Distribute the referral code to your friends and ask them to register with Tutorix using this referral code. Once we get 15 subscriptions with your referral code, we will activate your 1 year subscription absolutely free. Your subscribed friend will also get 1 month subscription absolutely free. Alkali metals The alkali metals get their name from the caustic substances containing compounds of group 1 metals that were originally extracted from the ashes of burnt plants, 'al kali' in Arabic. Potassium's symbol, K, comes from the same root. These metals, along with francium, occupy the far left group of the periodic table, group 1. Francium is radioactive and extremely rare and, consequently, is usually ignored. They are all soft, silvery reactive metals that have only one electron in the outer shell. Greater shielding from the inner electrons as the group is descended makes ionisation of the atoms easier, increasing the reactivity. This is the principal feature of their chemistry. top Physical properties The elements structure is metallic with a 'sea' of delocalised electrons surrounding a close-packed, regular array (lattice) of positive ions.
top Chemical properties The alkali metals react vigorously with oxygen, water and the halogens. The strength of reaction increases down the group. This is because the alkali metals are good reducing agents and always lose the outer shell electron when reacting, producing an ion. The ease of electron loss increases down the group due to the increased shielding between the nucleus and the outer shell electron. The consequence is a decrease in ionisation energy (the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms) and an increase in reactivity. The enthalpy of atomisation also decreases on descending the group (see physical properties above) favouring formation of ions. Finally, the smaller ions have a larger charge density and greater hydration enthalpy, favouring reaction. This last factor is of less importance than the previous energy changes. The energy steps needed to form an ion in solution from an alkali metal are as follows:
Comparison of actual values:
Hence, for reaction of lithium, sodium and potassium with a second species, the reaction with the most exothermic change (least endothermic) is that of potassium.
The values above are positive (endothermic), as no account has been taken of the energy changes of the reacting species. The table shows that less energy is required for potassium to react than for sodium, then for lithium. This is in agreement with the observed violence of the reactions. K > Na > Li. top Reaction with oxygen The alkali metals are all shiny, but tarnish rapidly in the air. The rate of tarnishing increases on descending the group. All of the metals react directly with oxygen producing oxides. The reactions become more energetic on descending the group.
The increasing reactivity down the group can be extrapolated to rubidium and caesium. top Reaction with halogens All of the alkali metals combine directly with halogens making halide salts. The reaction gets more vigorous on descending the group.
Fluorine is the most reactive of the halogens. The most exothermic combination is therefore caesium + fluorine top Reaction with water Perhaps one of the most well-known reactions in chemistry is the behaviour of the alkali metals in water. They all react with increasing vigour on descending the group, forming alkali metal hydroxide solutions and hydrogen gas.
The other alkali metals are even more aggressive in their reaction and for this reason, never carried out unless major safety precautions are taken beforehand. In all cases the reaction is as follows:
The metal hydroxide solution formed is a strong alkali (getting stronger as we down the group). Sodium hydroxide (caustic soda) is the most common of all the alkalis and is used in many areas of chemistry and industry. top Summary of group 1 - the alkali metals
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Page 3Definition The atomic radius is defined as the distance from the centre of an atom to the outer edge. However, as atoms almost never exist as independent units, the measurement is taken as either the dispersion, or covalent radius, or the metallic radius. If the element is in the solid form and the atoms are held together by dispersion forces, or covalent bonds only, as in the case of solid non-metals, then the radius is taken as one half the distance between the nuclear centres of neigbouring atoms.
Similary for metals, the measurement is taken as one half the distance between nuclear centres in neighbouring atoms within a metallic lattice. While definitions are consistent, size comparison may be made between atoms or other particles. top Atomic radius Atoms consist of electrons in shells surrounding a positive nucleus. There is attraction between the electrons and the nucleus and there is repulsion between the electrons themselves. The radius of an atom is determined by two factors.
The number of shells is the more important of the two factors, however across a period the number of shells does not change, it is the number of protons in the nucleus that affects the radius.
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