How heavy is an atom?

It is possible to determine the mass of a single atom in kilograms. But to do this, you would need special instruments and the values you would get would be very clumsy and difficult to work with. The mass of a carbon atom, for example, is about 1, 99 × 10−26 kg, while the mass of an atom of hydrogen is about 1, 67 × 10−27 kg. Looking at these very small numbers makes it difficult to compare how much bigger the mass of one atom is when compared to another. To make the situation simpler, scientists use a different unit of mass when they are describing the mass of an atom. This unit is called the atomic mass unit (amu). We can abbreviate (shorten) this unit to just u. Scientists use the carbon standard to determine amu. The carbon standard gives carbon an atomic mass of 12, 0 u. Compared to carbon the mass of a hydrogen atom will be 1 u. Atomic mass units are therefore not giving us the actual mass of an atom, but rather its mass relative to the mass of one (carefully chosen) atom in the periodic table. In other words it is only a number in comparison to another number. The atomic masses of some elements are shown in table 4.1. Element Atomic mass (u) Carbon (C) 12, 0 Nitrogen (N) 14, 0 Bromine (Br) 79, 9 Magnesium (Mg) 24, 3 Potassium (K) 39, 1 Calcium (Ca) 40, 1 Oxygen (O) 16, 0 Table 4.1: The atomic mass number of some of the elements. The actual value of 1 atomic mass unit is 1, 67 × 10−24 g or 1, 67 × 10−27 kg. An atom is therefore very very small. 68 Chemistry: Matter and Materials CHAPTER 4. THE ATOM 4.3 Rutherford’s alpha-particle scattering experiment ESAAW Radioactive elements emit different types of particles. Some of these are positively charged alpha (α) particles. Rutherford wanted to find out where the positive charge in an atom is. He carried out a series of experiments where he bombarded sheets of gold foil with alpha particles (since these would be repelled by the positive nucleus). A simplified diagram of his experiment is shown in figure 4.5. radioactive substance α particles C C B B A A Zinc Sulfide particles nucleus of gold atom (a) (b) gold sheet Figure 4.5: Rutherford’s gold foil experiment. Figure (a) shows the path of the α particles after they hit the gold sheet. Figure (b) shows the arrangement of atoms in the gold sheets and the path of the α particles in relation to this. See video: VPald at www.everythingscience.co.za Rutherford set up his experiment so that a beam of alpha particles was directed at the gold sheets. Behind the gold sheets was a screen made of zinc sulphide. This screen allowed Rutherford to see where the alpha particles were landing. Rutherford knew that the electrons in the gold atoms would not really affect the path of the alpha particles, because the mass of an electron is so much smaller than that of a proton. He reasoned that the positively charged protons would be the ones to repel the positively charged alpha particles and alter their path. If Thomson’s model of the atom was correct then Rutherford would have observed mostly path C in figure 4.5. (C represents alpha particles that are reflected by the positive nucleus). What he found instead was that most of the alpha particles passed through the foil undisturbed and could be detected on the screen directly behind the foil (path A). Some of the particles ended up being slightly deflected onto other parts of the screen (path B). The fact that most particles passed straight through suggested that the positive charge was concentrated in one part of the atom only.

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Ammeter ESAFF An ammeter is an instrument used to measure the rate of flow of electric current in a circuit. Since one is interested in measuring the current flowing through a circuit component, the ammeter must be connected in series with the measured circuit component. I A Ammeter Activity: Constructing circuits Construct circuits to measure the emf and the terminal potential difference for a battery. Some common elements (components) which can be found in electrical circuits include light bulbs, batteries, connecting leads, switches, resistors, voltmeters and ammeters. You have learnt about many of these already. Below is a table with Physics: Electricity and Magnetism 283 17.2 CHAPTER 17. ELECTRIC CIRCUITS the items and their symbols: Component Symbol Usage light bulb glows when charge moves through it battery provides energy for charge to move switch allows a circuit to be open or closed resistor resists the flow of charge OR voltmeter V measures potential difference ammeter A measures current in a circuit connecting lead connects circuit elements together Experiment with different combinations of components in the circuits. The table below summarises the use of each measuring instrument that we discussed and the way it should be connected to a circuit component. Tip A battery does not produce the same amount of current no matter what is connected to it. While the voltage produced by a battery is constant, the amount of current supplied depends on what is in the circuit. Instrument Measured Quantity Proper Connection Voltmeter Voltage In Parallel Ammeter Current In Series Activity: Using meters If possible, connect meters in circuits to get used to the use of meters to measure electrical quantities. If the meters have more than one scale, always connect to the largest scale first so that the meter will not be damaged by having to measure values that exceed its limits. 284 Physics: Electricity and Magnetism CHAPTER 17. ELECTRIC CIRCUITS 17.2 Example 1: Calculating current I QUESTION An amount of charge equal to 45 C moves past a point in a circuit in 1 second, what is the current in the circuit? SOLUTION Step 1 : Analyse the question We are given an amount of charge and a time and asked to calculate the current. We know that current is the rate at which charge moves past a fixed point in a circuit so we have all the information we need. We have quantities in the correct units already. Step 2 : Apply the principles We know that: I = Q ∆t I = 45 C 1 s I = 45 C · s −1 I = 45 A Step 3 : Quote the final result The current is 45 A. Example 2: Calculating current II QUESTION An amount of charge equal to 53 C moves past a fixed point in a circuit in 2 Physics: Electricity and Magnetism 285 17.2 CHAPTER 17. ELECTRIC CIRCUITS seconds, what is the current in the circuit? SOLUTION Step 1 : Analyse the question We are given an amount of charge and a time and asked to calculate the current. We know that current is the rate at which charge moves past a fixed point in a circuit so we have all the information we need. We have quantities in the correct units already. Step 2 : Apply the principles We know that: I = Q ∆t I = 53 C 2 s I = 26, 5 C · s −1 I = 26, 5 A Step 3 : Quote the final result The current is 26,5 A.

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The Proton FACT Scientists believe that the electron can be treated as a point particle or elementary particle meaning that it cannot be broken down into anything smaller. The electron carries one unit of negative electric charge (i.e. −1, 6×10−19 C, C is Coulombs). Each proton carries one unit of positive electric charge (i.e. +1, 6 × 10−19 C). Since we know that atoms are electrically neutral, i.e. do not carry any extra charge, then the number of protons in an atom has to be the same as the number of electrons to balance out the positive and negative charge to zero. The total positive charge of a nucleus is equal to the number of protons in the nucleus. The proton is much heavier than the electron (10 000 times heavier!) and has a mass of 1, 6726 × 10−27 kg. When we talk about the atomic mass of an atom, we are mostly referring to the combined mass of the protons and neutrons, i.e. the nucleons. The Neutron The neutron is electrically neutral i.e. it carries no charge at all. Like the proton, it is much heavier than the electron and its mass is 1, 6749 × 10−27 kg (slightly heavier than the proton). Chemistry: Matter and Materials 71 4.4 CHAPTER 4. THE ATOM proton neutron electron Mass (kg) 1, 6726 × 10−27 1, 6749 × 10−27 9, 11 × 10−31 Units of charge +1 0 −1 Charge (C) 1, 6 × 10−19 0 −1, 6 × 10−19 Table 4.2: Summary of the particles inside the atom Atomic number and atomic mass number ESABC The chemical properties of an element are determined by the charge of its nucleus, i.e. by the number of protons. This number is called the atomic number and is denoted by the letter Z. DEFINITION: Atomic number (Z) The number of protons in an atom You can find the atomic number on the periodic table (see periodic table at front of book). The atomic number is an integer and ranges from 1 to about 118.

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When we talk about current we talk about how much charge moves past a fixed point in circuit in one second. Think of charges being pushed around the circuit by the battery, there are charges in the wires but unless there is a battery they won’t move. See video: VPfva at www.everythingscience.co.za When one charge moves the charges next to it also move. They keep their spacing as if you had a tube of marbles like in this picture or looked at a train and its carriages. marble marble If you push one marble into the tube one must come out the other side, if a train locomotive moves all the carriages move immediately because they are connected. This is similar to charges in the wires of a circuit. The idea is that if a battery started to drive charge in a circuit all the charges start moving instantaneously.

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How heavy is an atom?

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