Atomic structure and the periodic table — GCSE Chemistry Revision
Everything you need to revise atomic structure and the periodic table for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Atomic Structure: Protons, Neutrons, Electrons
Atoms are the fundamental building blocks of matter. They consist of a tiny, dense nucleus containing protons (positive charge, mass 1) and neutrons (no charge, mass 1). Orbiting the nucleus are electrons (negative charge, negligible mass). In a neutral atom, the number of protons equals the number of electrons. The atomic number (Z) is the number of protons and defines the element. The mass number (A) is the total number of protons and neutrons in the nucleus.
Isotopes and Ions
Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons, leading to different mass numbers. For example, Carbon-12 and Carbon-14 are isotopes. Ions are atoms or groups of atoms that have lost or gained electrons, resulting in an overall electrical charge. Cations are positive ions (lost electrons), while anions are negative ions (gained electrons). The formation of ions is crucial in ionic bonding.
Development of the Atomic Model
Our understanding of the atom has evolved over time. Dalton proposed indivisible spheres. Thomson discovered electrons with his 'plum pudding' model. Rutherford's gold foil experiment led to the nuclear model, with a tiny, dense, positively charged nucleus. Bohr refined this, suggesting electrons orbit in specific energy shells. The modern model includes electrons existing in regions of probability, not fixed orbits, a more complex understanding of sub-atomic particles.
Electron Shells and Reactivity
Electrons occupy specific energy levels or shells around the nucleus. The inner shells fill first. For the first 20 elements, shells can hold 2, 8, 8 electrons respectively. The number of electrons in the outermost shell (valence electrons) determines an element's chemical properties and reactivity. Atoms tend to gain, lose, or share electrons to achieve a stable outer shell, typically with 8 electrons (octet rule, except for the first shell which prefers 2).
Key facts
- Atomic number (Z) = number of protons.|Mass number (A) = protons + neutrons.|Neutral atom: protons = electrons.|Isotopes: same protons, different neutrons.|Ions: charged atoms (lost/gained electrons).|Outer shell electrons determine reactivity.
Key terms
- Atomic Number::The number of protons in an atom's nucleus, unique to each element.|Mass Number::The total number of protons and neutrons in an atom's nucleus.|Isotopes::Atoms of the same element with the same number of protons but different numbers of neutrons.|Ion::An atom or group of atoms that has an electrical charge due to gaining or losing electrons.|Electron Shells::Specific energy levels or orbits where electrons are found around the nucleus.|Relative Atomic Mass (Ar)::The weighted average mass of an atom of an element, taking into account its isotopes and their relative abundances.
Common mistakes
- Confusing atomic number with mass number when calculating subatomic particles.|Incorrectly stating that the number of neutrons must equal the number of protons or electrons in an atom.|Not remembering that electrons have a negligible mass compared to protons and neutrons.|Assuming all atoms of an element are identical, forgetting about isotopes.
Exam tips
- Practice calculating protons, neutrons, and electrons for various atoms and ions.|Learn the key scientists and their contributions to the atomic model (Dalton, Thomson, Rutherford, Bohr).|Understand how electron configuration (especially outer shell electrons) relates to chemical reactivity.|Be able to define and differentiate between atomic number, mass number, isotopes, and ions clearly.
Quick quiz
1. Which subatomic particle has a relative mass of 1 and a relative charge of +1?
- Electron
- Neutron
- Proton
- Ion
Show answer
Proton — Protons have a relative mass of 1 and a relative charge of +1. Neutrons have a mass of 1 and no charge, while electrons have negligible mass and a charge of -1.
2. An atom has 11 protons, 12 neutrons, and 11 electrons. What is its mass number?
- 11
- 12
- 22
- 23
Show answer
23 — The mass number is the total number of protons and neutrons. In this case, 11 protons + 12 neutrons = 23.
3. Which statement correctly describes isotopes?
- Atoms of different elements with the same mass number.
- Atoms of the same element with different numbers of electrons.
- Atoms of the same element with different numbers of neutrons.
- Atoms of different elements with different atomic numbers.
Show answer
Atoms of the same element with different numbers of neutrons. — Isotopes are defined as atoms of the same element (same number of protons) but with different numbers of neutrons, leading to different mass numbers.
4. According to Rutherford's model, where is most of the atom's mass concentrated?
- Evenly distributed throughout the atom
- In the orbiting electrons
- In a tiny, dense, central nucleus
- In the empty space surrounding the nucleus
Show answer
In a tiny, dense, central nucleus — Rutherford's gold foil experiment led to the conclusion that most of the atom's mass and all of its positive charge are concentrated in a small, central nucleus.
5. An atom has 9 protons, 10 neutrons, and 10 electrons. What is the charge of this particle?
- 0
- +1
- -1
- +9
Show answer
-1 — The number of protons (positive charges) is 9, and the number of electrons (negative charges) is 10. The overall charge is 9 + (-10) = -1, indicating it is an anion.
Exam-style questions
Describe the structure of a neutral atom, including the relative charges and masses of its subatomic particles. [3 marks]
Show mark scheme
- An atom consists of a nucleus and orbiting electrons.
- The nucleus contains protons (relative mass 1, charge +1) and neutrons (relative mass 1, charge 0).
- Electrons (negligible mass, charge -1) orbit the nucleus, and in a neutral atom, the number of protons equals the number of electrons.
An atom of element X has 17 protons, 18 neutrons, and 17 electrons. Write down its atomic number and mass number. State whether it is an ion or a neutral atom, and explain your answer. [4 marks]
Show mark scheme
- Atomic number = 17.
- Mass number = 35 (17 protons + 18 neutrons).
- It is a neutral atom.
- Because the number of protons (positive charges) equals the number of electrons (negative charges).
Explain what isotopes are, referring to the subatomic particles involved. Give an example. [3 marks]
Show mark scheme
- Isotopes are atoms of the same element.
- They have the same number of protons but a different number of neutrons.
- Example: Carbon-12 and Carbon-14 (or any correct example like Chlorine-35 and Chlorine-37).
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