IGCSE Physics / Atoms and radioactivity
The structure of the atom
Edexcel International GCSE Physics (4PH1), chapter 22: Atoms and radioactivity, specification point 7.2, 7.1. Free lesson.
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Atoms have a tiny nucleus containing protons and neutrons, with negatively charged electrons orbiting far away. Knowing their charges and masses helps you explain why atoms are neutral and read nuclear notation.
What is inside an atom?
An atom has a tiny, dense centre called the nucleus. Protons and neutrons are packed into it. Electrons orbit a long way from the nucleus, so most of an atom is empty space. The nucleus is positively charged because it contains protons. Neutrons have no charge. Electrons are negatively charged.
What are the charges and masses of the particles?
A particle's relative charge compares its charge with a proton's. Its relative mass compares its mass with a proton's. The electron's mass is so small that nearly all an atom's mass is in its nucleus. Proton: in the nucleus, relative charge +1, relative mass 1. Neutron: in the nucleus, relative charge 0, relative mass 1. Electron: orbiting the nucleus, relative charge −1, relative mass about 1/2000.
Why does an atom have no overall charge?
The nucleus is positive, but that does not make the whole atom positive. A neutral atom has equal numbers of protons and electrons. Their charges cancel because each proton has charge +1 and each electron has charge −1. An atom has no overall charge when its numbers of protons and electrons are equal. The nucleus is positive, but the electrons outside it must be included when considering the atom's total charge.
How do you read nuclear notation?
Nuclear notation gives the element's symbol and two numbers. The mass number is written on top and counts protons and neutrons together. The atomic number is written below and counts protons only. The letter is the chemical symbol. For carbon-14, ¹⁴₆C, the mass number is 14, the atomic number is 6 and C is the symbol for carbon. The mass number is never smaller than the atomic number.
What are activity and count rate measured in?
The becquerel (Bq) is the unit of activity. One becquerel means one nucleus decays, or gives out radiation, every second. A detector's count rate is recorded as counts per second or counts per minute. Time in this topic is measured in seconds (s), minutes (min) or hours (h). Write activity in becquerels (Bq). Write a detector's count rate in counts per second or counts per minute.