).
Exam Tip: The number of protons never changes when an atom becomes an ion. Only the number of electrons changes.
6. Worksheet
1. Which of the following statements about isotopes is incorrect?
a) Isotopes have the same number of protons.
b) Isotopes have different numbers of neutrons.
c) Isotopes have different chemical properties.
d) Isotopes have different physical properties.
2. An atom of element X has a mass number of 39 and an atomic number of 19. How many neutrons are in the nucleus of this atom?
a) 19 b) 20 c) 39 d) 58
3. The relative atomic mass of an element is 24.3. Which of the following is not a possible explanation for this value?
a) The element has only one isotope with a mass number of 24.
b) The element has two isotopes with mass numbers of 24 and 25.
c) The element has two isotopes with mass numbers of 23 and 25.
d) The element has more than two isotopes.
4. A sodium ion (Na⁺) has:
a) 11 protons and 11 electrons.
b) 11 protons and 10 electrons.
c) 10 protons and 11 electrons.
d) 10 protons and 10 electrons.
5. Which of the following is the correct formula for calculating relative atomic mass?
a) Ar = Σ (Isotopic mass + Percentage abundance) / 100
b) Ar = Σ (Isotopic mass × Percentage abundance) / 100
c) Ar = Σ (Isotopic mass / Percentage abundance) × 100
d) Ar = Σ (Percentage abundance / Isotopic mass) × 100
6. An element has two isotopes: ⁶⁹X (60% abundance) and ⁷¹X (40% abundance). What is the relative atomic mass of X?
a) 69.8 b) 70.0 c) 70.2 d) 70.4
Answers:
- c)
- b)
- a)
- b)
- b)
- a)
