- Created by: DoRevision123
- Created on: 24-09-18 13:35
- Pure substances: e.g. sodium chloride, ethanol - cannot be broken down by any physical process, fixed together.
- Mixtures; combinations of pure substances, e.g. sea water --> water and salts such as sodium chloride not bonded to each other chemically but combined physically.
- Homogenous - well-mixed mixtures, concentrations of substances within are very uniform.
- Heterogenous - Not very mixed, concentrations of substances may vary between regions, even those such as different counties, e.g. soils will vary in types of salts and concentrations etc.
- Elements are the simplest substances that exist in nature - e.g. diamond as pure carbon, argon being the third most abundant atmospheric gas and gold, being very unreactive, often occurs by itself as a pure element.
- Elements can occur in molecules with others, e.g. carbon dioxide in the atmosphere and in compounds such as sodium chloride in the sea to make saltwater.
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- The smallest unit of an element; the building block of matter.
- Most atoms cannot change albeit this isn't a universal concept (e.g. during radioactive decay or nuclear reactions in plants).
- Most atoms are stable and stay the same.
- Nucleus makes up the majority of the atom's mass, with the relative mass of both neutrons and protons being 1.
- Electrons have a negligible mass that is a small fraction of the relative mass of neutrons/protons.
- Neutrons are present to keep protons together otherwise they'd repel and cause the atom to fall apart.
- Neutrons and protons exist in identical or similar numbers with isotopes resulting more than neutrons compared to protons (see isotopes).
- Electrons orbit around the nucleus of an atom, with numbers of electrons and protons being the same when no electrostatic charge is present therefore it is held together since opposite electrostatic attract.
- Electron orbit determines the size of the atom.
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- The number of protons determines the type of element an atom is.
- A (mass number) = Z (atomic number) + N (number of neutrons).
- Atomic number determines the type of element an atom is, number of neutrons determines its isotope.
- Isotopes of an element have the same number of protons but a different number of element meaning their mass numbers differ. Those with similar numbers of neutrons and protons are stable isotopes but the bigger the difference, the less stable, and therefore more radioactive, the isotope becomes. The latter undergo radioactive decay.
- E.g., magnesium.
- About 290 isotopes in total exist in nature, all but 23 elements in the periodic table can occur as mixtures of isotopes.
- "Heavy" Isotopes of hydrogen have esoteric names;
- 2^H = deuterium, 3^H = tritium.
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Climate Change Reconstruction.
- Variations of isotopic compositions in water through times provides an index of global temperature change through the millennia.
- Stable isotopes of oxygen (16^O, 17^O, 18^O) and hydrogen (1^H and 2^H/D) provide 9 possible isotopic combinations for molecular composition of water.
- Water molecules containing heavier isotopes (e.g. H^218^O and HD16^O) evaporate less readily than H^216^O.
- This means "light" water disproportionally evaporates from ocean waters compared to "heavy" water, therefore "heavier" water is left behind.
- During ice ages, there are low sea levels enriched with "heavy" water, larger continental ice sheet mass with most of the "light" water is locked up in the ice sheet masses.
- During interglacial periods, the continental ice sheet masses are relatively smaller and the sea level are higher as more "light" water is melted and returns to the ocean, diluting the "heavy" water concentration comparatively.
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