Unit 2: Chemical context of life

What is matter?

Anything that takes up space and has mass.

What is an element?

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

What is a compound?

A substance made of two or more different elements in a fixed ratio.

What are the four elements that make up about 96% of living matter?

 C — Carbon
H — Hydrogen
O — Oxygen
N — Nitrogen

What elements make up most of the remaining 4%?

• Calcium (Ca)

• Phosphorus (P)

• Potassium (K)

• Sulfur (S)

What is a radioactive isotope?

 An isotope that spontaneously breaks down and releases particles and energy.

What is potential energy?

 Energy stored because of an object's location or structure.

What determines an atom's chemical behavior?

The distribution of its electrons, especially its valence electrons.

What are valence electrons?

 Electrons in the outermost electron shell.

Why are valence electrons important?

They are involved in chemical bonding.

What is an electron shell?

An energy level where electrons are found around the nucleus

 What is a covalent bond?

A bond where atoms share electrons.

What is a molecule?

Two or more atoms held together by covalent bonds.

What is a nonpolar covalent bond?

Electrons are shared equally.

What is a polar covalent bond?

Electrons are shared unequally.

What is electronegativity?

 An atom's ability to attract electrons in a covalent bond.

 Which type of covalent bond creates partial charges?

Polar covalent bonds.

What is an ionic bond?

The attraction between a positively charged ion and a negatively charged ion.

What is a cation?

A positively charged ion.

What is an anion?

A negatively charged ion.

What happens when sodium and chlorine form an ionic bond?

Sodium transfers an electron to chlorine.

• Na loses an electron → Na⁺

• Cl gains an electron → Cl⁻

What is a hydrogen bond?

A weak attraction between a partially positive hydrogen and an electronegative atom, usually O or N in living cells.

What are van der Waals interactions?

Weak attractions between molecules caused by temporary uneven distributions of charge

Why are weak bonds important?

They help large molecules keep their shape and allow molecules to attach to each other reversibly.

What is a chemical reaction?

The making and breaking of chemical bonds.

What are reactants?

The molecules you start with in a chemical reaction.

What are products?

The molecules that are made by a chemical reaction.

Is matter created or destroyed during a chemical reaction?

No. Matter is conserved.

What is chemical equilibrium?

A state where forward and reverse reactions continue, but their rates are balanced, so there is no overall change in concentrations.

Molecular formula vs. structural formula?

• Molecular formula: tells you how many atoms of each element are present.

• Structural formula: shows how the atoms are connected.Example:
  Molecular → H₂
  Structural → H–H