Atomic Structure
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Press your hand flat on the table. It stops. Nothing sinks in, nothing passes through, and the surface feels as solid as anything in the world. Yet if you could shrink until a single atom of that table filled a school sports hall, the part of it that carries virtually all the weight would be one grain of sand hovering at the centre circle, and everything else in the hall would be empty. That is not a poetic exaggeration. It is a measured result, and this lesson is about the experiment that measured it.
You will start with two numbers, the radius of an atom and the radius of its nucleus, and find out how astonishingly far apart they are. Then you will follow the one piece of evidence the specification names by name: a narrow beam of alpha particles fired at a sheet of gold foil, and what the three things that happened to them force you to conclude about where an atom keeps its mass. After that you will meet the three particles by their relative masses and charges, count them for any atom you are handed, turn atoms into ions by taking electrons off them, and learn to read the notation that packs a whole nucleus into two small numbers written beside a chemical symbol.
Objetivos
- Atoms are very small, having a radius of about 10-10 metres. The simple model of an atom is a small central positively charged nucleus composed of protons and neutrons, surrounded by electrons. The radius of the nucleus is much smaller than that of the atom with almost all of the mass in the nucleus.
- The scattering of alpha particles by thin metal foil provides evidence of the distribution of mass in the atom.
- The relative masses and electric charges of protons, neutrons and electrons are as follows: Relative mass Relative charge proton 1 1 neutron 1 0 electron Very small -1
- In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no overall electrical charge.
- In each atom its electrons are arranged at various distances from the nucleus. Atoms may lose or gain outer electrons to form charged particles called ions.
- The atoms of a particular element always have the same number of protons, but have a different number of neutrons for each isotope. The total number of protons in an atom is called its proton number or atomic number. The total number of protons and neutrons in an atom is called its mass number. Atoms can be represented as shown: (Mass number) 23 (Atomic number) 11 Na
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- Which particles are found in the nucleus of an atom? A. Protons and electrons B. Neutrons and electrons C. Protons and neutrons D. Protons, neutrons and electrons Answer: C
- What is the approximate radius of an atom? A. 10^-6 m B. 10^-10 m C. 10^-15 m D. 10^-20 m Answer: B
- In an alpha-particle scattering experiment, a very small number of alpha particles are deflected straight back towards the source. What does this show? A. The atom is mostly empty space B. Electrons are much lighter than alpha particles C. Almost all the mass of the atom is concentrated in a tiny nucleus D. Alpha particles are positively charged Answer: C
- An atom of an element has 19 protons, 20 neutrons and 19 electrons. It then loses one electron. What is the overall charge on the particle formed? A. -1 B. 0 C. +1 D. +19 Answer: C
- Two atoms are isotopes of the same element. Which quantity must be the same for both? A. The number of neutrons B. The number of protons C. The mass number D. The total number of particles in the nucleus Answer: B
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