Module 5
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Lesson 2.1

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Lesson 2.1
Lesson 2.2
Lesson 2.3
Lesson 2.4
Lab 2.1
Lab 2.2
Lab 2.3
Lab 2.4
Project 5

Lesson 2.1 Atoms

Overview
This lesson deals with the basic building blocks of matter: Atoms, electrons, protons and neutrons. On completion of the lesson, you should be able to describe atoms, molecules, electrons, protons, neutrons and other subatomic particles. You should be able to discuss the development of models used to describe atomic structure. You should also be able to explain the difference between elements and isotopes.

MINI LAB

CHOICE OF ACTIVITIES

1. See how many times you can cut a piece of paper in half

EXPERIMENT #1 DIVIDING BY 2

Purpose: To show that there is a limit to the number of times that we can cut a piece of paper in half

Equipment:
Sheet of paper
Scissors

Question
If an atom has a size in the region of 1 x10^-7 mm in diameter, how many times would we need to cut a 215mm x 280mm sheet of paper in half?

Matter Consists Of Particles
The Greek philosophers of the pre-Christian era believed that if we continued to cut things into smaller and smaller pieces, we would eventually come to a particle that could not be split or divided. These particles they called "atmos". The basic unit of an element is an atom.

Elements
Elements are the fundamental kinds of atoms that make up the building blocks of matter. The most abundant elements in the universe are hydrogen and helium. These two elements make up about 80and 20 % of all the matter in the universe respectively.

Until fairly recently (1908), atoms of were considered to be the smallest, indivisible, particles. Atoms of various elements differ considerably in size but each type of atom was considered to be indivisible.

Compounds
Atoms combine chemically to form compounds. Compounds can consist of molecules or ions. The atomic, molecular or ionic particles have forces of attraction between them. The particles also contain energy that results in constant movement – vibration, rotation and migration. The net degree of movement depends on the amount of energy contained in the particles and the forces of attraction between the particles. This net degree of movement determines the state of the material: Solid, liquid, gas or plasma.

Subatomic Particles
Researchers have recently discovered a host of subatomic particles. In addition to protons, electrons and neutrons, extremely small particles such as quarks and gluons have been discovered or proposed. Quarks and gluons are reported to be responsible for holding the nucleus of an atom together. The many subatomic particles that have been proposed to date now outnumber the known elements. . Because many of the subatomic particles are much smaller than electrons, they are extremely difficult to detect. Many may still be figments of the imagination.

Dalton’s Model Of The Atom
John Dalton built his theory of the structure of matter on a theory proposed a century earlier by the great English scientist, Isaac Newton. Newton had this to say about elements: "It seems probable to me that God in the beginning formed matter in hard, impenetrable, moveable particles. And that these primitive particles are so very hard as never to wear or break in pieces." John Dalton is credited with naming atoms as the smallest particles of an element. According to Dalton, atoms of a particular element were like tiny balls. Dalton presented his theory in 1808. Many chemists accepted the new ideas, others did not. "The Manchester Quaker," they said, "is talking about something that he has never seen—and never will."

Thompson’s Model
J.J. Thomson (1897) changed the view of the atom with his discovery of the electron. Thomson's work suggested that the atom could be divided into smaller particles. This was a dramatic proposal and came out of Thompson’s work with the cathode ray tube. Thomson proposed that atoms looked like pieces of raisin bread, a structure in which clumps of small, negatively charged electrons – like raisins - were scattered inside a smear of positive charges - the bread.

Rutherford’s Model
In 1908, Ernest Rutherford proved Thompson’s model incorrect. He performed a series of experiments with radioactive alpha particles. Rutherford fired tiny alpha particles at solid objects such as gold foil. He found that most of the alpha particles passed right through the gold foil, a small number of alpha particles passed through at an angle (as if they had bumped up against something) and some bounced straight back like a tennis ball hitting a wall. Rutherford's experiments suggested that gold foil, and matter in general, was composed largely of empty space.

Neutrons
James Chadwick discovered a third type of sub-atomic particle in 1932. He named this the neutron. Neutrons help stabilize the protons in the atom's nucleus. Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons are about the same size as protons and are electrically neutral.

The Size of an Atom
Atoms are extremely small. One hydrogen atom (the smallest atom known) is approximately 5 x 10^-8 mm in diameter. Most of the space taken up by an atom is actually empty because the electron spins at a very far distance from the nucleus.

For example, if we were to build a scale model of a hydrogen atom and use a small ball with a diameter of 1 cm as the proton, the atom's electron would move around at an average distance of 0.5 km from the 1-cm ball.

The Mass of an Atom
The number of protons and neutrons in the atom roughly determines the mass of an atom. While protons and neutrons are about the same size, the electron is more that 1,800 times smaller than the two. Thus the electrons' weight is inconsequential in determining the weight of an atom. A proton has a mass of 1.6726 x10^-24 grams and a neutron has a mass of 1.6750 x10^-24 grams.

Atomic Structures
Different atomic structures affect the physical properties of different materials We can often predict what these physical properties will be by looking at the structures of the atoms
For example, metals have loosely held electrons in their outer orbitals and this makes them good conductors of heat and electricity

Orbitals
Scientists initially believed that atoms were like the sun and planets with electrons orbiting the nucleus like planets around the sun
They found that there were fixed regions around the nucleus in which the electron could exist. These are called orbitals
Atoms are more spherical than flat and it was thus proposed that the orbitals were 3-dimensional.

Elements & Isotopes
The universe as we know it is made up of different elements. Each element has a different type of atom that has a characteristic atomic number.
The atomic number of an element refers to the number of protons in its nucleus. There are atoms with the same atomic number that have different numbers of neutrons in their nuclei These are known as isotopes.

The Periodic Table
The elements of matter consist of atoms of a particular type. These atoms all have the same number of protons and the same number of electrons. We can list these elements according to the different sizes of their atoms. We can also list them in a way that indicates how they are likely to react Each element in the periodic table below is represented by it’s atomic mass, its chemical symbol and its atomic number. For example, Boron is in Group 16 and Period 2. Its atomic mass is 16, its symbol is O and its atomic number is 8. From this we can see that oxygen has 8 protons and 8 electrons. The atomic mass indicates that it also has 8 neutrons in its nucleus.

Chlorine has isotopes that have atomic masses of 35 and 36. Because the ratios of these isotopes are close, the average atomic mass of chlorine is 35.45.

The position of an element in the periodic table indicates properties - densities, metal or non-metal, reactivity and stability

The elements in the first column are all metals and they react quite rapidly with other substances.

The substances at the other end of the table (Group 18) are noble gases. They don’t react at all and we use them when we want to keep things from reacting in glowing lights, welding or if we want to have balloons that float in air but don’t explode.

Most of the elements are metals. The elements in Group 17 are non metals. There are elements that behave like metals under some conditions and like non-metals under different conditions. The most important of these are those that can be used as semiconductors.

Abbreviated Periodic Table of the Elements

Questions

1. How do the atoms of different elements differ from each other?
2. What is an isotope?
3. What is the difference between an element and a compound?
4. How many protons are there in a boron atom?
5. Could we expect that boron could have a lower density than aluminum?
6. 6.02 x10²³ carbon atoms have a combined mass of 12.011 grams. What is the mass of one carbon atom?
7. How do the orbitals of electrons situated around the nuclei of atoms differ from the orbits of planets around a sun?
8. In comparing the relative sizes of atomic particles, how large would the diameter of an atom be if the diameter of its nucleus was 1 cm?
9. How did the New Zealander physicist Ernest Rutherford discover in 1911 that almost all of an atom’s mass is packed into the central region of the atom known as the nucleus?
10. How many electrons are there in a Germanium atom?
11. The relative atomic mass of carbon is 12.011. Does this mean that all carbon atoms have 6 neutrons?
12. Can we expect carbon to have a greater density than silicon?