C2 – Discovering Chemistry
An atom is made up of a nucleus and surrounding electrons. Anything inside the nucleus is called a nucleon. The two large particles in the nucleus are protons and neutrons. Protons have a relative mass of one and relative charge of +one. Neutrons are neutral and have a relative mass of 1. Electrons have a charge of –one and a relative mass of 1/1860.
Electrons orbit the nucleus in energy levels, held in place by the opposite (positive) charge of the nucleus. In the first energy level (shell), there are 2 electrons and 8 in every level thereafter. It is an atom’s goal to have 8 electrons in its outer energy level (as the elements in group 0 do). The arrangement of electrons can be displayed using an electron configuration. For example oxygen has 8 electrons so its electron configuration is 2.6 (requires 2 more electrons to complete its outer level)
We can tell how many electrons are surrounding an atom because it is equal to the atomic number (bottom on the periodic table). This is also equal to the number of protons in the nucleus. The amount of neutrons can be found by subtracting the atomic number from the mass number (top).
Isotopes and Relative Atomic Mass
An isotope is different versions of the same element, with the same amount of protons but a different amount of neutrons. For example carbon-12 has 6 neutrons but carbon-14 has 8 neutrons. Isotopes are usually radioactive, spontaneously decaying over time. Carbon-12 is stable because it does not decay but carbon-14 does decay so it unstable. The mass number has increased by 2 but the atomic number has stayed the same because the amount of protons has not changed. The atomic number defines the element.
Relative atomic mass (Ar) is the average mass of an element’s mass, compared to carbon-12. You work it by using this formula:
Ar = (atomic mass*relative abundance)+(atomic mass*relative abundance)/RA+RA
Relative atomic masses of each element is shown above each element on the periodic table, hence “mass number”.
The Periodic Table of Elements
Dimitri Mendeleev originally ordered the known elements according to physical properties. Elements that are chemically related were placed in vertical groups. Mendeleev left gaps where he predicted new elements would be discovered and he was correct. Since then, we have discovered a whole new group, group 0, the Noble Gasses. We haven’t discovered them until relatively recently because they are very unreactive due to having a full outer shell of electrons.
The differences between now and then include: no gaps, new group (0) and contains more elements
The similarities are: ordered in groups by physical properties and has a similar shape
In ionic bonding, one atom gives 1 or 2 electrons to another atom so they have opposite charges and attract each other, forming an ionic lattice (a 3D cube). For example, sodium chloride, NaCl is an ionic substance. Sodium has given 1 electron to chlorine, leaving Na+…