Worksheet 8
Atoms and atomic structure[1]
Early ideas about atoms
The word atom comes from atomos, an ancient Greek word meaning indivisible. The Greek philosopher Demokritos (460-370 BCE) maintained that all matter could be divided and sub-divided into smaller and smaller units, and eventually there would be a tiny particle that could not be divided any further - an atom. This was remarkable because there was no way ancient Greeks could support this theory by observation or experiment.
[atom: All elements are made of atoms.]
John Dalton
John Dalton (1766-1844)
1. Understanding of atoms didn’t progress much beyond Demokritos’ theory until the English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules His most important conclusions are summarised below.
• all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces
• all the atoms of a particular element are identical to each other and different from the atoms of other elements
• atoms are rearranged in a chemical reaction
• compounds are formed when two or more different kinds of atoms join together
[molecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds ].
Questions:
a. Why isn´t Demokritus considered a scientist? Why is Dalton considered a scientist?
b. What conclusions did Dalton come to about atomic theory?
Molecules of oxygen - the two atoms are joined by a double bond
A molecule of carbon dioxide. Atoms are represented as spheres and are colour-coded - carbon (green) and oxygen (red). The atoms are joined by double bonds
Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.
2. Atoms and elements
The structure of the atom
Although the word 'atom' comes from the Greek word for indivisible, we now know that atoms are not the smallest particles of matter. Instead, they have a small central nucleus surrounded by even smaller particles called electrons.
An atom consists of a nucleus containing protons and neutrons, surrounded by electrons
All substances are made from atoms, and, as Dalton suggested, any given element is made of atoms of just one particular sort. The atoms of any element are different from the atoms of any other element. So iron contains a different sort of atoms from those of sulfur, and the atoms in carbon are different from those of oxygen.
3. Atoms and atomic structure
Atoms consist of electrons surrounding a nucleus that contains protons and neutrons
Protons and neutrons have a relative mass of 1 and electrons have a negligible mass.
Neutrons are neutral, but protons and electrons are electrically charged. Protons have a relative charge of +1 and electrons have a relative charge of -1.
Electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons.
Key words:nucleus: The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom.
relative mass: The relative mass is the number of times heavier a particle is, compared to another.
negligible: So small as to be not worth considering.
Question: List and explain all the parts of an atom
The history of the discovery of the atomic structure is outlined below.
· John Dalton (1808) proposed the Atomic Theory. According to Dalton, matter is made up of tiny particles called atoms. The atom is the smallest particle of matter that takes part in a chemical reaction. Atoms are indivisible and cannot be created or destroyed. Further, atoms of the same element are identical in every respect.
· J. J. Thomson (1897) discovered electrons in Cathode Ray experiments. According to Thomson, atoms are divisible. Atoms contain very tiny negatively charged particles called electrons.
· E. Goldstein (1900) discovered protons in Anode Ray experiments. According to Goldstein, atoms contain positively charged particles called protons. Since atoms contain negatively charged particles, they must contain positively charged particles for them to be electrically neutral.
· E. Rutherford (1911) discovered the nucleus and provided the basis for the modern atomic structure through his alpha particle scattering experiment. According to Rutherford, the atoms is made of two parts: the nucleus and the extra-nuclear part. His experiments proved that the atom is largely empty and has a heavy positively-charged body at the center called the nucleus. The central nucleus is positively-charged and the negatively-charged electrons revolve around the nucleus.
· James Chadwick (1932) disovered neutrons. According to Chadwick, atoms contain neutral particles called neutrons in their nucleus along with the subatomic particles (i.e., electrons and protons).
· N. Bohr (1940) provided the modern concept of the atomic model. According to Bohr, the atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged) revolve around the nucleus in different imaginary paths called orbits or shells.
Question: How many times is heavier a Sodium atom than a Hydrogen atom? What is the relative mass of Sodium and Hydrogen atoms?
4. Chemical symbols
The atoms of each element are represented by chemical symbols. These usually consist of one or two different letters, but sometimes three letters are used for newly-discovered elements. The first letter in a chemical symbol is always an UPPERCASE letter, and the other letters are always lowercase. So, the symbol for magnesium is Mg and not mg, MG or mG.
Every element has its own chemical symbol. For example, iron is Fe, sulfur is S, sodium is Na and oxygen is O.
5. The periodic table
There are more than 100 different elements. The periodic table is a chart showing all the elements arranged in a particular way. The vertical columns in the periodic table are called groups. Each group contains elements that have similar properties.
The modern periodic table
The periodic table has eight main groups. For example, group 1 contains very reactive metals such as sodium - Na - while group 7 contains very reactive non-metals such as chlorine - Cl.
Note that you will never find a compound in the periodic table, because these consist of two or more different elements joined together by chemical bonds.
6. Reactions and compounds
New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur - often spelt 'sulphur' - react together to form a compound called iron sulfide - often spelt 'sulphide' - and sodium and oxygen react together to form sodium oxide.
Mixture of iron (grey) and sulphur (yellow) powders.
The mixture is heated in a test tube.
A chemical reaction occurs and iron sulphide is formed.
Chemical bonds involve electrons from the reacting atoms. Bonds can form when:
· electrons are transferred from one atom to another, so that one atom gives electrons and the other takes electrons, or
· electrons are shared between two atoms.
Activity: Watch the video in this site, is very interesting http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/rocks/atomsact.shtml
7. Chemical formulae
The chemical formula of a compound shows how many of each type of atom join together to make the units that make the compound up. For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In
sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the 2 is written as a subscript, so Na2O would be wrong.
The diagram below shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide - its chemical formula should therefore be written as CO2.
Carbon dioxide units contain one carbon atom and two oxygen atoms
Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3:
· a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
· a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H
8. Test
1. The atoms in an element are:
All the same type
Two types joined together
About a hundred different types
2. An atom consists of:
An electron surrounded by a nucleus
A nucleus containing electrons
A nucleus surrounded by electrons
3. What is the correct chemical symbol for sodium?
NANana
4. What do we get when two or more different types of atom join together using chemical bonds?
A mixture A compound A new element
5. What are the groups in the periodic table?
A row of similar elements
A column of similar elements
The boxes in the table
6. What do chemical bonds involve?
A nucleus moving from one atom to another
Electrons being transferred from one nucleus to another
Electrons being shared between two atoms
7. A unit of magnesium carbonate contains one magnesium atom, one carbon atom and three oxygen atoms. What is its chemical formula?
MgC3O Mg(CO)3 MgCO3
8. Which statement about chemical reactions is correct?
The mass of products equals the mass of reactants
The mass of products is more than the mass of reactants
The mass of products is less than the mass of reactants
9. Equations
When elements are joined to cause a chemical reaction, no atoms are made or lost during the process - but at the end of it they are joined differently from the way they were at the start. This means that the mass of the substances at the start - the reactants - is the same as the mass of the substances at the end - the products.
Copper and oxygen reaction - getting a balanced equation
We use balanced equations to show what happens to the different atoms in reactions. For example: copper and oxygen react together to make copper oxide.
Take a look at the word equation for the reaction, here:
copper + oxygen → copper oxide
You can see that copper and oxygen are the reactants, and copper oxide is the product.
If we just replace the words shown above by the correct chemical formulae, we will get an unbalanced equation, as shown here:
Cu + O2 → CuO
Notice that we have unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, we need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides of the arrow.
Here is the balanced symbol equation:
2Cu + O2 → 2CuO
You can see that now we have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.
Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide
The correct balanced equation for the reaction between magnesium and oxygen is:
Mg + O ==> MgO2
Mg + O2 ==> Mg2O
2Mg + O2 ==> 2MgO
Worksheet 8. Activities.
Activity 1: Read carefully the following text.
What is an Atom?
All substances are made up of matter and the fundamental unit of matter is the atom. The atom constitutes the smallest particle of an element. The atom is made of a central nucleus containing protons (positively-charged) and neutrons (with no charge). The electrons (negatively-charged with negligible mass) revolve around the nucleus in different imaginary paths called orbits or shells.
What is an Element ?
An element is a substance made up of atoms of one kind. There are about 82 naturally-occurring elements and about 31 artificially-made elements as listed in the Periodic Table
What is Atomic Number and Atomic Weight ?
Atomic number of an element is the number of protons in the nucleus of an atom. Since atoms are electrically neutral, the number of protons equal the number of electrons in an atom.
Atomic weight (or relative atomic mass) of an element is the number of times an atom of that element is heavier than an atom of hydrogen. The atomic weight of hydrogen is taken to be unity [1].
Mass number of an element is the sum of the number of protons and neutrons in the nucleus of an atom.
The elements are arranged according to increasing atomic numbers (along with their atomic mass) in a table called the Periodic Table.
What is a Molecule ?
A molecule is formed when atoms of the same or different elements combine. A molecule is the smallest particle of a substance that can normally exist independently.
Examples:
Two atoms of oxygen combine to form a molecule of oxygen [O2].
One atom of carbon combines with two atoms of oxygen to form a molecule of carbon dioxide [CO2].
What is a Compound?
A compound is formed when atoms or molecules of different elements combine. In a compound, elements are chemically combined in a fixed proportion.
Examples:
· Hydrogen and oxygen are combined in a fixed proportion of 2:1 to form the compound water [H2O].
· Carbon and oxygen are combined in a fixed proportion of 1:2 to form the compound carbon dioxide [CO2].
Interaction of Atoms:
It's the electrons in orbit around the nucleus that allow one atom to interact with other atoms so they can be linked together.
For example, H2O consists of an Oxygen atom linked to 2 Hydrogen atoms. The linkage or interaction between the electrons of the Hydrogen and Oxygen atoms is called a Chemical Bond. More on these later.
Ions:
Sometimes atoms gain or lose electrons. The atom then loses or gains a "negative" charge. These atoms are then called ions.
Positive Ion - Occurs when an atom loses an electron (negative charge) it has more protons than electrons.
Negative Ion - Occurs when an atom gains an electron (negative charge) it will have more electrons than protons.