Chapter 4 Atoms 4.1 Atoms 4.2 Structure of atoms 4.3 Atomic number and mass number 4.4 Isotopes 4.5 Relative masses of atoms 4.6 Arrangement of electrons CONTENTS OF CHAPTER 4

4.1 ATOMS WHAT ARE ATOMS? Everything is made up of particles. There are in fact different types of particles that make up matter. The basic type is atom. An ATOM is the smallest part of an element which has the properties of that element. 4.1 ATOMS

copper atoms enlarged many, many times a copper wire copper atoms enlarged many, many times Figure 4.2 A copper wire consists of trillions of copper atoms. (1 trillion = 1 000 000 000 000 , i.e. 1  1012) 4.1 ATOMS

SIZE AND MASS OF AN ATOM It is difficult to imagine anything as tiny as an atom. You may get a better idea of its very small size by looking at Figure 4.4. Figure 4.4 The 10 small spheres as shown in a line would be 5 cm long, but 140 000 000 hydrogen atoms in a line would only be 1 cm long! 4.1 ATOMS

ELEMENTS AND ATOMS An element contains only one kind of atoms. For example, silver consists of silver atoms only; gold consists of gold atoms only. a silver wire silver atoms enlarged many, many times a gold wire gold atoms Figure 4.5 Silver consists of silver atoms; gold consists of gold atoms. 4.1 ATOMS

An ELEMENT is a substance that is made up of only one kind of atoms. Different elements have different properties because they consist of different kinds of atoms. Symbols for atoms Chemical symbols of elements are also the atomic symbols for their atoms. 4.1 ATOMS

(d) The element copper or a copper atom. (b) Br (c) N (d) The element copper or a copper atom. 4.1 ATOMS

4.2 STRUCTURE OF ATOMS WHAT ARE ATOMS MADE OF? Atoms are made up of three fundamental sub-atomic particles — protons, neutrons and electrons. ATOMS are made up of protons, neutrons and electrons. The protons (positively charged) and neutrons (neutral) are concentrated in the very tiny nucleus. The electrons (negatively charged) move around the nucleus. 4.2 STRUCTURE OF ATOMS

negatively charged electrons moving around the nucleus very tiny positively charged nucleus (containing protons and neutrons) A simple picture of the structure of an atom. 4.2 STRUCTURE OF ATOMS

MORE ABOUT PROTONS, NEUTRONS AND ELECTRONS Table 4.1 Data on the three fundamental sub-atomic particles. 4.2 STRUCTURE OF ATOMS

BUILDING UP DIFFERENT ATOMS FROM PROTONS, NEUTRONS AND ELECTRONS Different atoms have different numbers of protons, neutrons and electrons. neutron electron proton nucleus Figure 4.10 A diagrammatic representation of a helium atom. It has 2 protons, 2 neutrons and 2 electrons. 4.2 STRUCTURE OF ATOMS

4.2 STRUCTURE OF ATOMS

ATOMS ARE ELECTRICALLY NEUTRAL The commonest type of hydrogen atom. ATOMS ARE ELECTRICALLY NEUTRAL An atom is electrically neutral. This is because in an atom, the number of protons is equal to the number of electrons. 4.2 STRUCTURE OF ATOMS

(a) 91 electrons. Number of neutrons cannot be predicted. (b) It is not an atom. The numbers of protons and electrons are not equal. 4.2 STRUCTURE OF ATOMS

4.3 ATOMIC NUMBER AND MASS NUMBER The ATOMIC NUMBER of an atom is the number of protons in the atom. A4.4 A magnesium atom would be changed to a chlorine atom. 4.3 ATOMIC NUMBER AND MASS NUMBER

MASS NUMBER The MASS NUMBER of an atom is the sum of the number of protons and neutrons in the atom. EXAMPLE mass number Cl 35 mass number A atomic symbol atomic number Z atomic number 17 4.3 ATOMIC NUMBER AND MASS NUMBER

A4.5 silver, silver, silver A4.6 (a) Aluminium (b) 13Al (c) (i) 13 (ii) 13 (iii) 27 - 13 = 14 27 4.3 ATOMIC NUMBER AND MASS NUMBER

4.4 ISOTOPES WHAT ARE ISOTOPES? ISOTOPES are different atoms of the same element, with the same number of protons (and electrons) but different numbers of neutrons. For example, hydrogen has 3 isotopes: 1H, 1H and 1H. 1 2 3 4.4 ISOTOPES

RELATIVE ABUNDANCE OF ISOTOPES electron proton neutron Figure 4.12 The three isotopes of hydrogen. RELATIVE ABUNDANCE OF ISOTOPES Most elements consist of more than one isotope. 4.4 ISOTOPES

Isotopes of some elements in Nature. Table 4.4 Isotopes of some elements in Nature. 4.4 ISOTOPES

COMPARING PROPERTIES OF DIFFERENT ISOTOPES (a) 3 (b) 8O (or 16O, or oxygen -16) 16 COMPARING PROPERTIES OF DIFFERENT ISOTOPES Isotopes of the same element have the same chemical properties. 4.4 ISOTOPES

Relative isotopic mass  mass number 4.5 RELATIVE MASSES OF ATOMS RELATIVE ISOTOPIC MASS The carbon-12 scale The carbon-12 isotope has been chosen as the reference standard for comparing masses of atoms. On the 12C = 12.000 00 scale, Relative isotopic mass  mass number 4.5 RELATIVE MASSES OF ATOMS

RELATIVE ATOMIC MASS A4.8 (i) 37 (ii) 35 (iii) 4 (iv) 238 (v) We cannot tell from the given data. (The mass number is not given.) RELATIVE ATOMIC MASS The relative atomic mass of an element is determined by: (1) the relative isotopic masses and (2) the relative abundance of the natural isotopes present in the element. 4.5 RELATIVE MASSES OF ATOMS

(a) Relative atomic mass of sodium = mass number of the only type of The *RELATIVE ATOMIC MASS of an element is the weighted average of the relative isotopic masses of its natural isotopes on the 12C = 12.000 00 scale. A4.9 (a) Relative atomic mass of sodium = mass number of the only type of sodium atom = 23 (b) Relative atomic mass of neon = = 20.2 4.5 RELATIVE MASSES OF ATOMS

4.6 ARRANGEMENT OF ELECTRONS ELECTRONIC ARRANGEMENT Electrons in an atom are arranged in shells. The distribution of electrons in the various shells is called ELECTRONIC ARRANGEMENT (or electronic configuration). Arrangement of electrons in a sodium atom. 4.6 ARRANGEMENT OF ELECTRONS

Rules for finding electronic arrangement (1) The atomic number of the element is first found. This is equal to the number of protons, and hence the number of electrons present in an atom of the element. (2) Electrons go into the shells one by one, starting from the innermost shell. WAYS OF REPRESENTING ELECTRONIC ARRANGEMENT Electronic arrangement by numbering Electronic arrangement may be shown by numbering. 4.6 ARRANGEMENT OF ELECTRONS

2, 8, 1 Electronic arrangement by diagram 2, 8, 1 Number of 1st 2nd 3rd electrons in: shell shell shell Figure 4.16 Showing the electronic arrangement of a sodium atom by numbering. Electronic arrangement by diagram Besides numbering, electronic arrangement can also be represented by an electron diagram. 4.6 ARRANGEMENT OF ELECTRONS

ELECTRONIC ARRANGEMENTS OF THE FIRST 20 ELEMENTS Na Figure 4.17 The electron diagram of a sodium atom. ELECTRONIC ARRANGEMENTS OF THE FIRST 20 ELEMENTS 4.6 ARRANGEMENT OF ELECTRONS

4.6 ARRANGEMENT OF ELECTRONS

A4.10 (a) 17 (b) (i) 2, 8, 7 (ii) Cl 4.6 ARRANGEMENT OF ELECTRONS

SUMMARY 1. An atom is the smallest part of an element which has the properties of that element. 2. An element is a substance that is made up of only one kind of atoms. Different elements have different properties because they consist of different kinds of atoms. SUMMARY

(d) Electrons move around the nucleus in shells. 3. (a) An atom consists of three types of sub-atomic particles – protons, neutrons and electrons. (b) (c) An atom has an extremely small centre called nucleus. The protons and neutrons are in the nucleus. (d) Electrons move around the nucleus in shells. (e) An atom is electrically neutral. SUMMARY

4. Atomic number of an atom = number of protons in the atom Atomic number of an element = number of protons in an atom of the element 5. Mass number of an atom = number of protons + number of neutrons in the atom 6. Full atomic symbol mass number atomic number Z A atomic symbol Cl 17 35 EXAMPLE SUMMARY

8. Relative isotopic mass  mass number 7. Isotopes are different atoms of the same element, with the same number of protons (and electrons) but different numbers of neutrons. Different isotopes of the same element have the same chemical properties but slightly different physical properties. 8. Relative isotopic mass  mass number 9. Relative atomic mass = weighted average of the relative of an element isotopic masses of its natural Isotopes on the 12C = 12.000 00 scale SUMMARY

10. The electronic arrangement of an atom is the distribution of electrons in the various shells of the atom. (See Table 4.6 again.) SUMMARY