8th Grade Chapter 16 Notes
Section 1: Why do atoms combine?
1. The nucleus, containing protons and neutrons, is at the center of an atom and is
surrounded by the electron cloud an area of space around the nucleus where electrons
travel.
2. Electrons have a negative charge and do not travel in well defined orbits.
3. Each element has a different atomic structure consisting of a particular number of
protons, neutrons, and electrons.
4. The number and arrangement of electrons in the electron cloud determines the physical
and chemical properties of the element.
5. Electrons are arranged in different energy levels at different distances from the nucleus.
6. The farther an energy level is from the nucleus, the more electrons it can hold.
7. Electrons in the level closest to the nucleus have the lowest amount of energy; electrons
farthest from the nucleus have the highest amount of energy.
8. Data from the periodic table can be used to understand energy levels.
9. The atomic number is the same as the number of protons (or electrons) in an
electrically neutral atom. (no charge)
10. The number of electrons in an element’s outermost energy level increases from left to
right across a period.
11. The first energy level is complete with two electrons, and the first period has two
elements.
12. The last element in each of the other periods has eight electrons in its outer energy
level.
13. Each column in the periodic table contains one element family or group with similar
chemical properties.
14. The noble gases in Group 18 do not combine easily with other elements because their
energy levels are stable.
15. The halogens have 7 electrons in their outer energy levels; reactivity (ability to react
with other substances) decreases down the group.
16. The alkali metals in Group 1 have one electron in their outer energy level; reactivity of
the alkali metals increases down the group. (more reactive elements at the bottom)
17. An electron dot diagram is the symbol for the element surrounded by as many dots as
there are electrons in its outer energy level.
18. Dots are written on four sides of the element symbol. One dot represents a single
electron.
19. Paired electrons are represented by two dots.
20. A chemical bond is the force that holds two atoms together.
21. Electron dot diagrams can be used to show how atoms bond with each other.
22. Atoms bond with other atoms so that each atom has a stable energy level.  this is why
atoms bond!!!
Section 2: How Elements Bond
23.Atoms form bonds by losing electrons, by gaining electrons, by pooling electrons, or
by sharing electrons.
24.An atom that is no longer neutral because it has lost or gained an electron is called an
ion.
25.An ionic bond forms when positive and negative ions attract each other.
26.Sodium chloride (NaCl) is formed from sodium ions and chloride ions.
8th Grade Chapter 16 Notes
27.Two or more elements that are chemically bonded form a compound.
28.Some atoms can gain or lose more than one electron when they form ions.
29.Metallic bonds form when metal atoms share their pooled electrons.
30.A covalent bond forms between atoms that share electrons, no electrons are gained or
lost.
31.Atoms sharing electrons form a neutral particle called a molecule.
32.Covalently bonded compounds are called molecular compounds.
33. Two pairs of electrons are involved in a double bond.
34. A polar bond occurs when electrons are unevenly shared. (like water – electrons spend
more time around oxygen atom than hydrogen atom)
35. Polar molecules such as water have two opposite ends, or poles, like a magnet.
36. Nonpolar bonds form between atoms of the same element and the electrons spend
equal time around each atom.
37. Symbols are used to represent atoms and compounds.
38. Elements are represented by a one-, two-, or three-letter symbol.
39. Compounds are described using element symbols and numbers.
40. In the formula H2, the small 2 after the H is called a subscript and indicates the number
of atoms of hydrogen that are in the molecule.
41. A chemical formula is a combination of chemical symbols and numbers.
42. It tells which elements are present and how many atoms of each element are present.
43. No subscript means that one atom of that element is present.
44. Know the following chemical formulas:
• NaCl – sodium chloride or table salt
• H2O – water
• CO2 – carbon dioxide
• O2 – oxygen gas
• N2 – nitrogen gas
•
C6H12O6 – sugar or glucose