INTRODUCTION

INTRODUCTION

Extracted from Chemistry for You (Lawrie Ryan. Published by Nelson Thornes, 2^nd ed. 2001)

History of the atom

The word atom comes from a Greek word meaning something which can’t be split.

This fits in nicely with Dalton’s ideas about atoms. However, as you probably already know, atoms can be split

In the late 1800s and early 1900s, scientists had to think up new pictures of atoms to explain new observations.

For example, in 1897 J.J. Thomson put forward his ‘plum pudding’ theory. He thought atoms were balls of positive charge with tiny negative particles stuck inside. The negative particles were called electrons. He said they were like the currants in a bun or Christmas pudding.

This model explained Thomson’s experiments with electricity very well. However, later experiments using radioactive particles needed a new picture. By 1915, scientists, like Ernest Rutherford and Niels Bohr, had developed a model that is still useful today.

Democritus suggests that all things are made of particles.

John Dalton’s atomic theory. Atoms of the same element are all alike. They combine to make compounds.

J.J. Thomson finds the electron.

Ernest Rutherford discovers the proton.

Ernest Rutherford discovers the nucleus.

Niels Bohr suggests that electrons are found in shells around the nucleus.

James Chadwick proves that neutrons exist.

Inside the atom

There are 3 types of particle inside an atom. These are protons, neutrons and electrons.

The protons and neutrons are found squashed together in the middle of the atom.

The middle, called the nucleus, is incredibly small and dense.

The tiny electrons whizz around this nucleus.

Protons are positively charged. Electrons are negatively charged.

Neutrons have no charge. They are neutral.

Protons and neutrons are the heavy particles in an atom.

They each have a mass of 1 atomic mass unit, and are found in the nucleus (centre) of an atom.

We can ignore the tiny mass of the electrons.

The electrons orbit the nucleus in shells.

The 1st shell can hold 2 electrons

The 2nd shell can hold 8 electrons, as can the 3rd shell.

The atomic number (Z) = the number of protons (which equals the number of electrons).

The mass number (A) = the number of protons + the number of neutrons.

Isotopes are atoms with the same number of protons, but different numbers of neutrons.

ACTIVITIES

1. Copy and complete: chemical, physical, atom, molecule, compound, atoms, simpler

a) The smallest part of an element is called an …………………All the.................... in an element are the same. Elements can’t be broken down into ................... substances.

b) Atoms joined or bonded, together chemically are called……………. If a substance is made from more than one type of atom, it is called a ………………

c) In a.................... change, new substances are formed. However, no new substances are made in a ………………..change.

2. Look at the boxes below:

Which box contains:

a) one element

b) a mixture of elements

c) a pure compound

d) a mixture of elements and a compound? What might be happening in this box?

3. Complete the year with the name of the scientists and their contributions:

4. Copy and complete:

There are 3 types of particle found inside atoms:a)………….. b)……………...and c)……………..

This table shows their mass and charge:

The protons and neutrons are found in the…………….. of the atom, called the nucleus.

The ................. zoom around the nucleus in shells. The 1st shell, which is .................. the nucleus, can hold.... electrons, whereas the 2nd and 3rd shells can hold ...... electrons.

5. a) What is the atomic number of an atom?

b) What is the mass number of an atom?

c) What is the atomic number of the atom below? What is its mass number?

a) What is the atomic number of an atom?

b) What is the mass number of an atom?

c) What is the atomic number of the atom below? What is its mass number?

6. Give the numbers of protons, electrons and neutrons in the atoms below:

a) ¹⁴₇N; b) ²⁰₁₀Ne; c) ¹⁹₉F; d)³⁹₁₉K; e) ⁶⁰₂₇Co; f)²³⁵₉₂U

7. Draw fully labelled diagrams of the atoms below:

a) ⁴₂He; b) ⁹₄Be; c) ²⁷₁₃Al; d)⁴⁰₂₀Ca;

8. Copy this table and fill in the gaps.

Use Periodic Table to help you

Do you have to look up the atomic number of lithium, given the information in the table above? Explain your answer.

9. Hydrogen (atomic number 1) has 3 isotopes.

They can be shown as ¹₁H, ²₁H and ³₁H.

a) What are isotopes?

b) What is the difference between the 3 isotopes?

c) Hydrogen reacts with chlorine in sunlight, forming hydrogen chloride:

H₂+ Cl₂ → 2HCI

Would you expect the same reaction for each isotope of hydrogen? Why?

d) Chlorine exists naturally as 2 isotopes. 75% is ³⁵₁₇Cl, and 25% is ³⁷₁₇Cl. Show why the relative atomic mass of chlorine is 35.5.

e) The element chlorine is a gas. Its formula is Cl₂ How many different masses of the Cl₂ molecule would you expect to find in a sample of chlorine gas? Explain your answer.

10.Work out the relative formula mass of:

a)H₂O b) C₂H₅OH c) Na₂SO₄

(R.A.M.s: H = 1, O= 16, C = 12, Na = 23, S=32)

IN THE LABORATORY

www.chemistry.lmt.md/sezennur/ subjects/experiments/018_flame_test.pdf

Flame Test

PRE-LAB DISCUSSION

The normal electron configuration of atoms or ions of an element is known as the “ground state.” In this most stable energy state, all electrons are in the lowest energy levels available. When atoms or ions in the ground state are heated to high temperatures, some electrons may absorb enough energy to allow them to “jump” to higher energy levels. The element is then said to be in the “excited state.” T

This excited configuration is unstable, and the electrons “fall back to their normal positions of lower energy. As the electrons return to their normal levels, the energy that was absorbed is emitted in the form of electromagnetic energy. Some of this energy may be in the form of visible light. The colour of this light can be used as a means of identifying the elements involved. Such crude analyses are known as flame tests.

Only metals, with their loosely held electrons, are excited in the flame of a laboratory burner. Thus, flame tests are useful in the identification of metallic ions. Many metallic ions exhibit characteristic colours when vaporized in the burner flame. In this experiment, characteristic colours of several different metallic ions will be observed, and an unidentified ion will be identified by means of its flame test.

PURPOSE

Observe the characteristic colours produced by certain metallic ions when vaporized in a flame. Identify an unknown metallic ion by means of its flame test.

EQUIPMENT

graduated cylinder, 10-mL wire loop

platinum laboratory burner

glass-marking pencil test tubes

test tube rack

MATERIALS

HCI(conc.)

Unidentified solutions 0.5 M solutions of nitrates of: Na⁺,K⁺, Ca⁺², Sr⁺², Ba⁺², Cu⁺²

PROCEDURE

1. Measure 5 mL of tap water in a graduated cylinder and pour the water into a 13 x 100 test tube. Using a marking pencil, mark the outside of the tube to indicate the level of the water. Discard the water. Using the marked tube as a guide, mark seven clean test tubes at approximately the same level. Place the clean tubes in a test tube rack. Set the other test tube aside.

2. Into each of the clean test tubes, pour 5 mL of a different nitrate solution. Mark each test tube to indicate the metallic

ion it contains.

3. Pour about 10 mL of concentrated hydrochloric acid into a 50-mL beaker. CAUTION: Use extreme care in handling this acid. To clean the wire loop, dip the loop in the acid and then heat the loop in the outer edge of the burner flame. Continue to clean the loop in this manner until no colour is observed in the flame.

4. Dip the clean wire loop into one of the nitrate solutions. Place the loop in the outer edge of the burner flame and move the loop up and down (Figure). Note the colour in the flame. Record your observations in the data list provided.

5. Clean the wire loop as described in step 3. Repeat step 4 using a different nitrate solution.