SCNC1112 Chap.3 Atoms, Electrons and Chemical Reactivity
Atoms, Electrons and Chemical Reactivity
Module 1 opens with a claim that carries the whole of the chemistry in this course: the arrangement of electrons around a nucleus determines both the energy an atom or compound holds and how readily it reacts. Everything that follows, from why water dissolves salt to why oxygen is worth breathing, is that claim worked out at a different scale. The vocabulary is small and has to be exact.
The atomic number is the count of protons and fixes which element an atom is. The mass number counts protons and neutrons together, so subtracting one from the other gives the neutron count and distinguishes isotopes. Isotopes of one element behave almost identically in chemistry and differ in mass and in nuclear stability, which is precisely what makes radiometric dating possible later in the course.
Valence electrons are the electrons in the outermost occupied shell, and they are the ones transferred or shared when anything reacts. Bonding then has three common routes, and each leaves a fingerprint in the properties of the material. Electrons transferred from a metal to a non-metal leave charged ions attracting one another, giving a high melting point, brittleness and conduction once molten or dissolved.
Electrons shared between non-metal atoms give discrete molecules with lower melting points and poor conduction. Electrons pooled across a lattice of positive cores give a metal, which conducts and bends rather than shattering. Real bonds lie on a continuum between these, so a question asking which description fits best is asking you to place a substance rather than to recall a sorting rule.
The case that matters most for later chapters is unequal sharing. When two different atoms share a pair of electrons, the pair sits closer to whichever atom attracts electrons more strongly, and the bond becomes polar: slightly negative at one end and slightly positive at the other while the molecule stays neutral overall.
Water is built from two such bonds arranged at an angle, and nearly everything the next chapter says about water follows from that geometry. Finally, a chemical reaction is defined as a re-configuration of electronic structure. Atoms are conserved, so an equation must balance before any other argument can be made about it.
Breaking bonds costs energy and forming bonds releases it, and the difference between the two decides whether a reaction gives out energy or takes it in. Calling a substance a fuel is therefore a statement about the relative strength of the bonds broken and formed, not about the substance being special.
What this chapter covers
- 01
Atomic Number, Mass Number and Isotopes
- 02
Valence Electrons and Filled Shells
- 03
Ionic, Covalent and Metallic Bonding
- 04
Polarity From Unequal Sharing
- 05
Reactions as Bonds Broken and Formed
Deciding what a melting point and a conductivity test reveal
- 2Take the high melting point first. A large amount of thermal energy is needed to separate the particles, which points to a lattice of ions attracting one another rather than to discrete molecules.
- 2Use the conductivity. Conduction requires charged particles that can move. A molten ionic solid supplies free ions; a molecular liquid supplies nothing charged. The first solid is ionic and the second is covalent and molecular.
- 2Propose a further test with its expected outcome. Dissolve each in water and test the solution: the ionic solid should give a conducting solution and the molecular solid should not. A prediction that could fail is what makes it a test.
Key terms
- Atomic Number
- The number of protons in a nucleus, which fixes the identity of the element and cannot change without changing the element.
- Isotope
- An atom of a given element with a different number of neutrons, so its chemistry is nearly unchanged while its mass and nuclear stability are not.
- Valence Electron
- An electron in the outermost occupied shell of an atom, and the electron that is transferred or shared when a bond forms.
- Ionic Bond
- The attraction between oppositely charged ions formed when electrons transfer from one atom to another, typically between a metal and a non-metal.
- Polar Bond
- A covalent bond in which the shared electrons sit closer to one atom, leaving one end slightly negative and the other slightly positive.
Atoms, Electrons and Chemical Reactivity FAQ
Why are elements in the same column of the periodic table alike?
Because they have the same number of electrons in their outermost shell, and those electrons are what take part in bonding. Mass and size vary down a column while the outer arrangement repeats, which is why sodium and potassium behave similarly despite their different masses. The table is a map of outer-shell arrangements.
What does it mean to say a reaction releases energy?
It means the bonds formed in the products are collectively stronger than the bonds broken in the reactants, so more energy comes out in forming than went in to breaking. Nothing has been created: the difference was already stored in the arrangement of electrons, which is what makes a fuel a fuel.
Exam move
Practise reading a substance backwards from its properties: given a melting point, a conductivity result and a state at room temperature, name the bonding and say which single observation was decisive. Then balance three equations and, for each, name one bond broken and one bond formed so the energy statement has something concrete behind it.
