Atoms, ions and chemical bonds

The chemical level, atoms and the molecules they form, is the base that cells, tissues and organs are built from.

• The cellular level:
• The tissue level:
• Correct: The chemical level:

A molecule is two or more atoms joined together. Cells are built from enormous numbers of molecules.

• Correct: Two or more atoms joined together:
• A single cell:
• A group of similar cells:

Charge is protons minus electrons: +19 − 18 = +1. A positive ion is a cation, written K+. The proton count still says potassium.

• An anion with a charge of −1: An anion has more electrons than protons. This particle has one fewer electron than protons, so its net charge is positive, not negative.
• Correct: A cation with a charge of +1: Correct. One more proton than electrons gives a +1 charge, and a positively charged ion is a cation: K+.
• A neutral atom of a different element: The element is fixed by the number of protons. It is still 19, so this is still potassium, and with unequal protons and electrons it is not neutral.
• An isotope of potassium with one fewer neutron: Isotopes differ in neutrons, and the question describes a change in electrons. Losing an electron changes charge, not mass number.

Isotopes have the same number of protons and different numbers of neutrons. Carbon-12 has 6 neutrons; carbon-14 has 8. The mass number changes, the element does not.

• The number of protons in the nucleus: Both have 6 protons. The proton count is the atomic number, and it is what makes both of them carbon.
• The number of electrons in a neutral atom: A neutral atom has as many electrons as protons. Both have 6 protons, so both neutral atoms have 6 electrons.
• The element they belong to: Both are carbon, because both have 6 protons. Isotopes are versions of one element.
• Correct: The number of neutrons in the nucleus: Correct. Mass number is protons plus neutrons: 6 + 6 = 12 and 6 + 8 = 14.

KCl is held together by ionic bonds, which are weak in water. Water separates the ions, so the potassium circulates as free K+ ions and the chloride as free Cl− ions.

• As whole KCl units, each held together by a covalent bond: KCl is ionic, not covalent: potassium transferred an electron to chlorine rather than sharing one. In water the ions separate.
• As neutral potassium atoms that have picked up an electron from the water: Potassium stays a cation in body water. Nothing in blood hands it an electron back, and a lab measures it as the K+ ion.
• As small crystals of KCl carried along in the blood: A crystal forms only when there is little water between the ions. In blood, water separates the ions, so no crystals remain.
• Correct: As K+ ions, separated from the Cl− ions and surrounded by water: Correct. Ionic bonds break apart in water, releasing K+ and Cl− as separate electrolytes.

Inside one water molecule, oxygen and hydrogen share a pair of electrons: a covalent bond. Oxygen pulls the pair harder than hydrogen does, so the sharing is unequal and the bond is polar.

• A hydrogen bond: A hydrogen bond is the weak attraction between a hydrogen of one water molecule and the oxygen of another. The bond inside a molecule is a shared electron pair.
• Correct: A polar covalent bond: Correct. A shared pair makes it covalent; oxygen's stronger pull makes it polar.
• An ionic bond: No electron is transferred between oxygen and hydrogen and no ions form. They share electrons.
• A nonpolar covalent bond: Oxygen pulls shared electrons much harder than hydrogen, so the sharing is unequal. Equal sharing, as in H2 or O2, is nonpolar.

A hydrogen bond forms between a partly positive hydrogen, already covalently bonded to oxygen or nitrogen, and a partly negative oxygen or nitrogen nearby. Neighboring water molecules fit that description exactly.

• Correct: A hydrogen of one water molecule and the oxygen of its neighbor: Correct. The δ+ hydrogen of one molecule is attracted to the δ− oxygen of the next.
• The two hydrogen atoms within the same water molecule: The two hydrogens in one water molecule both carry partial positive charges. Like charges repel, so they do not attract each other.
• A carbon atom and one of its hydrogen atoms within a methane molecule: Carbon and hydrogen share electrons almost equally, so a C–H bond gives hydrogen almost no partial positive charge. The carbon–hydrogen link here is a covalent bond, not a hydrogen bond.
• A sodium ion and a chloride ion in a crystal: Na+ and Cl− carry full charges and are held by ionic bonds. A hydrogen bond involves partial charges and a hydrogen atom.

An organic compound has carbon covalently bonded to hydrogen. Methane is one carbon bonded to four hydrogens.

• Carbon dioxide (CO2), which you exhale: Carbon dioxide contains carbon but no carbon–hydrogen bond, so it is classed as inorganic. Containing carbon is not enough.
• Correct: Methane (CH4), a gas made by microbes in your gut: Correct. Four carbon–hydrogen bonds make methane organic.
• Water (H2O): Water has no carbon at all, so it is inorganic.
• Sodium chloride (NaCl): Sodium chloride has no carbon and is held by ionic bonds. It is inorganic.

Hydrogen bonds are weak, so modest heat breaks them. Many hydrogen bonds between different parts of a chain hold it folded; breaking them lets the chain unfold while the stronger covalent bonds of the chain survive.

• Correct: Hydrogen bonds between different parts of the chain: Correct. Weak hydrogen bonds hold the fold; they give way to heat first.
• Covalent bonds between the atoms of the chain: The chain stayed intact, so its covalent bonds did not break. Covalent bonds are far stronger than hydrogen bonds and need a chemical reaction to break.
• The attraction between electrons and protons in each atom: Gentle heat does not strip electrons from atoms. The atoms themselves are unchanged.
• Every bond in the molecule, which then re-formed as a straight chain: If every bond broke, the chain would fall apart into separate atoms. The chain survived, so only the weak attractions gave way.