Matter, Elements, and Chemical Bonding

Matter and Elements

Life is made up of matter, which occupies space and has mass.
Matter is composed of elements, which cannot be broken down chemically.
Elements are made of atoms with a constant number of protons and unique properties.
Each element has a chemical symbol (e.g., H, N, O, C, Na) and unique properties.

An atom is the smallest component of an element that retains its chemical properties.
Atoms combine to form molecules, which can interact to form cells, tissues, and organs in multicellular organisms.
Atoms contain protons, electrons, and neutrons (except the most common hydrogen isotope).
Proton: Positively charged, resides in the nucleus, mass of 1, charge of +1.
Electron: Negatively charged, travels around the nucleus, negligible mass, charge of -1.
Neutron: Resides in the nucleus, mass of 1, no charge.
In a neutral atom, the number of protons equals the number of electrons, resulting in a net zero charge.
The mass of an atom is equal to the number of protons plus the number of neutrons.

The atomic number is the number of protons in an element.
The mass number (atomic mass) is the number of protons plus the number of neutrons.
Number of neutrons can be determined by subtracting the atomic number from the mass number.

Isotopes are different forms of the same element with the same number of protons but a different number of neutrons.
Radioactive isotopes (radioisotopes) are unstable and lose subatomic particles or energy to form more stable elements.
Half-life: The time it takes for half of the original concentration of an isotope to decay to its more stable form.
Carbon-14 dating is used to age formerly living objects up to about 50,000 years old using the half-life of 14C.

Electrons exist at energy levels that form shells around the nucleus.
The closest shell holds up to two electrons, while the second and third shells can hold up to eight electrons.
Atoms are most stable when their outermost shell is filled (octet rule).
Chemical bonds are interactions between elements that result in the formation of molecules to achieve greater stability.
Atoms bond by sharing, accepting, or donating electrons.
An ion is an atom that does not contain equal numbers of protons and electrons, giving it a net charge.
Cations: Positive ions formed by losing electrons.
Anions: Negative ions formed by gaining electrons.
Electron transfer: The movement of electrons from one element to another.

Ionic bond: A bond between ions formed through electron transfer, resulting in positive and negative ions attracting each other.
Covalent bond: A strong bond formed when electrons are shared between two elements; the most common type of bond in living organisms.
Nonpolar covalent bond: Electrons are equally shared between atoms of the same element or different elements.
Polar covalent bond: Electrons are unequally shared, resulting in a slightly positive (δ+) or slightly negative (δ–) charge.
Hydrogen bond: A weak attraction between the δ+ charge of a hydrogen atom and a δ– charge of another molecule.
van der Waals interactions: Weak attractions between polar, covalently bound atoms in different molecules due to temporary partial charges.