1. Abundant elements

2. Hydrogen atoms make up about 90% of the total mass of the universe. they make up about 1% of the Earth’s crust most are combined with oxygen atoms in the form of H 2 O

3. Atoms in the elements of living things are very different from what is in the earth’s crust Living things contain at least 25 types of atoms – primarily oxygen, carbon, hydrogen, and nitrogen

4. Types of atoms in approximately the top 100 kilometers of Earth’s crust. Oxygen atoms account for more than half your body’s mass.

5. Isotopes Isotopes are atoms of the same element that have a different number of neutrons. Some elements have many isotopes, while other elements have just a few.

6. Protons and Neutrons are larger than electrons, most of the mass of the atom is found in the nucleus. The electron cloud consists of electrons, which have a very small mass, and empty space. *Remember*

7. A particular isotope is designated by the name of the element and the total number of its protons and neutrons.

8. You can find the number of neutrons in a particular isotope by subtracting the atomic number from the atomic mass number.

9. For example, chlorine-35 indicates the isotope of chlorine that has 18 neutrons. Chlorine-37 has 20 neutrons.

10. Every atom of an element has the same atomic number because it has the same number of protons. The atomic mass number is the average mass of all the isotopes. 17 Cl Chlorine 35.45

11. Ions An atom has an equal number of electrons and protons. Since each electron has one negative charge and each proton has one positive charge, atoms have no overall electrical charge.

12. Ions An ion is formed when an atom loses or gains one or more electrons. Because the number of electrons in an ion is different from the number of protons, an ion does have an overall electric charge.

13. Positive Ions The left side of the illustration represents a sodium (Na) atom. Its nucleus contains 11 protons and some neutrons. The electron cloud surrounding the nucleus consists of 11 electrons, so the positive and negative charges are balanced.

14. Positive Ions If the atom loses one electron, the charges are no longer balanced. There is one more proton than there are electrons. This sodium Ion has a positive charge (Na + )

15. Positive Ions If an atom loses more than one electron…. The overall charge is indicated in the superscript ex: Ca 2+ = a calcium atom that lost 2 electrons Al 3+ = an aluminum atom that lost 3 electrons

16. Negative Ions The nucleus of a chlorine atom contains 17 protons and some neutrons. The electron cloud has 17 electrons, so the atom has no overall charge.

17. Negative Ions When an electron is added to the chlorine atom, a negatively charged ion is formed. The extra electron increases the repulsion within the cloud, causing it to expand.

18. Negative Ions Negative ions are represented by placing a minus sign as a superscript to the element’s symbol The negative chloride ion in the example, therefore, would be written as Cl –.

19. Negative Ions If an ion has gained more than one electron, the number of added electrons is indicated by a number in front of the minus sign. ex: O 2- = Oxygen that gained 2 electrons