chemistry - atomic structure and the periodic table
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58 Terms
What is an atom, and what is an element?
An atom is the smallest part of an element that can exist. An element is a substance made of only one type of atom, represented by a chemical symbol.
What is a compound, and how is it different from a mixture?
A compound contains two or more elements chemically combined in fixed proportions. A mixture contains two or more elements or compounds not chemically combined, with unchanged chemical properties.
What are the key features of a chemical reaction?
Compounds are broken up or formed, at least one new substance is created, there is often an energy change, and no atoms are created or destroyed.
Write the word equation and the balanced symbol equation for sodium reacting with oxygen.
Word equation: sodium + oxygen -> sodium oxide. Balanced symbol equation: 4Na + O₂ -> 2Na₂O.
Why must symbol equations be balanced?
Atoms are not created or destroyed in a chemical reaction, so there must be the same number of atoms of each element on both sides.
Describe how filtration separates a mixture, and give a suitable example.
Filtration separates an insoluble solid from a liquid using filter paper where the solid is trapped and the liquid runs through. Example: sand and water.
Describe how crystallisation separates a solution into its solute and solvent.
Heat the solution so the solvent evaporates; crystals of the solute form, and the solvent can be collected by condensing it.
Describe how chromatography separates a mixture of dissolved solids, and what it is used for.
A drop of solution is placed near the bottom of chromatography paper and dipped in a solvent; different solutes move up at different speeds. Used for inks and dyes.
State the difference between simple and fractional distillation.
Simple distillation separates two liquids with different boiling points. Fractional distillation separates many liquids with different boiling points by slowly heating and collecting fractions.
Why are these separation techniques described as physical processes?
They do not involve chemical reactions and no new substances are made.
What was the model of the atom before the discovery of the electron?
Atoms were thought to be tiny spheres that could not be divided.
Describe the plum pudding model and say who proposed it.
Proposed by J. J. Thomson, it modelled the atom as a ball of positive charge with negative electrons embedded in it.
What did the alpha particle scattering experiment show, and why did it replace the plum pudding model?
Some alpha particles bounced back, showing mass and positive charge are concentrated at the nucleus, replacing the plum pudding model with the nuclear model.
What did Niels Bohr add to the nuclear model?
He added that electrons orbit the nucleus at specific distances.
What did later experiments and James Chadwick contribute to the model?
Later experiments identified protons in the nucleus, and Chadwick's work provided evidence for neutrons.
State the 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 0.0005.
Why does an atom have no overall electrical charge?
The number of electrons equals the number of protons, so positive and negative charges cancel out.
Define atomic number and mass number.
Atomic number is the number of protons. Mass number is the sum of protons and neutrons.
A sodium atom has mass number 23 and atomic number 11. Calculate its numbers of protons, neutrons and electrons.
Protons = 11, electrons = 11, neutrons = 12.
How large is an atom and its nucleus?
The radius of an atom is about 0.1 nm (1×10⁻¹⁰ m). The nucleus is less than 1/10,000 of that, about 1×10⁻¹⁴ m.
What is an isotope, and what is relative atomic mass?
Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass is an average value accounting for isotope abundance.
Chlorine exists as 75% ³⁵Cl and 25% ³⁷Cl. Calculate its relative atomic mass.
Ar = (0.75 × 35) + (0.25 × 37) = 35.5.
Name the three isotopes of hydrogen and state how many neutrons each has.
Protium (0 neutrons), deuterium (1 neutron), and tritium (2 neutrons).
How are electrons arranged in an atom?
Electrons occupy the lowest available energy levels (shells) innermost first (max 2 in first shell, 8 in subsequent shells).
Write the electronic structure of sodium, and say what it means.
Sodium is 2,8,1, meaning two electrons in the first shell, eight in the second, and one in the third.
How is the periodic table arranged, and why is it called "periodic"?
Arranged in order of atomic number, with similar properties in groups. Called periodic because similar properties occur at regular intervals.
Why do elements in the same group have similar chemical properties?
They have the same number of electrons in their outer shell.
How did Newlands and Mendeleev contribute to the periodic table?
Newlands devised an early incomplete table. Mendeleev arranged elements by atomic weight, left gaps, and predicted properties of undiscovered elements.
How are metals and non-metals distinguished, and where are they found in the periodic table?
Metals form positive ions (lose electrons); non-metals gain or share electrons. Metals are on the left and bottom; non-metals on the right and top.
Give the typical physical properties of metals and of non-metals.
Metals have high melting/boiling points, are good conductors, and are solids (except mercury). Non-metals have lower melting points and are poor conductors.
Why are the noble gases unreactive, and how does helium differ from the rest?
They have stable outer shells (eight electrons, or two for helium), so they do not easily gain or lose electrons.
State the trends in boiling point and density down Group 0.
Both boiling point and density increase going down the group.
Give two uses of noble gases and explain each.
Light bulbs (unreactive with hot filament) and balloons (low density).
Why are the alkali metals so reactive, and what happens to reactivity down the group?
They have a single outer electron easily lost. Reactivity increases down the group as the outer electron is further from the nucleus.
Describe what you would observe when lithium, sodium and potassium react with water.
The metal floats, melts, moves around, produces hydrogen bubbles, and forms a metal hydroxide.
Describe the reactions of the first three alkali metals with oxygen and with chlorine.
With oxygen, they turn dull and burn with colored flames (red, yellow/orange, lilac). With chlorine, they form metal chlorides.
Why do the halogens react in similar ways, and what ion do they form?
They have seven outer electrons and gain one electron to form a negative ion.
Why do halogens exist as diatomic molecules?
They share electrons to form molecules made of pairs of atoms.
State the trends in relative molecular mass, melting point, boiling point and reactivity down Group 7.
Mass, melting point, and boiling point increase down the group; reactivity decreases down the group.
Explain why reactivity decreases down Group 7.
Atoms gain more electron shells, placing the outer shell further from the nucleus and reducing attraction.
What is displacement, and which halogen would displace which?
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
What type of compound forms when a halogen reacts with a metal?
A salt, where the metal atom transfers its outer electron to the halogen atom.
Name the compound with formula Na₂O, and write the formula of potassium chloride.
Na₂O is sodium oxide; potassium chloride is KCl.
Write a balanced half equation for a chlorine atom forming a chloride ion.
Cl₂ + 2e⁻ -> 2Cl⁻.
Predict the reactivity of an element from its position in the periodic table.
Group 1 elements become more reactive down the group, while Group 7 elements become less reactive down the group.