to describe the composition of the atomic nucleus on the basis of mass and atomic numbers.
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Before you start, answer the questions.
What is matter made of?
What is the smallest indivisible particle of matter?
Structure of an atom
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An atom is a piece of matter that uniquely defines an element. Atoms have very complex structurestructurestructure.
Almost all of the mass of an atom is concentrated within a tiny and extremely dense nucleusnucleusnucleus which carries a positive electric charge. The value of this charge is known as the atomic numberatomic numbernumber of the element.
The empty space around the nucleus is occupied by electrons carrying negative electric charge of 1,6022 · 10Indeks górny -19-19 C. This charge is called an elementary charge. The mass of an electron is extremely small and is 9,109390 · 10Indeks górny -31-31 kg. Electrons behave as quantum particles and that means that its location at any moment cannot be determined. Instead of electrons location we speak of the probabilityprobabilityprobability of founding an electron in an atom space. The atom radiusatom radiusatom radius is a measure of the size of the atom and usually is the mean distance from the centre of nucleusnucleusnucleus to the boundary in which the electron can be found with 95% probability. Atomic radii are typically 30‑300 pm.
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Nucleus
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The nucleusnucleusnucleus is composed of two types of particles: protons and neutrons. Protons are positively charged and their charge is exactly the same as the value of the electron charge, but of opposite sign. In every electrically neutral atom the number of protons and electrons is equal.
Mass of proton is 1,6726 · 10Indeks górny -27-27 kg. Proton is 1837 times heavier than electron.
Neutron has no charge. Its mass is almost the same as that of proton: 1,674929 · 10Indeks górny -27-27 kg. Most nuclei contain roughly equal numbers of neutrons and protons.
Quarks
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A quarkquarkquark is an elementary particle, that is the smallest indivisible particle of matter. Protons and neutrons are composed of quarks. There are six quarks and they are considered in terms of three pairs: up/down, charm/strange, and top/bottom. Protons are made of two up and one down quark. The neutron is made of two down and one up quark. The up quarks have a charge of of the elementary charge, and the down quarks have a charge of of the elementary charge.
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Task 1
List the parts of the atom
List the parts of the atom
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Label the parts of the atom shown in the picture below.
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Atomic Number (Z)
The number of protons in the nucleusnucleusnucleus is called the atomic numberatomic numberatomic number and is denoted by the symbol Z. The Z protons within the nucleus correspond to the Z electrons in outer part of an atom.
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Mass Number (A)
The sum of the number of protons and the neutrons in the nucleusnucleusnucleus is called the mass numbermass numbermass number. It is represented by the symbol A.
where: Z - is the number of protons, N - the number of neutrons.
If an element is denoted by X, then the following notation is in use:
If the mass numbermass numbermass number and atomic numberatomic numberatomic number of an element is known, it is very easy to calculate the number of neutrons in nuclei.
In some notation only mass number occurs.
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Different styles of the atomic structurestructurestructure notation for oxygen: (from left) the periodic table; mass numbermass numbermass number and atomic numberatomic numberatomic number are shown; only mass number is shown.
Isotopes
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Nuclides are atomic nuclei with a certain atomic number Z and the mass number A. Two nuclides with the same atomic numberatomic numberatomic number but different mass numbers are known as isotopes.
More than 300 nuclides are found naturally on Earth.
Some examples of isotopes.
The simplest element - hydrogen – has two heavy isotopes, called deuterium and tritium. They have own symbols: D (deuterium) and T (tritium).
Carbon occurs mainly as . IsotopeisotopeIsotope is a little more than 1%. All other carbon isotopes are radioactive.
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Task 2
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Answer the following questions.
1. What part of the atom has no charge? 2. What is the relation between the number of protons and electrons in an atom? 3. What are the constituents of proton and neutron? 4. What determines the mass numbermass numbermass number?
Exercise 1
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Do following steps and determine to which element the following atom belongs.
1. Draw five protons in the nucleusnucleusnucleus of the atom. Mark their charge with the appropriate symbol. 2. Draw six neutrons in the nucleus of the atom. 3. Draw two electrons in the first energy level. Mark their charge with the appropriate symbol. 4. Draw three electrons in the second energy level. Mark their charge with the appropriate symbol. 5. What element is represented by the diagram?
Boron
Remember
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A nucleus consists of protons and neutrons.
Protons and neutrons are made up of quarks.
Every nuclidenuclidenuclide is characterised by mass number and atomic numberatomic numberatomic number.
Two nuclides with the same atomic number but different mass numbers are known as isotopes.
Exercises
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Exercise 2
Exercise 3
Answer the question and write them to your notebook.
1. What does the atomic number represent? 2. What does the mass number represent? 3. How can be the number of neutrons in an atom figured out?
1. Protons and electrons. 2. The sum of protons and neutrons. 3. The difference of mass number and atomic number.
Exercise 4
Describe in English the structurestructurestructure of an atom.