Flashcards · 5070 · Topic 2

Atoms, elements and compounds flashcards

38 cards on Topic 2, subtopics 2.1 to 2.7. Pick a subtopic or study them all.

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Element: A substance made of only one type of atom.
Compound: A substance made of two or more elements chemically combined.
Mixture: Two or more substances that are not chemically combined.
How can you tell a mixture from a compound?: A mixture can be separated by physical methods (such as filtration or distillation), and each part keeps its own properties. A compound has different properties from its elements and can only be split by a chemical reaction.

Relative charge and relative mass of a proton, a neutron and an electron: Proton: charge +1, mass 1. Neutron: charge 0, mass 1. Electron: charge −1, mass 1/1836 (negligible).
Proton number (atomic number): The number of protons in the nucleus of an atom.
Mass number (nucleon number): The total number of protons and neutrons in the nucleus of an atom.
Electronic configuration of sodium (proton number 11): 2,8,1
Electronic configuration of chlorine (proton number 17): 2,8,7
What do the group number and the period number tell you about an atom’s electrons?: Group number (I–VII) = number of outer-shell electrons. Period number = number of occupied electron shells.

Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
How many neutrons are in chlorine-35 and chlorine-37? (proton number 17): Chlorine-35: 18 neutrons. Chlorine-37: 20 neutrons.
Why do isotopes of an element have the same chemical properties?: They have the same number of electrons, and so the same number of outer-shell electrons (the same electronic configuration).
Chlorine is 75% chlorine-35 and 25% chlorine-37. Calculate its relative atomic mass.: (75 × 35 + 25 × 37) ÷ 100 = 35.5

Ion: An atom or group of atoms that has gained or lost electrons, so it carries a charge.
Cation and anion: Cation: a positive ion (electrons lost). Anion: a negative ion (electrons gained).
How an ionic bond forms: The metal loses electrons (forming a cation) and the non-metal gains them (forming an anion). Both reach a full outer shell.
Ionic bond: A strong electrostatic attraction between oppositely charged ions.
How do sodium and chlorine atoms form sodium chloride?: Each sodium atom loses one electron to form Na⁺. Each chlorine atom gains one electron to form Cl⁻. The oppositely charged ions attract.
Electrical conductivity of an ionic compound: Poor when solid. Good when molten or dissolved in water.
Structure of an ionic compound such as sodium chloride: A giant lattice: a regular arrangement of alternating positive and negative ions.
Explain why ionic compounds have high melting points: The strong electrostatic attractions between oppositely charged ions act throughout the giant lattice, so a lot of energy is needed to overcome them.
Explain why an ionic compound conducts when molten but not when solid: In the solid the ions are held in fixed positions in the lattice. When molten (or in solution) the ions are free to move and carry charge.

How a covalent bond forms: Two non-metal atoms share a pair of electrons, so each reaches a full outer shell.
How many covalent bonds are there in a molecule of methane, CH₄, and of water, H₂O?: Methane: four single C–H bonds. Water: two single O–H bonds.
Properties of simple molecular substances: Low melting and boiling points. Poor electrical conductivity.
Explain why simple molecular substances have low melting and boiling points: Only the weak intermolecular forces between molecules have to be overcome, which needs little energy. The covalent bonds inside the molecules are not broken.
Explain why simple molecular substances do not conduct electricity: They have no ions or free electrons, so there are no mobile charged particles.
Bonding in carbon dioxide and in nitrogen: CO₂: two C=O double bonds (O=C=O). N₂: one N≡N triple bond.

Structure of diamond: A giant covalent lattice in which each carbon atom is bonded to four others in a tetrahedral arrangement.
Structure of graphite: Layers of hexagonal rings. Each carbon atom is bonded to three others, with weak forces between the layers and one delocalised electron per atom.
Why is diamond used in cutting tools?: It is very hard, because every atom is held by strong covalent bonds in a rigid three-dimensional lattice.
Why is graphite used as a lubricant and as an electrode?: Lubricant: the layers slide over each other easily. Electrode: its delocalised electrons move through the structure, so it conducts electricity.
Structure of silicon(IV) oxide, SiO₂: A giant covalent structure similar to diamond: each silicon atom is bonded to four oxygen atoms and each oxygen atom to two silicon atoms.

Metallic bonding: An electrostatic attraction between a lattice of positive metal ions and a “sea” of delocalised electrons.
How metallic bonding forms: Metal atoms lose their outer electrons into a delocalised sea, leaving positive ions.
Explain why metals conduct electricity: The delocalised electrons can move through the structure and carry charge.
Explain why metals are malleable and ductile: The layers of positive ions can slide over each other without the metallic bonding being broken.