Number of electrons contributed by an atom for

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Number of electrons contributed by an atom for sharing The number of electrons contributed by an atom for sharing is equal to what that atom needs to obtain a stable electronic arrangement. For example, a hydrogen atom (1) needs one more electron to obtain the electronic arrangement of a helium atom (2), so it contributes 1 electron for sharing. A nitrogen atom (2,5) needs three more electrons to obtain the electronic arrangement of a neon atom (2,8), so it contributes 3 electrons for sharing.
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Topic 2 Microscopic World I Unit 8 Covalent bonds Table 8.1 Number of electron(s) contributed for sharing by atoms of Period 2 non-metals during covalent bond formation Period 2 non-metal C N O F Electronic arrangement of atom 2,4 2,5 2,6 2,7 Number of electron(s) needed to obtain the electronic arrangement of a noble gas atom 4 3 2 1 Number of electron(s) contributed for sharing 4 3 2 1 8.4 Writing chemical formulae of covalent compounds We can use the following steps to work out the chemical formulae of covalent compounds. Table 8.2 Steps for working out the chemical formulae of covalent compounds Step Compound formed from hydrogen and sulphur Compound formed from silicon and chlorine 1 Write down the electronic arrangements of the atoms involved. H S 1 2,8,6 Si Cl 2,8,4 2,8,7 2 Decide the number of electrons each atom needs to obtain a stable electronic arrangement. Write down the number on the top of each atom. A hydrogen atom needs 1 electron, while a sulphur atom needs 2 electrons. 1 2 H S A silicon atom needs 4 electrons, while a chlorine atom needs 1 electron. 4 1 Si Cl 3 Decide the number of each type of atoms in one molecule (cross multiply the numbers and the symbols). 1 2 H S =H 2 =S 1 4 1 Si Cl =Si 1 =Cl 4 4 Combine the symbols and simplify the ratio if necessary. H 2 S (Omit the number of 1 for S.) SiCl 4 (Omit the number of 1 for Si.)
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Topic 2 Microscopic World I Unit 8 Covalent bonds 8.5 Predicting the formation of ionic and covalent compounds When a metal combines with a non-metal, an ionic compound forms. When non-metals combine, a covalent compound forms. When a non-metal conbines with a semi-metal, a covalent compound usually forms 8.6 Dative covalent bonding (Not included in the assessment) We have looked at a covalent bond in which each of the two atoms provides one electron to form the bond between them. However, it is possible for just one atom to provide both bonding electrons. This atom is called the donor atom , and the two electrons it provides come from a lone pair on that atom.
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