What are atoms made of?
Atoms are made up of protons,neutrons and electrons. These smaller particles are called subatomic particles.
Summary of Key Ideas
The table below summarises the key ideas from above.
The Atom
The number of electrons in an atom is the same as the number of protons. Thus, - the negative charges cancel out the positive charges - an atom is electrically neutral
Proton Number
The proton number of an atom refers to the number of protons in the atom.
Nucleon Number
The nucleon number of an atom is the total number of protons and neutrons in the atom. It is also called the mass number, as the mass of an atom depends on the number of protons and neutrons in its nucleus. The mass of electrons in the atom is said to be negligible. The nucleon number is represented by the symbol A.
Representing the Proton and Nucleon Numbers
The nucleon and proton numbers can be included when representing an element in symbols.
What are isotopes?
Isotopes are atoms of the same element with the same number of protons and electrons, but different numbers of neutrons. In other words, isotopes have the same proton number but different nucleon numbers.
Properties of Isotopes
Isotopes have the same chemical properties but slightly different physical properties.
How are electrons arranged in an atom?
The electrons in an atom move around the nucleus in regions known as electron shells. Each electron shell - corresponds to a specific energy level - can only hold a certain number of electrons
Electronic Configuration
The arrangement of electrons in an atom can be represented using electronic structure or electronic configuration. The electronic structure of a magnesium atom is shown in the figure below.
What are valence electrons?
The valence shell or outer shell of an atom refers to the shell that is furthest away from the nucleus of the atom. The electrons in the outer shell of an atom are known as valence electrons or outer electrons. The outer electronic structure of an atom shows only the electrons in the outer shell. An example is shown for magnesium below.
Electronic Structures of Some Elements
The electronic structures of the first 20 elements in the periodic table are shown below. In the periodic table, - elements are arranged in order of increasing proton number - the (horizontal) rows of elements are called periods - the (vertical) columns of elements are called groups Elements in the same group have the same number of valence electrons and elements in the same period have the same number of electron shells.
Atomic Structure (Video)
Refer to the video below for a summary of the chapter (starting from 3:45).
How are noble gases different from other elements?
The noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).
Electronic Configuration of a Noble Gas
The figure below shows the outer electronic structures of the noble gases. Noble gases are unreactive as they have fully filled outer shells. Thus, an atom is stable if it achieves the electronic configuration of a noble gas.
How do atoms achieve the electronic configuration of a noble gas?
Atoms of elements (except the noble gases) do not have fully filled outer shells, and hence are reactive. They achieve the stable electronic configuration of a noble gas by - losing outer electrons - gaining outer electrons - sharing outer electrons
Formation of Positive Ions (Cations)
Atoms of metals tend to lose electrons to form cations, attaining the electronic configuration of a noble gas. When an atom loses electrons, it has more protons than electrons. A positive ion or cation is formed. The charge on a cation corresponds to the number of electrons the atom loses.
Formation of Positive Ions (Example)
The diagram below shows the formation of a sodium ion.
Formation of Negative Ions (Anions)
Atoms of non-metals usually have more than four outer electrons. Hence, they tend to gain electrons to form anions. When an atom gains electrons, it has more electrons than protons. A negative ion or anion is formed. The charge on an anion corresponds to the number of electrons the atom gains.
Formation of Negative Ions (Example)
The diagram below shows the formation of an oxide ion.
Common Cations and Their Charges
The table below shows the names and formulae of some cations.
Common Anions and Their Charges
The table below shows the names and formulae of some anions.
Polyatomic Ions
The ions below are polyatomic ions. They are ions composed of two or more covalently bonded atoms.
Arrangement of Electrons in Molecules (Hydrogen)
The formation of a covalent bond can be shown by a ’dot and cross’ diagram. The diagram below shows the formation of a hydrogen molecule.
Arrangement of Electrons in Molecules (Oxygen)
The diagram below shows the formation of an oxygen molecule.
Arrangement of Electrons in Molecules (Nitrogen)
The diagram below shows the formation of a nitrogen molecule.
Arrangement of Electrons in Compounds
When atoms of different elements are joined together by covalent bonding, a covalent compound or molecular compound is formed. Water, methane and carbon dioxide are examples of covalent compounds. The diagram below shows the formation of a water molecule.
Naming a Compound
The table below shows some general rules for naming compounds.
General Rules for Writing Chemical Formulae
The table below shows some general rules for writing chemical formulae of ionic compounds.
Determining the Chemical Formula of an Ionic Compound
All the positive charges must be equal to all the negative charges in an ionic compound. Hence, the formula of an ionic compound can be constructed by balancing the charges on the positive ions with those on the negative ions. Consider the ionic compound calcium chloride. The ions present are Ca2+ and Cl-. The Ca2+ion has two positive charges but the Cl- ion has only a single negative charge. Thus, there must be two Cl- ions to balance out the two positive charges. The chemical formula od calcium chloride is thus CaCl2.
Determining the Chemical Formula of an Ionic Compound (Example)
Below is an example of how the chemical formula of an ionic compound can be determined.
