Week 2 - Atoms, Molecules and Ions Flashcards
Size of an atom (notes)
The diameter of a single atom ranges from 0.1 to 0.5 nanometre or 1 – 5 Å (1 nm = 1 × 10-9 m, 1 Å = 1 × 10-10 m).
Structure of an atom
- Each atom consists of nucleus surrounded by electrons.
- The nucleus contains protons and neutrons but does not contain electrons.
- In a neutral atom, the positive charge of the nucleus is exactly offset by the negative electrons.
- Atomic number of an element is the number of protons in the nucleus of an atom of that element. It is the identity of that element.-
Define Isotope
Atoms of an element having the same atomic number but different atomic mass are called isotopes of that element. That is, atoms of the various isotopes of an element have the same number of protons and electrons but different numbers of neutrons.
For example: Three isotopes of hydrogen (atomic number 1) are known. Each has one proton in the nucleus and one electron. The first isotope (protium) has no neutron with mass number of 1; the second (deuterium) has one neutron and mass number of 2; the third isotope (tritium) with two neutrons has a mass number of 3.
Another example: For example, potassium atoms contain 19 protons and 19 electrons. To make a potassium ion, we remove one electron, leaving 19 protons and only 18 electrons. This gives an ion with a positive one (+1) charge, symbolled K+.
How to calculate neutrons
The mass number minus the atomic number gives the number of neutrons
Finding average atomic mass from differing isotopes
The average mass can be calculated by multiplying the atomic mass of each isotope by the fraction of each isotope present and adding the results. The calculation for copper is:
(62.9298 amu)(0.6909) + (64.9278 amu)(0.3091) = 63.55 amu
Particular arrangement of the periodic table
Each horizontal row in the periodic table is called a period.
The element with similar chemical properties in columns is called groups.
Some groups in the periodic table have their special names, for example, noble gases, alkali metals, alkaline earth metals and halogens
Difference between metals, non-metals and metalloids
The metals are solids at room temperature (mercury is an exception). They have high lustre, and are good conductors of heat and electricity. They are malleable and ductile.
Non-metals are not lustrous, have relatively low melting points and densities, and are generally poor conductors of heat and electricity.
Several elements (boron, silicon, germanium, arsenic, antimony, tellurium, and polonium) are classified as metalloids, and have properties that are intermediate between those of metals and those of non-metals.
What is a compound
A compound is a distinct substance that contains two or more elements chemically combined in a definite proportion by mass.
Compounds, unlike elements, can be decomposed chemically into simpler substances – that is, into simpler compounds and/or elements.
Atoms of the elements in a compound are combined in whole-number ratios, never as fractional parts.
Compounds fall into two general types: molecular and ionic.
Difference between molecule and ion
A molecule is the smallest uncharged individual unit of a compound formed by the union of two or more atoms.
An ion is a positively or negatively charged atom or group of atoms. An ionic compound is held together by attractive forces that exist between positively and negatively charged ions.
Names of a positively and negatively charged ion
A positively charged ion is called a cation, a negatively charged ion is called an anion.
Rules for writing formulas for ionic compounds
Write the formula for the metal ion followed by the formula for the nonmetal ion.
Combine the smallest number of each ion needed to give the charge sum equal to zero.
Write the formula for the compound as the symbol for the metal and nonmetal, each followed by the subscript of the number determined in step 2.
(Watch video on lesson 11 for worked example)
