The Atom & the Periodic Table

Atom - All matter is made of tiny particles called atoms - An element contains only one type of atom - An atom is the smallest part of an element that still retains the properties of that element

- Inside every atom there are 3 different sub-atomic particles: - protons - neutrons - electrons - These sub-atomic particles are so tiny their mass cannot be measured in grams so the atomic mass unit (a.m.u.) is used

Particle: Mass : Charge: Location: Proton 1 +1 nucleus Neutron 1 0 nucleus Electron 1/1840 -1 in shells orbiting nucleus

- The atoms of elements have different amounts of each sub-atomic particle in them - This is what makes each element different from the rest

Element: Proton: Neutron: Electron: He 2 2 2 O 8 8 8 Ca 20 20 20 Cu 29 35 29 Hg 80 121 80 - So an O atom is 4 times bigger than a He atom, but a Hg atom is 40 times bigger

- The atomic number is the number of protons in the nucleus of an atom - The mass number is the number of protons plus the number of neutrons in the nucleus of an atom

- no. protons = no. electrons = Atomic no. - no. neutrons = Mass no. – Atomic no. - All atoms are neutral because they have equal numbers of protons (+1) & electrons (-1)

e.g., 3 atomic no. (no. protons) Li 7 mass no. - In the periodic table the atomic no. is above the symbol for the element & the mass no. is below it e.g., 3 atomic no. (no. protons) Li 7 mass no. (no. protons + no. neutrons) - Li has 3 protons (atomic no.) Li has 4 neutrons (mass no. – atomic no.) Li has 3 electrons (no. protons = no. electrons)

Isotopes - The atoms of an element are not always the same, e.g., carbon (C) - There are 3 types of C atom: - carbon-12 - carbon-13 - carbon-14 (used in archaeology to carbon date)

- All these carbon atoms have the same number of protons (6) & electrons (6) because their atomic number is the same (6) - They each have different amounts of neutrons so their mass numbers are different - carbon-12 = 6 neutrons (12-6) - carbon-13 = 7 neutrons (13-6) - carbon-14 = 8 neutrons (14-6)

- Isotopes are atoms of the same element that have different mass numbers because of the number of neutrons in the nucleus - So carbon-12, carbon-13 & carbon-14 are all isotopes of carbon - Other elements also have isotopes, e.g., hydrogen - H-1, H-2 & H-3 chlorine - Cl-35 & Cl-37

- carbon-14 has 6 protons (atomic no.) - Isotopes are shown in their nuclear formula so the mass no. is above the symbol & the atomic no. is below it e.g., 14 mass no. C 6 atomic no. - carbon-14 has 6 protons (atomic no.) carbon-14 has 8 neutrons (mass no. – atomic no.) carbon-14 has 6 electrons (no. protons = no. electrons)

Periodic Table

- The periodic table is a list of all the elements in order of increasing atomic number - All the elements that behave similarly are in columns called groups

- Group I = alkali metals Group II = alkaline earth metals Group VII = halogens Group VIII = noble gases - All the elements in the same group behave similarly because they have the same number of electrons in the outer shells of their atoms

- All the elements are in rows called periods (symbol = n) - The row that an element is in is equal to the number of shells in an atom of that element, e.g., H - row 1 (n=1) so its atoms have 1 shell Li - row 2 (n=2) so its atoms have 2 shells Na - row 3 (n=3) so its atoms have 3 shells K - row 4 (n=4) so its atoms have 4 shells

- The number of elements in each period (row) is equal to the maximum number of electrons allowed in that shell, e.g., Row 1 has 2 = max 2 electrons in shell 1 Row 2 has 8 = max 8 electrons in shell 2 Row 3 has 8 = max 8 electrons in shell 3 Row 4 has 18 = max 18 electrons in shell 4

Electron configuration - The arrangement of electrons in an atom is called the electron configuration Atomic no: Element: n=1: n=2: n=3: n=4: Elec. Conf.: 1 H 1 1 2 He 2 2 3 Li 2 1 2, 1 4 Be 2 2 2, 2 : : : : :

Atomic no: Element: n=1: n=2: n=3: n=4: Elec. Conf.: 11 Na 2 8 1 2, 8, 1 12 Mg 2 8 2 2, 8, 2 : : : : : : : : : : : : 19 K 2 8 8 1 2, 8, 8, 1 20 Ca 2 8 8 2 2, 8, 8, 2

Bohr diagrams Hydrogen: Atomic no.= 1 1proton, 1 electron Mass no. = 1 1-1 = 0, 0 neutron In row 1 so has 1 shell Elec. Conf. = 1 1p 0n

Helium: Atomic no. = 2 2 protons, 2 electrons Mass no Helium: Atomic no.= 2 2 protons, 2 electrons Mass no. = 4 4 - 2 =2, 2 neutrons In row 1 so has 1 shell Elec. Conf. = 2 2p 2n

Lithium: Atomic no. = 3 3 protons, 3 electrons Mass no Lithium: Atomic no.= 3 3 protons, 3 electrons Mass no. = 7 7 - 3 = 4, 4 neutrons In row 2 so has 2 shells Elec. Conf. = 2, 1 3p 4n

Beryllium: Atomic no. = 4 4 protons, 4 electrons Mass no Beryllium: Atomic no.= 4 4 protons, 4 electrons Mass no. = 10 10 - 4 = 6, 6 neutrons In row 2 so has 2 shells Elec. Conf. = 2, 2 4p 6n