Topic 1 Lesson 3:

• The reactivity of elements changes in a predictable pattern across the periodic table.

• Most things in the world are not made up of individual elements; they are made up of compounds, or substances that form when two or more elements bond together.

• The electrons of an atom are found in different energy levels. The first energy level is closest to the nucleus and it can hold a maximum of 2 electrons. The second energy level can hold up to 8 electrons.

• The valence electrons in an atom are the electrons that have the highest energy. Each atom has a certain number of valence electrons which is specific to each element.

• The number of valence electrons increases from left to right across each period. Each period begins with an element that has 1 v.e. and ends with one that has 8 v.e.’s. Elements within a group have the same number of v.e.’s As a result, electrons in the same group have similar properties.

• Reactivity is based on the interactions of valence electrons.

• Metals generally have 1-3 v.e.’s They don’t react with other metals but they do react with other elements. Many of these elements tend to lose their v.e.’s easily to other atoms, making them highly reactive.

• Properties of metals include: malleable, ductile, conducting well, lustrous, low specific heat, high density, and solid state at room temp.

• The easy movement in metals is what makes them malleable and ductile. The movement of valence electrons also makes most metals good thermal and electrical conductors.

• Nonmetals have a wider variety of properties compared to metals. Atoms of nonmetals react with both metals and other nonmetals. Some nonmetals are essential to life while others can be poisonous.

• These levels of reactivity are based on the number of valence electrons in the atoms of the elements.

• Nonmetals are poor conductors of electric current and heat. Solid nonmetals tend to be dull and brittle. (Many nonmetals tend to be gaseous at room temp.)

• All metalloids are solids at room temp. Metalloids are also brittle, hard, and somewhat reactive.

• Metalloids’ most useful prop is their ability to conduct electricity. The conductivity varies depending on temp, exposure to light, or the presence of impurities.

• Ionic bonding is one-way elements form compounds. Ionic bonding involves the attraction between ions of opposite charge.

• When a neutral atom loses a valence electron, it loses a negative charge and it becomes a positive ion. When a neutral atom gains an electron, it gains a negative charge and becomes a negative ion.

• When atoms that easily lose electrons react with atoms that gain electrons, they transfer electrons to form ionic bonds.

• Ionic bond: total positive charge of all the positive ions = total negative charge of all the negative ions.

• Ionic bonds usually form when a metal combines with a non-metal. Covalent bonds usually form between nonmetal atoms.

• The attractions between the shared electrons and the protons in the nucleus of each atom hold the atoms together in a covalent bond.

• Atoms of some elements pull more strongly on the shared electrons of a covalent bond than atoms of other elements. As a result, the atoms are shared unequally.

• Unequal sharing of electrons - causes covalently bonded atoms to have slight electric charges on different parts of the molecule. The overall molecule has no charge.

• If 2 atoms pull equally on the electrons, neither atom becomes charged. A covalent bond in which electrons are shared equally is a nonpolar.

• A molecule is polar If it has a positively charged end and a negatively charged end. However, not all molecules containing bonds are polar overall.

• Opposite charges attract, as charges repel.

• Ionic compounds have high melting points. They conduct electric current when dissolved in water or melted.

• The ions in the crystal have to break apart for an ionic compound to melt. It takes a huge amount of energy to separate the ions in a crystal because the attraction between the positive and negative ions is so great. As a result, their melting point is so great.

• Ionic compounds form solids by building up repeating patterns of ions. Ions form an arrangement called a crystal.

• Electric current involves the flow of charged particles. When ionic crystals dissolve in water or melt, the ions are free to move about, and the solution can conduct current. In contrast, ionic compounds in solid form do not conduct current well. The ions in a solid crystal are tightly bound to each other and cannot move from place to place.

• Covalent compounds exist as molecules. Because of this, they are known as molecular compounds. Because bonds do not have to be broken for molecular compounds to melt, their melting points are relatively low.

• Molecular compounds somewhat differ in their properties depending on whether they are polar or nonpolar. This is because of differences in attraction between their molecules.