Atomic Structure and the History of the Atom
An atom has a nucleus containing protons and neutrons, surrounded by electrons in shells, and is neutral overall because the number of protons equals the number of electrons. The atomic number is the number of protons and defines the element; the mass number is the total number of protons and neutrons; and isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Learn the particle table exactly: proton, relative mass 1, charge +1, in the nucleus; neutron, relative mass 1, charge 0, in the nucleus; electron, relative mass very small (about 1/2000), charge -1, in shells around the nucleus.
- Read any nuclear symbol the same way: the bottom number is the atomic number (protons), the top number is the mass number, and neutrons = mass number - atomic number. For a neutral atom, electrons = protons.
- Fill electron shells in order, 2 then 8 then 8, and write the configuration with commas, for example sodium with 11 electrons is 2,8,1. The number of electrons in the outer shell gives the group number.
- Define an isotope precisely: atoms of the same element with the same number of protons but a different number of neutrons. Isotopes have identical chemical properties, because chemical reactions depend on the electrons, and only their masses differ.
- Calculate relative atomic mass from isotopic abundances as a weighted mean: multiply each isotope's mass number by its percentage abundance, add the products, then divide by 100.
- Learn the history as evidence changing a model, since that is how it is examined: plum pudding (a ball of positive charge with electrons in it), then alpha scattering (most particles passed straight through, so the atom is mostly empty space; a few deflected back, so there is a small, dense, positively charged nucleus), then Bohr (electrons in fixed shells at set distances), then Chadwick (evidence for the neutron).
Worked example
Chlorine exists as two isotopes: 75 percent of atoms are chlorine-35 and 25 percent are chlorine-37. Calculate the relative atomic mass of chlorine to one decimal place.
- Set up the weighted mean: relative atomic mass = (percentage of isotope 1 x mass 1 + percentage of isotope 2 x mass 2) / 100.
- Substitute: (75 x 35) + (25 x 37), all divided by 100.
- Calculate the first product: 75 x 35 = 2625.
- Calculate the second product: 25 x 37 = 925.
- Add and divide: (2625 + 925) / 100 = 3550 / 100 = 35.5.
- Final answer: the relative atomic mass of chlorine is 35.5, which is why the periodic table shows a value that is not a whole number.
Practice questions
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Q1State the relative mass and relative charge of a neutron.Show answer
Answer: Relative mass 1, relative charge 0.
Q2An atom has an atomic number of 17 and a mass number of 37. How many neutrons does it have?Show answer
Answer: 37 - 17 = 20 neutrons.
Q3Why is an atom electrically neutral?Show answer
Answer: It has equal numbers of protons and electrons, so the positive and negative charges cancel.
Q4Write the electronic structure of a magnesium atom (12 electrons).Show answer
Answer: 2,8,2.
Q5Define an isotope.Show answer
Answer: Atoms of the same element with the same number of protons but different numbers of neutrons.
Q6Why do isotopes of an element react in the same way?Show answer
Answer: Chemical reactions involve the electrons, and isotopes have the same number and arrangement of electrons.
Q7What did the alpha scattering experiment show that the plum pudding model could not explain?Show answer
Answer: That most alpha particles passed straight through, so the atom is mostly empty space, and that a few were deflected back, so the positive charge and most of the mass are concentrated in a tiny nucleus.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain how the results of the alpha particle scattering experiment led scientists to replace the plum pudding model.
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Boron has two isotopes: boron-10 with an abundance of 20 percent and boron-11 with an abundance of 80 percent. Calculate the relative atomic mass of boron.
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Free printable worksheet
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This topic is chapter 12 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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