Atomic Structure & The Periodic Table (WJEC GCSE Science (Double Award): Chemistry): Flashcards

The number of neutrons is calculated by subtracting the atomic number from the mass number.

Number of neutrons = mass number − atomic number.

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. They have the same atomic number but different mass numbers.

False.

Isotopes have the same number of electrons in their outer shells, so they have the same chemical properties. They differ only in the number of neutrons, which affects mass.

The mass number is the total number of protons and neutrons in the nucleus. The atomic number is the number of protons only.

A beryllium atom has 5 neutrons.

Number of neutrons = mass number − atomic number = 9 − 4 = 5.

The mass number is the total number of protons and neutrons in the nucleus of an atom. It is always the larger of the two numbers shown for an element on the periodic table.

Isotopes of the same element have the same chemical properties because they have the same number of outer shell electrons. Chemical behaviour is determined by electron configuration, not by the number of neutrons.

In the PEN system: P = number of protons, E = number of electrons, N = number of neutrons. This is a useful checklist for working out the subatomic composition of an atom.

A group is a vertical column in the periodic table. Elements in the same group have the same number of outer shell electrons and therefore similar chemical properties.

The period number tells you how many occupied electron shells an atom of that element has. For example, elements in Period 3 have three electron shells.

Elements in the periodic table are arranged in order of increasing atomic number. Vertical columns are called groups and horizontal rows are called periods.

Metals are located on the left and centre of the periodic table. Non-metals are on the right-hand side. A 'stair line' starting above aluminium separates the two regions.

False.

Metallic character decreases from left to right across a period. Elements on the left are metals and tend to lose electrons, while elements on the right are non-metals that gain electrons.

Metalloids are elements that display properties of both metals and non-metals. They are found along the stair-line boundary between metals and non-metals on the periodic table.

Elements in the same group have similar chemical properties because they all have the same number of outer shell electrons. The number of outer electrons determines how an element reacts.

Elements are arranged in order of increasing atomic number. This arrangement groups elements with similar properties into the same vertical column (group).

An electronic configuration is a shorthand notation showing the number of electrons in each shell of an atom, written as numbers separated by commas (e.g. 2,8,3 for aluminium).

The first shell holds up to 2 electrons, the second shell holds up to 8 electrons, and the third shell holds up to 8 electrons (for the first 20 elements).

Carbon has 6 electrons. Its electronic configuration is 2,4. Sodium has 11 electrons. Its electronic configuration is 2,8,1.

The valence shell is the outermost occupied electron shell of an atom. Atoms are more stable when this shell is completely full of electrons.

False.

The period number tells you how many occupied electron shells an atom has. It is the group number that tells you how many outer shell electrons an element has.

Chlorine's electronic configuration is 2,8,7.

Period 3 means 3 occupied shells. Group 7 means 7 electrons in the outer shell.

The last number in the electronic configuration gives the number of outer shell electrons, which equals the group number. For example, an element with configuration 2,8,6 is in Group 6.

A sodium ion has the electronic configuration 2,8.

A sodium atom has 11 electrons (2,8,1). It loses one electron to form Na⁺, leaving 10 electrons arranged as 2,8.

Electrons fill the innermost shell first because it has the lowest energy level. Once a shell is full, additional electrons must occupy the next shell, which has higher energy.

The alkali metals are the elements in Group 1 of the periodic table. They are soft metals with low densities and low melting points that react vigorously with water to form alkaline solutions.

The melting points of Group 1 metals decrease going down the group. This is because the attractive forces between the outer electrons and positive ions become weaker as the atoms get larger.

False.

Chlorine is a pale yellow-green gas at room temperature. Bromine is the red-brown liquid, and iodine is a grey-black solid.

The halogens are the non-metal elements in Group 7 of the periodic table. They are diatomic, poisonous and become denser with darker colours as you go down the group.

The melting and boiling points increase going down Group 7. This is because the atoms become larger and the intermolecular forces between molecules get stronger, so more energy is needed to overcome them.

Reactivity increases down Group 1 because each successive element has more electron shells. The outer electron is further from the nucleus and experiences greater shielding from inner electron shells, so the attractive force holding it is weaker and it is lost more easily.

Group 1 metals form ions with a 1+ charge by losing one electron.

Group 7 non-metals form ions with a 1- charge by gaining one electron.

Reactivity decreases down Group 7 because each successive element has more electron shells. The incoming electron is further from the nucleus and experiences greater shielding from inner electron shells, so the attraction between the nucleus and the gained electron is weaker, making it harder to gain an electron.

Group 7 elements form 1- ions because they have 7 electrons in their outer shell and need to gain one electron to achieve a full outer shell of 8 electrons. This makes them chemically stable like a noble gas.

At room temperature: chlorine is a pale yellow-green gas, bromine is a red-brown liquid, and iodine is a grey-black solid. Density increases going down the group.

Chlorine displaces bromine from potassium bromide solution because chlorine is more reactive than bromine. The solution turns from colourless to orange-brown as bromine is produced.

When an alkali metal reacts with water, a metal hydroxide and hydrogen gas are produced. The metal hydroxide dissolves to form an alkaline solution.

Alkali metals are stored in oil to prevent them from reacting with oxygen and water vapour in the air. Contact with air causes them to tarnish or react vigorously.

When potassium reacts with water, it burns with a lilac flame and reacts more vigorously than sodium. The products are potassium hydroxide and hydrogen gas.

True.

All alkali metals react with water to produce a metal hydroxide solution. Metal hydroxides are alkaline (pH > 7), which is why these metals are called alkali metals.

sodium + water → sodium hydroxide + hydrogen

The sodium hydroxide dissolves to give an alkaline solution. Vigorous fizzing is the observable sign that hydrogen gas is released.

When an alkali metal reacts with oxygen, a metal oxide is formed. For example, sodium forms sodium oxide (Na₂O). A dull oxide coating forms on the metal surface.

When an alkali metal reacts with chlorine gas, a metal chloride salt is formed. For example, sodium reacts with chlorine to form sodium chloride (NaCl). A white solid is produced.

The reactions become more vigorous going down Group 1. Lithium reacts relatively slowly, sodium reacts more vigorously, and potassium reacts very vigorously, burning with a lilac flame.

Sodium produces a yellow (orange) flame when it reacts with a halogen. Lithium produces a red flame and potassium produces a lilac flame.

The squeaky pop test is the standard test for hydrogen gas. A lit splint is held at the mouth of a test tube of gas. A positive result produces a loud squeaky pop sound.

Hold a lit splint at the open end (mouth) of a test tube containing the gas. If hydrogen is present, the gas burns with a loud squeaky pop. Do not insert the splint fully into the tube.

False.

A lit splint is used to test for hydrogen. A glowing splint is used to test for oxygen. The lit splint causes hydrogen to combust rapidly, producing the characteristic squeaky pop.

Hydrogen gas is tested using a lit splint held at the mouth of the test tube. A positive result produces a loud squeaky pop.

The squeaky pop occurs because hydrogen combusts rapidly when it meets the flame from the lit splint. It reacts with oxygen in the air to produce water, and this rapid combustion creates the popping sound.

The splint must be held at the mouth of the tube rather than inserted because hydrogen needs air (oxygen) to burn. If the splint is pushed in fully, there is not enough oxygen available for the gas to combust.

Chlorine is used to sterilise drinking water and to kill bacteria in swimming pools. The levels must be carefully monitored because chlorine is toxic.

A halide displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide salt. Reactivity decreases down Group 7.

Halogens react with alkali metals to form metal halide salts, which are ionic compounds. For example, sodium reacts with chlorine to form sodium chloride (NaCl).

Iodine is used as an antiseptic in plasters and sterilising sprays. It is a dark grey solid that can form a purple vapour when warmed, and is found in Group 7.

False.

Bromine cannot displace chloride ions because chlorine is more reactive than bromine. Only a more reactive halogen can displace a less reactive one.

The solution turns yellow-orange. Chlorine displaces bromide ions from the solution because it is more reactive, and the yellow-orange colour is due to the formation of bromine.

The reactions become less vigorous going down Group 7. Fluorine reacts with cold iron wool, chlorine and bromine react with hot iron wool vigorously, and iodine reacts only slowly with hot iron wool.

Chlorine reacts with iron to form iron(III) chloride. The symbol equation is: 2Fe + 3Cl₂ → 2FeCl₃.

Reactions with halogens must be carried out in a fume cupboard because halogens are toxic. Inhaling halogen vapours is dangerous to health.

chlorine + potassium bromide → potassium chloride + bromine

Chlorine displaces bromine because it is higher in Group 7 and therefore more reactive.

The noble gases are the elements in Group 0 (Group 8) of the periodic table. They are colourless, monoatomic gases with full outer electron shells, making them chemically inert.

Noble gases are chemically unreactive because they have full outer electron shells. This electronic configuration is extremely stable so they have no tendency to gain, lose or share electrons.

Helium is used in airships and weather balloons because it has a very low density. Argon is used in light bulbs (and welding) because it provides an inert atmosphere.

The electronic configuration of argon is 2,8,8.

Argon has 18 electrons arranged across 3 shells, with 8 in the outer shell, giving it a stable full outer shell.

Neon is used in advertising signs because it emits light when an electric current is passed through it. It is also safe to use because it is chemically inert.

False.

Most noble gases have 8 electrons in the outer shell, but helium is the exception — it has only 2 electrons, which fills its first (and only) shell.

All Group 0 elements are colourless, monoatomic gases at room temperature. They all have very low melting and boiling points and are chemically inert.

Helium is used in airships because it has a very low density and is chemically inert. Hydrogen is also less dense than air but is highly flammable, making it dangerous to use.

A flame test is a method used to identify metal cations by the colour of flame they produce when held in a Bunsen burner flame using a nichrome or platinum wire loop.

The wire loop must be cleaned in dilute acid to remove any contaminating ions from a previous test. If two ions are present on the wire, their flame colours could mix and one colour could mask the other, making identification unreliable.

1. Li⁺ produces a red flame.

2. Na⁺ produces a yellow-orange flame.

3. K⁺ produces a lilac flame.

Barium ions (Ba²⁺) produce an apple-green flame. This distinctive colour must be described accurately — 'green' alone is not sufficient.

True.

Ca²⁺ ions produce a brick-red flame. This is distinct from the red flame of lithium (Li⁺), so the specific colour must be stated accurately.

The wire must be held in the blue flame because it is hotter than the yellow flame. The higher temperature is needed to excite the metal ions enough to emit their characteristic colour of light.

To test for halide ions, add silver nitrate solution.

Ag⁺ (aq) + Cl^- (aq) → AgCl (s)

Silver chloride forms as a white precipitate. The potassium and nitrate ions are spectator ions and do not take part in the reaction.

Spectator ions are ions that are present in solution but do not take part in the reaction. In the reaction of potassium chloride with silver nitrate, the potassium ions and nitrate ions are spectator ions.

The full symbol equation shows all reactants and products: KCl (aq) + AgNO₃ (aq) → KNO₃ (aq) + AgCl (s).

The ionic equation shows only the ions that react: Ag⁺ (aq) + Cl^- (aq) → AgCl (s). Spectator ions (K⁺ and NO₃^-) are left out.