04 Particles (1) Atoms 2009

THE PARTICLES OF MATTER (PART I): ATOMS The Nature of the Particles of Matter So far we have talked about “particles”. Evidently, the particles forming water must be different to the particles of iron or sugar. There are millions of different particles (as many as different substances). To begin with, particles can be sorted into three different classes: atoms, molecules and ions. Atoms: electrically balanced particles that consist in one positively charged centre called the nucleus surrounded by a “cloud” of negatively charged particles called the electrons, to exactly balance the nuclear charge. Molecules: electrically balanced particles with more than one positive centres (nuclei). Molecules are sets of bonded atoms that act as a unit. Ions: atoms (or groups of atoms) with unbalanced charges As it can be appreciated, atoms seem to be the essential particles for any kind of matter. They were thought to be the simplest possible particles, with no possibility of having “parts” or being broken. We will learn how these ideas have changed during the past 200 years. Dalton's Atomic Theory

                It was in the early 1800s that John Dalton came up with his atomic theory. The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it was supported by careful chemical measurements behind it. Up to this moment, Democritus’ ideas have been tacitly accepted by Galileo, Newton, Boyle and other scientists, but Dalton’s theory relied on experimental facts: it wasn't just a philosophical statement that there are atoms because there must be atoms. Less than twenty years earlier, in the 1780's, Lavoisier had started a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy. He established that matter cannot be created or destroyed during a chemical change (the Law of Conservation of Mass). By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition: when elements combine to form a compound they do it always in the same proportion. In 1803 Dalton noted that oxygen and carbon combined to make two compounds one had exactly twice as much oxygen as the other. This fact of elements combining in definite amounts was proved to hold in many other cases. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.

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This theory stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. Atoms of an element could not be created, destroyed or divided. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together.

Dalton’s theory was not compatible with some experimental results of Gay-Lussac’s so it had to be modified. Avogadro did that admitting Dalton’s ideas but with a slight change: he hypothesised that the elements are not formed by atoms but rather by sets of equal atoms stuck together. He called this new entity a molecule. So although atoms are the ultimate particles involved in chemical reactions, the physically separated particles in a gas are the molecules. Previous to Avogadro’s assumptions it was admitted that the number of particles (atoms) in say 1 dm3 of any gaseous element was the same (provided temperature and pressure were the same for both). According to Avogadro’s ideas this still holds but the number of particles is not the number of atoms but the number of molecules. The First Atomic Model: Thomson’s “Plum Pudding” The next great step forward in the understanding of atoms was accomplished by John Thomson. Using a cathode ray tube, Thomson determined that all matter, whatever its source, contains particles of the same kind that are much less massive than the atoms of which they form a part. They are now called electrons, although he originally called them corpuscles. His discovery was the result of an attempt to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out.

Thomson was able to put forward a convincing argument that these rays were composed of particles. Furthermore, these rays seemed to be composed of the same particles, or corpuscles, regardless of what kind of gas carried the electric discharge or what kinds of metals were used as conductors. From these

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ideas he developed the idea that atoms are made of negative electrons embedded in a gel of positive charge (a "plum pudding" model). Rutherford’s Planetary Model Rutherford overturned Thomson's atom model in 1911 with his “gold foil” experiment. Rutherford used alpha particles emitted by a radioactive element as atomic bullets to probe the unseen world of atomic structure. He beamed them through the foil and detected them as flashes of light or scintillations on a screen. The gold foil was only a few hundreds of atoms thick. If the Thomson model of atoms was correct, then the alpha particles should pass through with some deflection (bending) in their paths and they will strike the fluorescent screen behind the foil. What happened was absolutely different. Most alpha particles were observed to pass straight through as if there were no atoms at all (empty space). But a few were scattered at very large angles, even bouncing back toward the source Results can best explained by a model for the atom as a tiny, very dense, positively charged core called a nucleus, around which the light negative electrons, circulate at some distance, much like planets revolving around the Sun. That’s why the Rutherford atomic model has been alternatively called the nuclear atom, or the planetary model of the atom. Bohr’s electron shells A spinning electron should produce EM radiation (light) according to the laws of electromagnetism. Consequently it would lose energy and spiral into nucleus, crashing against it, i.e. atoms should be very unstable and could not exist. Classical physics fails to describe the properties of atoms. To explain this and other experimental facts (too complicated to explain briefly) Bohr proposed a shell model for the atom using the same basic structure as Rutherford, but restricting the location of electrons to definite zones with specific energies and at definite distances from it. The electrons in the shells do not behave as tiny balls spinning around the nucleus but rather as bubbles or onion skins. The first shell holds just up to 2 electrons, the second and third up to 8 electrons. For the sake of clarity they are represented as dots.

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The Atomic Nucleus By the time of Thomson’s discovery of the electron, Goldstein found in a related set of experiments rays formed by streams of positive particles. He supposed they were the remains of atoms as you strip some electrons from them. Later, Rutherford, who was still bombarding matter with alpha particles, found that if he beamed nitrogen particles with them, some hydrogen nuclei (hydrogen is the lightest atom) were formed. He checked this with other particles and concluded that complex nuclei were formed by the aggregation of hydrogen nuclei. He called this particle a proton (protos=first one). Now, what keeps protons stuck together if they strongly repel each other because of their charges being equal? This was not clearly established until 1932 when Chadwick detected a particle with no charge and practically the same mass of the proton. He named this particle a neutron. Neutrons are the cementing stuff in the nucleus. The force between protons and neutrons is called the strong nuclear interaction. It is attractive and some 100 fold stronger that electric repulsion The sub-atomic particles The following chart summarises the essential facts about the particles in the atom

proton neutron electron

relative mass 1 1 1/1836

relative charge +1 0 -1

The nucleus is at the centre of the atom and contains the protons and neutrons. Protons and neutrons are collectively known as nucleons. Virtually all the mass of the atom is concentrated in the nucleus, because the electrons weigh so little. Atomic and Mass Numbers Nr of protons = ATOMIC NUMBER of the atom ( Z ) The atomic number is also given the more descriptive name of proton number. The number of protons will determine the number of electrons necessary to balance the atom’s charge. Nr of protons + Nr of neutrons = MASS NUMBER of the atom ( A ) The mass number is also called the nucleon number. This information can be given simply this way

→

How many protons and neutrons has this atom got? The atomic number counts the number of protons (9); the mass number counts protons + neutrons (19). If there are 9 protons, there must be 10 neutrons for the total to add up to 19. A question arises: what is the meaning of the “F” in the figure? To answer this question we must know what an element is.

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Elements An element is formed by all the nuclei that have the same number of protons. Elements show distinct chemical and physical properties and are given different names and symbols. There are 92 different stable natural elements: the lightest element (Z = 1) is hydrogen and the heaviest (Z= 92) is uranium. Scientists have synthesised some 20 trans-uranic elements but they are unstable and decay to lighter atoms. The following table shows the names and the symbols of the first 20 elements. You must recall these symbols but not the order of the elements. Z 1 5 9 13 17

Name Hydrogen Boron Fluorine Aluminium Chlorine

Symbol H B F Al Cl

Z 2 6 10 14 18

Name Helium Carbon Neon Silicon Argon

Symbol He C Ne Si A

Z 3 7 11 15 19

Name Lithium Nitrogen Sodium Phosphorus Potassium

Symbol Li N Na P K

Z 4 8 12 16 20

Name Beryllium Oxygen Magnesium Sulphur Calcium

The atomic number is tied to the position of the element in the Periodic Table (the system used by chemists to classify atoms) and therefore the number of protons defines what sort of element you are talking about. So if an atom has 8 protons (atomic number = 8), it must be oxygen. If an atom has 12 protons (atomic number = 12), it must be magnesium. Similarly, every chlorine atom (atomic number = 17) has 17 protons; every uranium atom (atomic number = 92) has 92 protons. Atomic properties that depend on the nucleus Three properties of the atoms depend on their nuclei: • Stability: not all possible nuclei exist. As the number of nucleons increase they become more and more unstable and break down into pieces or emit particles until they get to an adequate number of neutrons and protons. This is called radioactivity. Moreover, the ratio between neutrons and protons is restricted to certain values to render the nucleus stable. • Mass: electrons are about 2.000 times less massive than protons and neutrons. Thus, the total mass of the electrons can be neglected and the mass of an atom depends completely on its nucleus. • Number of electrons around the nucleus: atoms are particles with balanced charges and they have as many electrons as protons in the nucleus to achieve neutrality. The Electrons Atoms are electrically “neutral”, and the positive charge of the protons is balanced by the negative charge of the electrons. It follows that in an atom: Nr of electrons = Nr of protons So, if an oxygen atom (atomic number = 8) has 8 protons, it must also have 8 electrons; if a chlorine atom (atomic number = 17) has 17 protons, it must also have 17 electrons.

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Symbol Be O Mg S Ca

The arrangement of the electrons As we have studied before, the electrons are found at considerable distances from the nucleus in a series of energy levels also called shells. Each energy level (shell) can only hold a certain number of electrons. The first level (nearest the nucleus) will only hold 2 electrons, the second up to 8, and the third also seems to be full when it has 8 electrons. These shells or levels can be thought of, as getting progressively further from the nucleus. Electrons will always go into the lowest possible energy level (nearest the nucleus) - provided the level is not full. Atomic properties that depend on the electron cloud Two properties of the atom depend on their electrons: • Size: the volume of an atom does not depend on the nucleus that is a small dot in the centre of it. The electrons around the nucleus and reaching at different distances according to the shells they occupy are responsible of giving the atom a volume, a region in space of its own where other atoms cannot penetrate because of the electron-to-electron repulsion. • Chemical properties: as we will see later, the arrangement of the electrons and in particular their number in the outer shell determines an atom’s behaviour towards other atoms. In a vast number of situations chemistry is just about describing what happens at the atoms’ outer shells. The Periodic Table Classification criterion The periodic table is a chart where scientists have arranged the elements according to their increasing proton or atomic number Z. It was first proposed by Dimitri Mendelejeff around the 1860’s although he actually used the mass number as a classification criterion. He stated that if elements were arranged according to increasing atomic masses, chemical and physical similarities would appear every eight elements. Following this, elements should form eight columns or groups, divided in sub-groups. Each row of eight members was called a period. He had to allow for some oddities that could not be clearly explained by his time (atoms were supposed to be kind of balls with no internal structure and nobody suspected the existence of protons, electrons etc.) so many scientists didn’t quite agree with him. Further advances suggested that the classification should be based on the number of charges of the nuclei rather than their masses (Moseley’s law), and finally the groups were separated in different blocks. Current aspect of the Periodic Table The current shape of the Periodic Table is shown below this paragraph. All elements in the same column belong to the same group and have similar behaviour. These properties vary “smoothly” from group to group. The first two columns are called the reactive metals. The rest of the pink elements are all metals too. They are classified as the transition, poor and rare earth metals. The yellow elements are the non metals. The elements in the last column (usually included in this set) are called the noble gases (because of their low to almost none reactivity they show as noble metals do). The

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metals as they appear as pure substances are all good conductors of electricity and heat, malleable, ductile, sonorous, form high melting point solid oxides with oxygen. All but three of them are solid at room temperature and most are hard and very dense. The non metals are poor conductors of heat and electricity (except for a form of carbon called graphite), half of them are gaseous, one is liquid and the rest are dull, brittle solids. Their oxides, if solid, have low melting points. The pale green zigzagging set between metals are non metals share properties with both main groupings. They are sometimes called the semi-metals. Hydrogen poses a problem because despite having one electron in its outer shell (group I) it is a typical non metal (a non conducting, light gas that bonds to non metals covalently (see later)). Its special behaviour is due to the fact that loosing its outer shell electron as other metals do, will form … a loose proton! So this doesn’t normally happen.

Group and period numbers In the table shown groups are numbered from 1 to 18. In many tables the “tall” groups are numbered from 1A to 7A , the noble gases belonging either to group 8A or 0. The “short” groups are numbered starting at 3B (headed by Sc) to 8B (Warning!! Group 8B has three columns headed by Fe, Co and Ni) and ending with Zn in group 2B (there are historical reasons for this messy numbering). Periods are numbered from 1 (the H period, a very short period indeed with just two members) to 7. Elements past uranium are all man-made elements: all of them are unstable and break down with half lives sometimes as short as milliseconds or less. If you look at the patterns in this table: • •

• • •

The number of electrons in the outer level is the same as the group number. (Except for helium which has only 2 electrons). This pattern extends throughout the Periodic Table for the main or “tall” groups (i.e. not including the transition elements). So if you know that barium is in group 2, it has 2 electrons in its outer level; iodine (group 7) has 7 electrons in its outer level; lead (group 4) has 4 electrons in its outer level. Noble gases have full outer levels. The noble gases are also usually called group 0 - not group 8. The number of the period equals the number of shells totally or partially occupied.

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To work out the electronic arrangement of an atom knowing its group and period •

• •

Look up the atomic number in the Periodic Table - making sure that you choose the right number if two numbers are given. The atomic number will always be the smaller one. This tells you the number of protons, and hence the number of electrons. Arrange the electrons in levels, always filling up an inner level before you go to an outer one. QUESTIONS AND PROBLEMS

1- Name one difference between an atom and an ion and one difference between an atom and a molecule. 2- List the essential ideas of Dalton’s Atomic Theory. 3- Assume as Dalton that all oxygen atoms have the same mass. The ozone molecule is formed by three oxygen atoms (O3) and the oxygen molecule just by two (O2). a- Which is the heaviest particle? b- Are there more particles in 10 g of ozone or in 10 g of oxygen? c- Are there more particles in two litres of ozone or in two litres of oxygen? (at the same pressure and temperature) d- Are 5 litres of ozone heavier or lighter than 5 litres of oxygen?(same P & T) e- (Bonus) How many litres of oxygen have the same mass as 1 litre of ozone? (same P & T) 4- In the paragraph about Rutherford’s model a lithium atom is shown as modelled by him. Draw the lithium atom according to Bohr’s model. 5- Find the number of protons, neutrons and electrons for the following atoms (values of A and Z are given) a- A = 40 Z = 18 b- A = 23 Z =11 c- A = 35 Z = 17 d- A = 16 Z = 8 e- A = 28 Z = 14 f- A = 37 Z = 17 g- A = 39 Z = 19 h- A = 19 Z = 9 6- Write the atoms of exercise (5) in order of increasing mass and in order of increasing nuclear charge. 7- Find the electron configuration (distribution) for the elements of exercise (5) a- Which belong to the same period? b- Which belong to the same group? c- Which are metals, which non metals? Is there any noble gas in the group? d- Which belong to the same element?

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