Unit 3: Elements, Compounds, and Reactions
1. atom – the smallest particle of an element that still has the same chemical
properties of the element. Atom comes from the Greek word atomos which means
“cannot be divided” or “indivisible”
2. electron – a particle with a single negative charge outside the nucleus of an atom.
3. proton – Inside the nucleus of an atom that has a positive charge.
4. neutron – a neutral particle in the nucleus of an atom.
5. nucleus – the center of an atom, which contains most of the atom’s mass.
6. valence electrons – the electrons in the outermost level involved in chemical
bonding.
7. electron cloud – an area around an atomic nucleus where an electron is most likely
to be found.
8. element – pure substance that cannot be broken down into any simpler substances.
9. atomic number – the number of protons in an atom of an element
10. isotopes – atoms of the same element that have different numbers of neutrons.
11. mass number – the sum of the number of protons and neutrons in an atom
12. periodic table – a chart that shows the elements in order of increasing atomic
number.
13. groups or families – the periodic table is arranged in vertical columns numbered
1-18.
14. periods – the periodic table is arranged in horizontal rows numbered 1-7.
15. compound – a substance made of two or more elements that are chemically
joined in a specific combination.
16. ion – an atom that is no longer electrically neutral because it has lost or gained
electrons.
17. ionic bond - an attraction that holds oppositely charged ions close together.
18. covalent bond - is a chemical bond that forms when atoms share valence
electrons
19. exothermic reaction – a chemical reaction that releases energy.
20. endothermic reaction - is a chemical reaction that absorbs energy.
21. catalyst – is a substance that speeds up a chemical reaction.