chapter 2

Trace elements are required by an organism in only minute quantities

An atom is the smallest unit of matter that still retains the properties of an element

Atoms are composed of subatomic particles

No electrical charge

Positive charge

Negative charge

Electrons form a "Cloud" of negative charge around around the nucleus

Electrons are so small they are ignored when calculating the total mass of an atom

The Atomic Nucleus is made up for protons and neutrons

Neutrons and protons mas are almost identical and measured in daltons

An element's atomic number is the number of protons in its nucleus

An elements mass mass number is the sum of protons plus neutrons in the nucleus

Atomic mass, the atom's total mass, can be approximated by the mass number

Isotopes are two atoms of an element that differ in the number of neutrons

Radioactive isotopes decay spontaneously, giving off particles and energy

Radioactive Isotopes are often used as diagnostic tools in medicine

Radioactive tracers can be used to track atoms through metabolism

In radiometric dating, scientists measure the ratio of different isotopes and calculate how many half-lives have passed since the fossil or rock formed

A "parent" isotope decays into its "daughter" isotope at a fixed rate, expressed as the half-life of the isotope

Half-life values vary from seconds or days for some isotopes to billions of years for others

Energy is the capacity to cause change

Potential energy is the energy that matter possesses because of its location or structure

Electrons are found in different electron shells, each with a characteristic average distance and energy level

Valence electrons are those in the outermost shell, or valence shell

The chemical behavior of an atom is mostly determined by the number of valence electrons

An orbital is the three- dimensional space where an electron is found 90% of time

No more than 2 electrons can occupy a single orbital

These interactions usually result in atoms staying close together, held by attractions called chemical bonds

Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms

A covalent bond is the sharing of a pair of valence electrons by two atoms

In a covalent bond, the shared electrons count as part of each atom’s valence shell

Covalent bonds can form between atoms of the same element or atoms of different elements

A molecule consists of two or more atoms held together by covalent bonds

A single covalent bond, or single bond, is the sharing of one pair of valence electrons

A double covalent bond, or double bond, is the sharing of two pairs of valence electrons

Bonding capacity is called the atom’s valence

Electron negativity is an atom’s attraction for the electrons in a covalent bond

The more electron negative an atom is, the more strongly it pulls shared electrons toward itself

In a non-polar covalent bond, the atoms share the electron equally

In a polar covalent bond, one atom is more electron negative, and the atoms do not share the electron equally

The two resulting oppositely charged atoms or molecules are called ions

A positively charged ion is called a cation

A negatively charged ion is called an anion

Anions and cations attract each other; this attraction is called an ionic bond

Compounds formed by ionic bonds are called ionic compounds, or salts

Salts, such as sodium chloride (NaC l; table salt), are often found in nature as crystals

NaC l itself is not a molecule; the formula for an ionic compound indicates the ratio of elements in a crystal of the salt

Most salts are quite stable when dry, but dissociate quite easily in water

A hydrogen bond forms when a hydrogen atom covalently bonded to one electron negative atom is also attracted to another electron negative atom

In living cells, the electron negative partners are usually oxygen or nitrogen atoms

Van der Waals interactions are attractions between molecules that are close together as a result of these charges

Chemical reactions are the making and breaking of chemical bonds

The starting molecules of a chemical reaction are called reactants

The resulting molecules of a chemical reaction are called products

Chemical equilibrium is reached when the forward and reverse reactions occur at the same rate

At equilibrium the relative concentrations of reactants and products do not change