We sometimes have reactions between individual atoms. During such reactions atoms give, take or share electrons in order to achieve a more stable electronic arrangement.
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After exchanging electrons atoms come together and form strong bonds between them known as chemical bonds.
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Oxygen reacts with berylium to form a new compound called beryllium oxide. Oxygen takes two electrons from beryllium to achieve a more stable electronic configuration. Notice how oxygen ends up having a 2,8 configuration and beryllium a 2 configuration. These type of configurations are very stable and you can see more detailed explanations by clicking on the links on the left.