Atomic Structure
Name Mass Charge Location
Protons 1 +1 Nucleus
Neutrons 1 0 Nucleus
Electrons 1/1837 -1 Orbit nucleus in outer shells called
“nucleons”
The number of protons equals the atomic number. This is the number which defines the element (i.e. all elements have different atomic numbers). The atomic number can be written as a subscript (below the line) in front of the symbol (e.g. 6C).
In a neutral atom, the number of electrons equals the number of protons.
The mass number is equal to the number of nucleons. The mass number can be written as a superscript before the symbol (e.g. 12C).
Isotopes are when a single element has atoms with the same number of protons, but different number of neutrons (and therefore mass number).
Example: Chlorine comes in two different forms. One form has a mass number of 35 and the other has a mass number of 37:
35Cl - 17 protons 18 neutrons
37Cl - 17 protons 20 neutrons
Relative atomic mass (Ar) – this is the mass of an “average” atom of an element divided by one atom of 1H. The Ar takes into account the relative abundance of the naturally occurring isotopes of the element….ok, so basically, you find the average of the masses of the different isotopes! The mass number is always an integer (that’s a “whole number” for all you non-mathematicians) BUT the Ar does not have to be.
Example:
What is the Ar of chlorine given that there are two isotopes: 35Cl (which is 75% abundant) and 37Cl
(which is 25% abundant)?
To do this, we need to look at the abundance to work out the mass of an “average atom” of Cl. As 35Cl is 75% abundant, we multiply 35 by 75% (i.e. 0.75) and as 37Cl is 25% abundant, we multiply 37 by 25% (i.e. 0.25). The two numbers are then added together and divided by the mass of one atom of 1H (i.e. 1 – so this step is pointless…):
(35 × 0.75) + (37 × 0.25) = 35.5
(If you didn’t understand that…don’t worry, here’s an easier way…just for you…simply remember this formula:
The Periodic Table
There are approximately 100 elements. Everything is made up of one or more elements. The periodic table shows elements in increasing atomic number.
Electrons are accommodated in shells. The maximum in the first shell is 2, the maximum in the second shell is 8 and the maximum in the third shell is also 8. Electrons always fill lower energy shells (i.e. closer to the nucleus) first.
.
.
..
..
..
.
.
2nd electron shell (can hold up
to 8 electrons) 1st electron shell (can hold up
to 2 electrons)
nucleus – with protons and neutrons The first 20 elements:
Atomic
Number Symbol Element Shell 1 Shell 2 Shell 3 Shell 4
1 H Hydrogen 1 2 He Helium 2 3 Li Lithium 2 1 4 Be Berillium 2 2 5 B Boron 2 3 6 C Carbon 2 4 7 N Nitrogen 2 5 8 O Oxygen 2 6 9 F Fluorine 2 7 10 Ne Neon 2 8 11 Na Sodium 2 8 1 12 Mg Magnesium 2 8 2 13 Al Aluminium 2 8 3 14 Si Silicon 2 8 4 15 P Phosphorous 2 8 5 16 S Sulphur 2 8 6 17 Cl Chlorine 2 8 7 18 Ar Argon 2 8 8 19 K Potassium 2 8 8 1 20 Ca Calcium 2 8 8 2
Other important elements include: Atomic
number Symbol Element
26 Fe Iron
29 Cu Copper
30 Zn Zinc
35 Br Bromine
Atomic
number Symbol Element
47 Ag Silver
53 I Iodine
56 Ba Barium
In the periodic table, elements are arranged in rows named periods when a single electron shell is filling. We start a new period when we start filling the next electron shell.
At the top of each column or group, there is a number showing how many electrons are in the outer shell (e.g. group 7 has 7 electrons in the outer shell).
Element
An element is a pure substance which cannot be broken down any simpler by chemical means. Atoms of the same element have the same atomic number. A sample of an element may be made up of atoms or molecules.
Compound
Compounds are pure substances containing two or more elements chemically bonded together in fixed proportions by mass.
Example: H2O
2g Hydrogen, 16g oxygen
1g Hydrogen, 8g oxygen
The properties of a compound are different to those of its constituent element. The chemical bonds in compounds are either ionic or covalent (we will look at this a bit later…fun fun fun!). Compounds can be either ions or molecules.
Mixtures
Vary in composition and can be physically separated. Mixtures and Compounds – the difference!
Iron and sulphur mixed together looks like yellow powder with grey bits in it…(a mixture) You can separate then by:
- using a magnet (the smart way of getting a needle from a haystack!)
- dissolving the sulphur in a solvent (e.g. toluene) and filtering when hot. Then let the solution cool and the sulphur crystallises.
If the mixture is heated, an exothermic reaction (a reaction that gives out heat) takes place. There are 3 things to prove that a reaction has taken place:
1. It continues to glow red hot even when you stop heating it
2. The product is a brittle grey solid, which does not look like the mixture
3. The mixture reacts with dilute hydrochloric acid to produce hydrogen whereas the compound (after heating) reacts with hydrochloric acid to make foul smelling hydrogen sulphide.
Iron(s) + Sulphur (l) → Iron (II) Sulphide(s) Fe(s) + S(l) → FeS(s)
Atoms of different elements can combine to form compounds by the formation of new chemical bonds.
Magnesium(s) + Oxygen(g) → Magnesium Oxide(s) 2Mg(s) + O2 (g) → 2MgO(s)
Ions
Ions are formed from atoms by the gain or loss of electrons. The nucleus is left unaffected. The maximum that can be lost or gained is 3. Generally we form an ion which has a noble gas configuration (i.e. all full outer shells).
Positive ions are called cations. Negative ions are called anions.
Atoms of elements in group 1 LOSE one electron to form a 1+ ion of noble gas configuration:
Na → Na+ + 1e– (electron)
2, 8 (same electron configuration as neon) 2, 8, 1
Atoms of elements in group 2 LOSE two electrons to form a 2+ ion of noble gas configuration:
Ca → Ca2+ + 2e– (electrons)
2, 8 (same electron configuration as argon) 2, 8, 8, 2
Atoms of elements in group 3 LOSE three electrons to form a 3+ ion of noble gas configuration:
Al → Al3+ + 3e– (electrons)
2, 8 (same electron configuration as neon) 2, 8, 3
Atoms of elements in group 7 GAIN one electron to form a 1– ion of noble gas configuration:
(electron) e– + Cl → Cl–
2, 8, 8 (same electron configuration as argon) 2, 8, 7
Atoms of elements in group 6 GAIN two electrons to form a 2– ion of noble gas configuration:
(electrons) 2e– + O → O2–
2, 8 (same electron configuration as neon) 2, 6
Formulae of Ionic compounds
The ones in red / green need to be learnt, the rest can be worked out by looking at their position in the periodic table.
2– 1– 1+ 2+ 3+
O2– (oxygen) F– (fluoride) Li+ (lithium) (magnesium) Mg2+ Al3+ (aluminium III) S2– (sulphide) Cl– (chloride) Na+ (sodium) Ca2+ (calcium)
Br– (bromide) K+ (potassium) Ba2+ (barium) I– (iodide) Ag+ (silver) Zn2+ (zinc)
H+ (hydrogen) Pb2+ (lead) Cu+ (copper I) Cu2+ (copper II)
Fe2+ (iron II) Fe3+ (iron III) SO42– (sulphate) OH– (hydroxide) CO32– (carbonate) NO3– (nitrate) NH4+ (ammonium) SO32– (sulphite) HCO3 – (hydrogencarbonate) CH3CO2–(ethanoate)
Radical Ions (2 or more atoms covalently bonded together carrying a
charge. they behave as a single unit)
Both copper and iron can form ions of more than one charge. We have to write the charge in roman numerals in the name to distinguish between the compounds.
Cu2O = Copper (I) oxide CuO = Copper (II) oxide FeCl2 = Iron (II) chloride FeCl3 = Iron (III) chloride
Learning how to write formulae is VERY important…lots of chemistry is about formulae... Basically, all you have to do is take the symbol of each element and put it together making sure that the charges cancel out and that you put the positive ion first.
So…three steps:
1. In name and formulae, the positive ion goes first 2. The total charge must equal 0
3. If you have to multiply a radical ion, put it in brackets Examples:
Sodium Chloride (Na+ and Cl–) → NaCl Calcium Chloride (Ca2+ and Cl–) → CaCl2 Calcium Nitrate (Ca2+ and NO3–) → Ca(NO3)2
(notice that 2 Cls are needed to cancel the 2+ of the Ca)
Try some for yourself (just to see if you really understood it and aren’t just pretending to): 1. Sodium sulphate 2. Aluminium chloride 3. Calcium sulphide 4. Aluminium nitrate 5. Sodium nitrate 6. Ammonium chloride 7. Ammonium sulphide 8. Aluminium sulphide
A GCSE Chemistry word-fill worksheet on
"ATOMIC STRUCTURE, ISOTOPES and ELECTRON STRUCTURE"
15 2 2 2.7 2.8.5 21 21 22 26 26 2nd 3 30 4th 5 5th 7 8 8 9 atomic atomic electrons electrons electrons group iron isotopes
mass mass neutrons neutrons nucleus number period shell shells three
Q1(a) Atoms are made of fundamental particles called protons
(+), (0) and (-). (b) The centre of the atom is called
the . (c) It consists of protons and and contains most
of the mass of the atom.
Q2(a) The number of protons in the atom is called the
number. It also equals the number of in a neutral
atom. (b) The is the sum of the protons and neutrons in
the nucleus.
Q3 An atom of mass number 43, and atomic number 21, consists of
protons, neutrons and electrons.
Q4 (a) In the symbol for an atom of shown on the left,
the atom consists of protons, neutrons and
electrons. (b) Atoms of the same element with a different number due to different numbers of neutrons are called
.
Q5(a) Electrons are arranged around the nucleus in energy levels or
. (b) The 1st shell can have a maximum of electrons, the
shell a maximum of electrons and the 3rd shell a maximum
of electrons. (c) The 19th and 20th electrons go in the
shell. (d) The electron arrangement, showing the number of
electrons in each can be written out in shorthand eg 2.8.6 for
Q6(a) Apart from hydrogen and helium in period 1 of the Periodic Table the last number in the arrangement is equal to the
number in the Periodic Table. (b) The number of shells used
containing electrons is equal to the number. (c) The number
of electrons in an atom is equal to the or proton number.
Q7(a) Phosphorus has an atomic number of 15 so the atoms have
protons or 15 . (b) The electron arrangement will be
. (c) In the Periodic Table it will be a member of Group
and it will be the element along Period .
Q8(a) Fluorine has an atomic number of so the atoms have 9
protons or electrons. (b) Its electron arrangement is and it
belongs to Group on Period .
Ionic Equations – Questions (A) Translate into symbols and balance each equation: 1. calcium carbonate breaks down on heating to give calcium oxide and carbon dioxide. 2. sulphur burns in oxygen to form sulphur dioxide gas. 3. heating magnesium and sulphur powders together produces solid magnesium sulphide. 4. sodium hydroxide and hydrochloric acid react to form sodium chloride and water. 5. hydrogen gas and oxygen gas burn together to form water. 6. zinc metal and copper sulphate solution react to produce copper and zinc sulphate solution. (B) Balance these equations and add state symbols:
1. CuO + HNO3 to Cu(NO3)2 + H2O
2. Al + Cl2 to AlCl3
3. N2 + H2 to NH3
4. Na + H2O to NaOH + H2
5. K2CO3 + H2SO4 to K2SO4 + H2O + CO2
6. Fe + H2SO4 to FeSO4 + H2
7. CH4 + O2 to CO2 + H2O 8. Pb(NO3)2 + NaCl to PbCl2 + NaNO3 (C) Balance these equations:
1. MnO2 + HCl to MnCl2 + Cl2 + H2O
2. HBr + H2SO4 to SO2 + Br2 + H2O
3. HI + H2SO4 to I2 + H2S + H2O
4. KMnO4 + HCl to KCl + MnCL2 + Cl2 + H2O
Equation Answers
A. .
1. CaCO3(s) CaO(s) + CO2(g)
2. S(s) + O2(g) SO2(g)
3. Mg(s) + S(s) MgS(s)
4. NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
5. 2H2(g) + O2(g) 2H2O(l)
6. Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)
B.
1. CuO(s) + 2HNO3(aq) Cu(NO3)2(aq) + H2O(l)
2. 2Al(s) + 3Cl2(g) 2AlCl3(s)
3. N2(g) + 3H2(g) 2NH3(g)
4. 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)
5. K2CO3(s) + H2SO4(aq) K2SO4(aq) + H2O(l) + CO2(g)
6. Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g)
7. CH4(g) + 2O2(g) CO2(g) + 2H2O(g)
8. Pb(NO3)2(aq) + 2NaCl(aq) PbCl2(s) + 2NaNO3(aq)
C. 1. MnO2 + 4HCl MnCl2 + Cl2 + 2H2O 2. 2HBr + H2SO4 SO2 + Br2 + 2H2O 3. 8HI + H2SO4 4I2 + H2S + 4H2O 4. 2KMnO4 + 16HCl 2KCl + 2MnCl2 + 5Cl2 + 8H2O 5. 10FeSO4 + 2KMnO4 + 8H2SO4
5(Fe)2(SO4)3 + K2SO4 + 2MnSO4 +
8H2O
