Atoms

¨    An atom is the basic unit of matter.

¨    Atoms are made up of subatomic particles, which are even smaller.

¨    The subatomic particles that make up atoms are protons, neutrons, and electrons.

¨    Protons and neutrons have the same mass.

¨    Protons carry a (+) positive charge and neutrons carry no charge.

¨    The nucleus, which is at the center of the atom, contains both protons and neutrons.

¨    The electron is a (-) negatively charged particle.

¨    Electrons are in constant motion in the space surrounding the nucleus.

¨    Atoms are neutral because the subatomic particles have equal but opposite charges.

Elements and Isotopes

¨    An element is a pure substance that consists entirely of one type of atom.

¨       More than 100 elements are known, but only about

2 dozen are commonly found in living organisms.

¨       Elements are represented by a one- or two-letter symbol.

Ex) H for hydrogen, C for carbon, Na for sodium

¨       The number of protons in an atom of an element is the element’s atomic number.

Ex) Carbon’s atomic number is 6 so, carbon has 6 protons and 6 electrons.

Isotopes

¨       An isotope is an atom of the same element that differs in the number of neutrons they contain.

¨       The sum of the protons and neutrons in the nucleus of an atom is called its mass number.

¨       Isotopes are identified by their mass numbers.

Ex) carbon-12, carbon-13, carbon-14

¨       The weighted average of the masses of an element’s isotopes is called its atomic mass.

¨       The number of electrons does not change in an isotope so; all the chemical properties are the same.

Radioactive Isotopes

¨       Radioactive isotopes have unstable nuclei that break down at a constant rate over time.

¨       The radiation given off can be dangerous but also has some practical scientific uses.

v      Geologists can determine the ages of rocks and fossils.

v      Treatment of cancer and kill bacteria that cause food to spoil

v      “tracers” that follow the movements of substances within organisms

Chemical Compounds

¨       A chemical compound is a substance formed by the chemical combination of two or more elements in definite proportions.

¨       A chemical formula is used to illustrate the composition of compounds.

Ex) Water – two atoms of hydrogen for each atom of oxygen has the chemical formula of H₂O

¨       The physical and chemical properties of a compound are usually very different from those of the elements from which it is formed.

Ex) hydrogen and oxygen are gases at room temperature, which combine explosively to form liquid water

Chemical Bonds

¨       The atoms in compounds are held together by chemical bonds.

¨       The main types of chemical bonds are ionic bonds and covalent bonds.

Ionic Bonds

¨       An ionic bond is formed when one or more electrons are transferred from one atom to another.

¨       An ion is a positively or negatively charged atom.

¨       Oppositely charged ions have a strong attraction.

¨       The attraction between oppositely charged ions is an ionic bond.

Covalent Bonds

¨       A covalent bond forms when electrons are shared between atoms.

¨       When the atoms share two electrons, the bond is called a single covalent bond; four electrons form a double bond; six electrons form a triple bond.

¨       The structure that results when atoms are joined together by covalent bonds is called a molecule.

¨       A molecule is the smallest unit of most compounds.

Ex) water is a molecule with each hydrogen atom forming a single covalent bond with the oxygen atom.