# Write electronic formulas of elements with serial numbers: a) 9, 38; b) 42, 20; c) 6, 29. Specify valence electrons

To perform these works, you need to know the formula for filling energy levels with electrons: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p.

And how many electrons each sublevel contains: on the ‘s’ sublevel there can be up to 2 electrons, on ‘p’ – up to 6, on ‘d’ – up to 10 and on ‘f’ up to 14.

Electrons located on the outer, and sometimes on the penultimate, electron layer of an atom can take part in the formation of chemical bonds. Such electrons are called valence electrons.

9. Element with serial number 9 is fluorine. Is in the second period of the seventh group. Let’s write its electronic formula. We take only 1 and 2 levels from the large formula: 1s → 2s → 2p. We fill it, knowing that there can be up to 2 electrons on the ‘s’ sublevel, and up to 6 electrons on ‘p’.

2 electrons on the 1s-sublevel

2 electrons on the 2s-sublevel

5 electrons in the 2p sublevel

1s²2s²2p⁵

The second level contains 7 electrons. Fluorine is in the seventh group. The valency of fluorine is equal to one, because one electron is not enough to fill the P-sublevel, this number of bonds forms fluorine with other elements.

38. The element strontium Sr. We also fill in the formula, just take 5 sublevels, because the fifth period and only S, since second group. The electronic valency is equal to two.

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2

42 – molybdenum (Mo – ordinal number 42), the electronic formula is:

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 4d5

There are 6 electrons in the valence orbitals of the Mo atom, and it has six valence electrons.

Molybdenum atoms in compounds exhibit valences VI, V, IV, III, II, I. The valence of molybdenum characterizes the ability of the Mo atom to form chemical bonds. Valence follows from the structure of the electron shell of an atom; the electrons involved in the formation of chemical compounds are called valence electrons.

20. Element calcium. ; period, second group, Electronic valence 2, t. to. the outer level contains two electrons, which calcium gives up.

1s22s22p63s23p64s2.

6. Carbon is in the second period in the fourth group. Electronic valence 4.

1S22s22p2

Carbon atoms in compounds exhibit valence IV, III, II, I.

The valence of carbon characterizes the ability of the C atom to form chemical bonds.

29. Copper or Cuprum. Copper atoms in compounds exhibit valence IV, III, II, I. The presence of one unpaired electron at the S sublevel indicates that copper exhibits electronic valence I in its compounds: 1s2 2s2 2p6 3s2 3p6 4s1 3d10

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