Atomic structure & the periodic table flashcards

Key points

• Element: a substance made of only one type of atom, shown by a symbol such as O or Fe.
• Compound: two or more elements chemically combined in fixed proportions, such as water, H₂O.
• A compound can only be split up by a chemical reaction.
• Mixture: substances together but not chemically joined. Each keeps its own properties.

Key points

• Filtration: an insoluble solid from a liquid, such as sand from water.
• Crystallisation: a dissolved solid from its solution, such as salt crystals from salt water.
• Simple distillation: a liquid from a solution, such as pure water from seawater.
• Fractional distillation: liquids with different boiling points, such as the fractions of crude oil.
• Paper chromatography: dissolved substances, such as the dyes in an ink.

Key points

• Proton: mass 1, charge +1, in the nucleus.
• Neutron: mass 1, charge 0, in the nucleus.
• Electron: almost no mass (about $\frac{1}{2000}$), charge −1, in shells around the nucleus.
• An atom has as many electrons as protons, so it has no overall charge.
• An atom's radius is about 0.1 nm; the nucleus is tiny but holds almost all the mass.

Key points

• Atomic number: the number of protons. It decides the element, and equals the electrons in an atom.
• Mass number: the number of protons and neutrons added together.
• Neutrons = mass number − atomic number.
• In the periodic table, the atomic number is the smaller of the two numbers.

Sodium: atomic number 11, mass number 23

Key points

• Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.
• They have the same electrons, so they react in the same way.
• Relative atomic mass is the average mass of an element's atoms, allowing for how common each isotope is.

75% chlorine-35, 25% chlorine-37

Key points

• Dalton: atoms were tiny solid spheres that could not be divided.
• Thomson discovered the electron: the plum pudding model, a ball of positive charge with electrons dotted through it.
• Rutherford's alpha scattering experiment: a tiny, dense, positive nucleus with electrons around it.
• Bohr: electrons orbit the nucleus in shells at fixed distances.
• Later, the nucleus was found to hold protons, and Chadwick discovered the neutron.

Key points

• Positive alpha particles were fired at very thin gold foil.
• Most went straight through: most of the atom is empty space.
• A few were deflected, and a very few bounced back: they came close to a concentrated positive charge and were repelled.
• So the mass and positive charge are packed into a tiny nucleus. The plum pudding model could not explain this.

Key points

• Fill the shells from the inside out: 2 in the first, then up to 8, then up to 8.
• The number of outer-shell electrons is the group number.
• The number of occupied shells is the period number.
• The outer electrons decide how an element reacts.

Chlorine has 17 electrons

Key points

• An ion is an atom or group of atoms with an electric charge.
• Metal atoms lose their outer electrons to form positive ions.
• Non-metal atoms gain electrons to form negative ions.
• Both end up with a full outer shell, like a noble gas.

Sodium and chlorine

Key points

• Elements are in order of atomic number (the number of protons).
• Columns are groups: the same number of outer electrons, so similar reactions.
• Rows are periods: the same number of occupied electron shells.
• Metals are on the left and in the centre, non-metals on the right, with a staircase between them.

Key points

• He ordered the elements by atomic weight, because protons had not been discovered.
• He swapped some elements so that ones with similar properties stayed in the same group.
• He left gaps for elements not yet discovered, and predicted their properties.
• When gallium and germanium were found and matched his predictions, chemists accepted his table.
• Isotopes later explained why ordering by atomic weight did not always work.

Key points

• They have 1 outer electron, which they lose when they react.
• With water they make a metal hydroxide (an alkali) and hydrogen.
• Down the group the outer electron is further from the nucleus, so it is less strongly attracted and lost more easily.
• Lithium fizzes steadily, sodium whizzes around and potassium bursts into a lilac flame.

Sodium in water

Key points

• They have 7 outer electrons, gain 1 when they react, and exist as molecules of two atoms, such as Cl₂.
• Down the group they get less reactive: the outer shell is further from the nucleus, so an electron is gained less easily.
• Boiling points rise down the group: chlorine is a gas, bromine a liquid and iodine a solid.
• A more reactive halogen displaces a less reactive one from a solution of its salt.

Chlorine water and potassium bromide

Key points

• They have a full outer shell: helium has 2 electrons, the others 8.
• So they do not need to gain, lose or share electrons, and are very unreactive.
• They exist as single atoms.
• Boiling points rise down the group, as the atoms get heavier.