Atoms, Elements, and Matter

Definition and Properties of Matter

Matter is anything that has mass and takes up space. All living organisms are composed of matter, which is made up of atoms.

• Atom: The smallest unit of matter that retains the properties of an element and cannot be separated by normal chemical means.

• Element: A substance that cannot be broken down into other substances by chemical reactions.

• Compound: A substance consisting of two or more elements combined in a fixed ratio.

• Emergent Properties: Compounds have properties different from those of their constituent elements.

Essential elements (C, H, O, N) make up about 96% of living matter. Trace elements are required in minute quantities for life.

Subatomic Particles

Atoms are composed of subatomic particles: protons, neutrons, and electrons.

Atomic Number and Atomic Mass

• Atomic number: Number of protons in the nucleus of an atom.

• Mass number: Sum of protons and neutrons in the nucleus.

Atoms of the same element can have different numbers of neutrons, resulting in isotopes.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Radioactive isotope: An isotope that decays spontaneously, emitting particles and energy.

Energy Levels and Electron Shells

Energy and Potential Energy

• Energy: The capacity to cause change or do work.

• Potential energy: Energy possessed due to position or structure.

Electrons are arranged in electron shells around the nucleus. Electrons fill the lowest available energy levels first.

• The lowest electron shell can hold 2 electrons.

• The next shell can hold 8 electrons.

Valence Electrons and Chemical Properties

The chemical properties of an atom are largely determined by the number of electrons in its outermost shell, called the valence shell.

Chemical Bonds

Molecules and Compounds

Molecules and compounds are formed through chemical bonding between atoms. Atoms with unfilled valence shells can share or transfer electrons to achieve stability.

• Molecule: Two or more atoms held together by covalent bonds.

• Compound: A molecule containing atoms of different elements.

Covalent Bonds

• Covalent bond: Sharing of a pair of valence electrons between two atoms.

• Double covalent bond: Sharing of two pairs of valence electrons.

• Nonpolar covalent bond: Electrons are shared equally.

• Polar covalent bond: Electrons are shared unequally due to differences in electronegativity.

Electronegativity is the attraction of an atom for electrons. The greater the electronegativity, the more strongly an atom pulls electrons toward itself.

Ionic Bonds

• Ionic bond: Attraction between oppositely charged ions formed by the transfer of electrons.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

Salts, such as NaCl, are examples of compounds formed by ionic bonds. Ionic compounds dissociate in water.

Weak Chemical Interactions

• Hydrogen bond: Weak attraction between a hydrogen atom in a polar covalent bond and another electronegative atom.

• Hydrogen bonds are crucial in biological systems, despite their relative weakness.

Molecular Shape and Function

The shape and charge of a molecule are determined by the arrangement of its atoms and the types of bonds present. Molecular architecture is essential for biological function.

• Molecular shape affects how molecules interact and respond to other molecules.

• Examples include the shapes of oxygen and nitrogen atoms in cells.

Chemical Reactions

Making and Breaking Chemical Bonds

Chemical reactions involve the making and breaking of chemical bonds.

• Reactants: Starting molecules in a chemical reaction.

• Products: Resulting molecules of a chemical reaction.

• Many reactions are reversible; products can become reactants in the reverse reaction.

Chemical equilibrium occurs when the forward and reverse reactions happen at the same rate.

Key Equations

• Mass number equation:

• Chemical reaction equation (general):

Additional info: These notes expand on the original content by providing definitions, examples, and context for key terms and concepts in general biology, suitable for college-level study.