Atoms and Molecules
Elements are chemical compounds that cannot be broken down. Each element has a chemical symbol. For example, H is the chemical symbol for oxygen.
An atom is the smallest unit of an element.
The tiny particles of atoms are called matter, anything that has mass and takes up space.
An electron is a subatomic particle that has a negative charge, 1/1800 amu, and is located outside the nucleus.
A proton is a subatomic particle that has a positive charge, 1 amu, and is located in the nucleus.
A neutron is a subatomic particle that has no charge, 1 amu, and is located in the nucleus.
Together, protons and neutrons make up the nucleus of an atom.
The atomic number of an atom is the number of protons an element has.
The number of protons and neutrons makes up the atomic mass unit. The atomic unit mass tells how much matter an atom has.
An isotope is an atom that has the same number of protons and electrons but a different number of neutrons.
Electrons move through orbitals. Each orbital has two electrons.
The chemical behavior of an atom is associated with the number and arrangement of the atom’s valence electrons.
A chemical compound contains two or more different elements.
A molecule is two or more atoms joined together.
A chemical formula describes the chemical composition of a substance. There are three types of chemical formulas: simplest formula, molecular formula, and structural formula.
Simplest formula shows the atom ratio. For example, NH3shows a 1:3 ratio of nitrogen to hydrogen.
Molecular formula shows the atom number. For example, the molecular formula for water, H2O, shows that there are two atoms of hydrogen and one atom of oxygen.
Structural formula shows the arrangement of atoms and their connections. For example, the structural formula of water is H--O--H.
Avogadro’s number says that the number of molecules in one mole of a substance equals 6.02 x 10^23.
In a chemical equation, the reactants participate in the reaction and the products are the substances produced by the reaction.
At equilibrium, the rates of the forward and reverse reactions are equal and are represented by double arrows.
Chemical bonds are what holds atoms together.
Bond energy is the energy necessary to break a chemical bond.
Covalent bonds are the sharing of electrons between atoms so that each atom can fill its valence shell. The valence shell is the outermost shell of an atom.
Electronegativity is the ability of an atom to attract shared electrons to itself.
Covalent bonds can be nonpolar where the covalently bonded atoms have similar electronegativities and the electrons are shared equally.
Covalent bonds can also be polar where the covalently bonded atoms have differing electronegativities.
An ionic bond is a bond between the negative charge of an anion and the positive charge of a cation.
A hydrogen bonds is a weak bond that forms between an atom with a partial negative charge and an H atom that is covalently bonded to oxygen or nitrogen.
Van der Waal interactions are weaker than covalent, ionic, and hydrogen bonds. These interactions may interact in regions of weak positive and negative charge and they operate over short distances.