What is inside an atom, and how do protons, neutrons and electrons define an element?
Structure of the atom: describe protons, neutrons and electrons, atomic number and mass number, and the historical development of the atomic model from Dalton to the modern view.
A focused Virginia SOL Chemistry answer on standard CH.2: the subatomic particles, atomic number and mass number, how they define an element and its ions, and the development of the atomic model from Dalton, Thomson and Rutherford to Bohr and the modern model.
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What this topic is asking
Standard CH.2 opens with the atom itself. Virginia expects you to describe the three subatomic particles and where they sit, to use atomic number and mass number to count them, to see how the atomic number defines the element, and to outline how the atomic model developed as scientists gathered evidence. The periodic table provided on the test supplies the atomic number and atomic mass for every element.
The subatomic particles
Almost all the mass of an atom is in the nucleus, but almost all the volume is the electron cloud, so an atom is mostly empty space. Because protons and electrons carry equal and opposite charge, a neutral atom has equal numbers of each.
Atomic number and mass number
In a neutral atom the number of electrons equals the atomic number. If electrons are gained or lost the atom becomes an ion, but the atomic number (and so the identity of the element) never changes; only a change in the number of protons would change the element. Nuclide notation writes the mass number as a superscript and the atomic number as a subscript before the symbol.
How the atomic model developed
The picture of the atom changed as new experiments produced new evidence, a clear illustration of CH.1 in action:
- Dalton (early 1800s) proposed that matter is made of tiny, indivisible atoms, identical for a given element, that combine in whole-number ratios.
- Thomson discovered the electron using cathode rays and proposed the "plum-pudding" model: a ball of positive charge with electrons scattered through it.
- Rutherford fired alpha particles at thin gold foil. Most passed straight through, but a few bounced sharply back, showing that the positive charge and most of the mass are concentrated in a tiny nucleus, with the atom mostly empty space.
- Bohr refined this by placing electrons in fixed circular energy levels (shells), explaining the line spectrum of hydrogen.
- The modern (quantum-mechanical) model keeps energy levels but treats electrons as occupying regions of probability (orbitals) rather than fixed orbits.
Each model kept what still fit the evidence and revised what did not, which is exactly how scientific theories develop.
Try this
Q1. How many neutrons are in an atom with atomic number and mass number ? [1 point]
- Cue. neutrons.
Q2. Which experiment provided evidence for a small, dense, positively charged nucleus? [1 point]
- Cue. Rutherford's gold-foil (alpha-scattering) experiment.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksAn atom has 17 protons, 18 neutrons and 17 electrons. What is its mass number? (A) 17 (B) 18 (C) 35 (D) 52Show worked answer →
The answer is (C) 35.
The mass number is the total number of protons plus neutrons in the nucleus: . Electrons are not counted because their mass is negligible. The atomic number (17) is the number of protons alone, which identifies the element as chlorine. Because protons equal electrons (17 each), the atom is neutral.
The trap is adding the electrons or using only the proton count; the mass number is protons plus neutrons only.
SOL (technology-enhanced, drag and drop)3 marksMatch each contribution to the correct scientist: (a) proposed that atoms are indivisible spheres; (b) discovered the electron and the plum-pudding model; (c) discovered the small, dense, positive nucleus from the gold-foil experiment. Choose from Dalton, Thomson, Rutherford.Show worked answer →
A 3-point matching item on the history of the atomic model.
(a) Dalton (1 point): proposed atoms as solid, indivisible spheres (atomic theory).
(b) Thomson (1 point): discovered the electron and pictured the atom as positive matter with embedded electrons (the plum-pudding model).
(c) Rutherford (1 point): the gold-foil experiment showed most of the atom is empty space with a tiny, dense, positively charged nucleus.
Markers reward correct pairing. The sequence Dalton then Thomson then Rutherford then Bohr then the modern model shows how evidence refined the picture of the atom.
Related dot points
- Isotopes and average atomic mass: define isotopes, write nuclide notation, and calculate the weighted average atomic mass of an element from its isotopes.
A focused Virginia SOL Chemistry answer on isotopes under CH.2: what isotopes are, how to read nuclide notation, and how to calculate the weighted average atomic mass of an element from the masses and natural abundances of its isotopes.
- Electron configuration and energy levels: describe how electrons occupy energy levels, write electron configurations, identify valence electrons, and relate ground and excited states to spectra.
A focused Virginia SOL Chemistry answer on electron arrangement under CH.2: energy levels and sublevels, writing electron configurations, counting valence electrons, and the difference between ground state and excited state and how it produces line spectra.
- The periodic table and periodic trends: describe the organization of the periodic table and the trends in atomic radius, ionization energy, electronegativity and reactivity across periods and down groups.
A focused Virginia SOL Chemistry answer on the periodic table under CH.2: how it is organized into groups, periods, metals, nonmetals and metalloids, and the trends in atomic radius, ionization energy, electronegativity and reactivity and why each runs the way it does.
- Nuclear chemistry and radioactivity: describe alpha, beta and gamma decay, balance nuclear equations, distinguish fission from fusion, and use half-life.
A focused Virginia SOL Chemistry answer on nuclear processes under CH.2: alpha, beta and gamma decay, balancing nuclear equations, the difference between fission and fusion, and using half-life to find how much of a sample remains.
- Scientific investigation and experimental design: plan and conduct safe investigations, identify independent, dependent and controlled variables, and distinguish hypothesis, theory and law.
A focused Virginia SOL Chemistry answer on standard CH.1: planning a safe, fair investigation, identifying independent, dependent and controlled variables, the difference between accuracy and precision, and how hypothesis, theory and law differ in science.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)