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4.2 Structure of the Atom The Nucleus is: • Positively charged • Very Dense and at the center portion of the atom – Contains most of the atom’s mass, but . . . – Occupies only a tiny fraction of its volume Figure 3.9: A nuclear atom viewed in cross section. Composition of the Nucleus: • A Proton is a – Positively charged subatomic particle that is found in the nucleus of an atom – Mass of 1.673 x 10-24 grams • Approximately 1800 times that of the electron • A Neutron is a neutral subatomic particle that is found in the nucleus of an atom. – No electrical charge (neutral) – Mass approximately equal to the proton Composition of the Nucleus: • In 1932, the English physicist James Chadwick designed an experiment to show that neutrons exist. • Neutron’s mass is approximately equal to the proton. • Electrons move around the nucleus and are relatively far away from the nucleus • Most of the atom is empty space • The nucleus contains protons & neutrons Protons, Neutrons, and Electrons Symbol Relative Mass Charge Electron e- Proton p+ 1 +1 Neutron no 1 0 1/1800 -1 Atomic number and Mass number The atomic number (“Z”) of an element equals the number of protons in an atom of that element – The atomic number identifies the atom of that elements – Atoms of different elements have different numbers of protons Atomic number and Mass number • The mass number (“A”) of an atom is the sum of the protons and neutrons in the nucleus of that atom. • Mass number identifies the particular isotope • In an atom, electrons equal the number of protons • Atoms are electrically neutral Isotopes of Atoms • Isotopes are atoms of the same element with different mass numbers • Isotopes have the same atomic number but different mass numbers because they have different numbers of neutrons Two isotopes of sodium. How to calculate the number of neutrons in an atom? • Number of neutrons= mass number – atomic number • Nuclear Symbol: See next slide • Hyphen Notation: Bromine- 80 Nuclear Symbol Mass Number 65 29 Cu Atomic Number (Z) Number of Protons 29 Electrons Protons + Neutrons 36 Neutrons 65 29 Cu Protons Remember!! Atoms are neutral!! Practice • Write the nuclear symbol for bromine-80 – How many protons, electrons, and neutrons are in this atom? Practice • Write the hyphen notation for the element that has 6 protons and 7 neutrons – How many electrons does this atom have? – Write the nuclear symbol for this atom