The Atom
? Smallest particle that can take place in a chemical reaction
? Consists of three sub-atomic particles, electrons, protons, neutrons
Electrons
? Negatively charged, found in energy shells outside the central part of the atom
? Move at very high speeds in orbit
? Have very negligible mass
? Mass is 0.0005 of a proton
Protons
? Are positively charged and found in the nucleus of an atom
? Have a unit mass of one (ie. 1 a.m.u)
Neutrons
? Have no charge at all and are located in the nucleus
? Have a unit mass each
1 atomic mass unit (1 a.m.u) = 1.67 x 10-27 kg)
Energy Shells
? Shells increase as they become full
? The first shell only contains 2 electrons and following shells contain 8 each
Chemical Stability
? Electrons are spaced out regularly at intervals as shown below. The presence of unpaired electrons determines the chemical reactivity of the atom.
? The electrons in the outer shell of an atom are referred to as valence electrons
? If the valence electrons for the outer shell are not enough to fill the shell, then the atom is chemically reactive and unstable. (E.g. Sodium Na+)
? Atoms with fully filled outer shells such as noble gases, are chemically unreactive and stable.
Noble Gases
? Also called inert gases have fully filled outer shelled atoms (E.g. Helium, Neon, Argon, Krypton)
Electronic Configuration
? A group of numbers which show the arrangement of electrons in their various shells
? The numbers represent number of electrons in each shell starting with the innermost shell, separating each number (shell) by a comma
E.g. He: 2, Ne: 2,8, O: 2,6
Isotopes
? Atoms of the same element having different mass #s but the same atomic number (ie. Different neutrons)
? Most elements exist as a mixture of their isotopes
? Hydrogen has 3 isotopes: Protium (0n), Deuterium (1n) and Tritium (2n).
These are called H-1, H-2 and H-3
The two types of isotopes are: (i) Radioactive and (ii) Non-radioactive isotopes
? Radioactive isotopes are isotopes with an unstable nucleus (the general term for radioactive substances since they are all isotopes).
? Non-Radioactive isotopes are stable and unreactive
Medical uses:
? Isotope cobalt-60 emits gamma radiation which can be used to sterilize medical equipment
Industrial Uses:
? Uranium-238 can be used to estimate the ages of rocks
Differences between Elements, Mixtures and Compounds
? An Element is a pure and simple substance which cannot be broken down into any simpler substances other than itself by any ordinary chemical means.
? A Mixture is made up of two or more substances physically mixed together.
The components in a mixture can always be separated by physical means
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Mixture |
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Compound |
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No heat or light is given out or absorbed when mixing occurs |
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Heat and sometimes light is usually given out or absorbed when a compound is made. |
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The substance in a mixture can be separated by physical means |
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the elements in a compound cannot be separated by physical means |
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the properties of a mixture are the average of the properties of thes substances in it |
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the properties of a compound are quite different from those of the elements in it |
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the substances in a mixture can be presented in any proportions by mass |
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the elements in a compound are combined together in definite proportions by mass (we have not proved this yet). |
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Metals |
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Non-Metals |
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good conductors of heat and eletricity |
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poor conductors of heat and electricity |
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shiny |
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dull |
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malleable |
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brittle |
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strong |
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weak |
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react with oxygen to form basic oxides |
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react with oxygen to form acidic oxides |
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usually high density |
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usually low density |
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usually high melting points except alkali |
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usually low melting points except carbon |
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many react with dilute acid to produce hydrogen |
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no reaction with dilute acid |
Alloys
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Name of Alloy |
Composition |
Uses |
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Brass |
Copper, Zinc |
To make musical instruments (especially wind instruments). Utensils |
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Bronze |
Copper, Tin |
Utensils, metal, statues, decorating pieces of art works |
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Duralumin |
Aluminium, magnesium, copper, manganese, silicon |
manufacturing parts of aircraft (e.g. the wings) |
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Steel |
iron, carbon (and sometimes manganese) |
machine parts, cooking utensils |
Ion Formation
? Atoms that have more than ? the max # of electrons in their outer shell (ie. 4 +) will tend to gain electrons needed to make 8 (an octet) and hence a stable atom
? If less than ? then it will tend to lose electrons
? 17Cl has 17e (Cl: 2,8,7) and requires one electron to achieve chemical stability
? The new atom has one electron in excess, compared to protons. It will thus carry an extra charge of -1 (minus) and becomes Chloride Ion.
? Ions with negative charges are called anions
? Similarly, metals, having less than 4 valence electrons, will tend to lose electrons to become stable.
? This way they end up having an excess +ve charge.
? Positively charged ions are known as cations.
? Ions are electrically charged particles formed when atoms lose or gain one or more electrons to form a stable outer shell.
Bonding
? A chemical union developed due to attraction when atoms lose, gain or share electrons in order to acquire stable (full) outer shells.
? Atoms are held together by bonds
? Bond is formed by the use of a valence electron (ie. The electron in the outer shell of an atom.)
? Types are (i) Ionic (ii) Covalent (iii) Metallic
Ionic Bonding
? The attraction between the positive and negative ions (anions and cations) formed by loss/gain of electrons (ie. Metals and non-metals)
? This attraction results in a chemical union known as bonding
? The union involves transfer of electrons
? It is exhibited by elements far apart in the periodic table (metals and non-metals)
? In the formation of sodium chloride (Na+Cl- are the ions in the compound) the metallic element (sodium) burns in chlorine (the non-metallic element).
? Sodium atom has 1 valence electron and will tend to lose this. Each sodium atom will lose on electron to become a sodium ion. Similarly, Chlorine has 7 valence electrons and needs to gain one to become chloride ion.
? Sodium atom outer shell and Chlorine outer shell now both contain 8 electrons. However, the two atoms are now bonded and share opposite charges. They is hence and electrostatic force between them holding them together.
?
? Electrovalent bonds exist between electrovalent compounds. These compounds can conduct electricity when melted or dissolved.
? Magnesium Fluoride is also an ionic compound
? Magnesium (2,8,2) has two electrons in its outermost shell. It needs to lose these to gain a stable, full outer shell. Fluorine needs to gain one electron. One magnesium atom needs two fluorine atoms to accept two electrons. Formula is Mg2+2F- or MgF2.
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