Ch 2.1 Elements combine to form compounds

Compounds have different properties from elements Elements have individual properties that help us identify them. Although, when these elements combine to form compounds a whole new set of properties are created. Example: carbon (C) and oxygen (O) alone are harmless, but carbon monoxide (CO) is deadly.

Atoms combine in predictable numbers Ammonia (NH3) will always have 3 hydrogen atoms for every nitrogen; 3:1 3 hydrogen atoms with 1 electron in the last energy level and 1 nitrogen atom with 5 electrons in the last energy level. Sharing these will give nitrogen 8 electrons & hydrogen 2 electrons, filling the last energy levels!

Water (H2O) will always have 2 hydrogen atoms for every oxygen. 2 to 1 ratio of hydrogen to oxygen 2:1 2 hydrogen atoms with 1 electron in the last energy level and 1 oxygen atom with 6 electrons in the last energy level. Sharing these will give oxygen 8 electrons & hydrogen 2 electrons, filling the last energy levels!

Forming compounds Atoms can be described in one of two ways: They tend to gain or share electrons to become stable. They tend to lose or share electrons to become stable.

Example: Hydrogen Hydrogen has only 1 electron- it needs two to fill its energy level. Hydrogen atoms tend to gain or share 1 electron to become stable. If two hydrogen atoms come together they will become stable by sharing their electrons. This will give each atom 2 electrons.

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Ionic Bonds An ion is a charged atom caused by the loss or gain of electrons A positively charged ion has lost electrons; mainly metals. A negatively charged ion has gained electrons; mainly non-metals. An ionic bond forms from the attraction of positive and negative ions.

Example of ionic bonding Na + Cl NaCl Na + Cl salt Sodium, with one outer electron will become a positive ion when it gives its one electron to chlorine. Chlorine will become a negative ion.

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Covalent Bonds A pair of shared electrons between two atoms. Non-metals usually form bonds with each other by sharing electrons. Covalent atoms are neutral because they do not gain or loss electrons These electrons spend time in the outer energy levels of both atoms.

Covalent Bonds When atoms share electrons, the shared electrons move around the nuclei of the atoms that are involved. Step by step: 1. The atoms approach each other

2. The shared electrons begin to move around the nuclei. 3. To form a hydrogen molecule in which each hydrogen atom shares its electrons.

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CHEMICAL BONDS When two atoms share their electrons, they join to form a chemical bond. Atoms are held together by chemical bonds when one or more of their electrons are attracted to, and move around the nuclei of both atoms.

CHEMICAL FORMULAS A chemical formula of a compound tells how much of each element is present. Examples: CO₂ shows one atom of carbon and two atoms of oxygen.

SUBSCRIPTS Subscripts are small numbers written slightly below the element symbol to show how many atoms of that element are in the compound. When there is no subscript the number of atoms is understood to be one.

SUBSCRIPTS Examples of subscripts: MgCl₂ is the formula for magnesium chloride. It is made up of one atom of magnesium and two atoms of chlorine. H₂O is the formula for water. It is made up of two atoms of hydrogen and one atom of oxygen.

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